Sizing a ductless min-split for basement

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airahcaz
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Sizing a ductless min-split for basement

Postby airahcaz » Mon Mar 06, 2017 12:01 pm

How may BTU ductless mini-split is needed to heat a well insulated 1000 sq. foot basement? (say temp in basement averages mid to high 50's now, and would like to get it to 70?)

Some have said 15 btu per 1000 sq ft which makes it a 15,000 btu unit, others 30?
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

mhalley
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Re: Sizing a ductless min-split for basement

Postby mhalley » Mon Mar 06, 2017 1:27 pm

You might want to have a professional do a manual j calculation. Buying too much or too little could be a costly mistake. I see several places online that offer room by room for a fee.
This site has a quick and dirty free one.
https://www.furnacecompare.com/perl/est ... at_loss.pl
This is from the Mitsubishi site. It seems to talk more about cooling, but perhaps it applies to heating also.
Getting a Professional Load Calculation
A qualified Mitsubishi Electric contractor can perform a professional cooling load calculation on any rooms in your home that are designated for ductless split system installation. Utilizing industry standard software called Manual J, the contractor will input data such as the square footage of the room as well as information relating to the number and size of windows, the type and amount of insulation in the walls and ceiling, the number and kind of light bulbs and the usual number of occupants in the room. With this information, the software calculates the cooling load—the precise amount of BTUs of heat energy that must be extracted from the room to maintain a comfortable temperature, usually in the mid-70s. The contractor can then use the cooling load figure to recommend a ductless unit with the BTU capacity range to most effectively and efficiently cool that space.

In the absence of a cooling load calculation performed by a professional, homeowners asking, “what size mini split system do I need?” can make a ballpark estimate of the capacity required to cool a room by using a simple DIY formula. Although it’s not a replacement for a load calculation by a trained technician with sophisticated software, this method can at least serve as a basis to conduct preliminary comparison shopping among different Mitsubishi Electric ductless air conditioning systems. Here’s how to make a rough estimate of the cooling load in any room in your home.

Measure the room where the air handler will be installed. You’ll need to know the total square footage, which means multiplying the length of the room times the width. Don’t include closets in the calculation. If the room is not a perfect square or rectangle, divide it into rectangular areas, figure the square footage of each, then add all the square footage results together for the total.
Figure the BTU requirements to determine how much cooling capacity will be needed to keep the room comfortable. The general formula is to multiply the total square footage of the room by 25 to determine the BTU capacity of a ductless unit to cool the space. Therefore, a room that is 200 square feet in size would require a cooling capacity of 5,000 BTUs per hour.
The baseline BTU-per-hour figure above should be modified to incorporate factors that will increase the cooling load. Make allowances for such variables as typical number of occupants and any use of major appliances. As an approximation, when estimating the cooling load for bedrooms add 450 BTUs to the total for each person who will normally occupy the room. To accommodate major appliances when sizing a ductless unit for a kitchen, add 4,000 BTUs.
When calculating the cooling load for a multi-split system where one outdoor heat pump will serve multiple indoor ductless air handlers, add the BTU estimates for each room where an air handler will be installed to determine the total system requirement.
This might be a better forum to ask your questions
http://hvac-talk.com/vbb/

ralph124cf
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Re: Sizing a ductless min-split for basement

Postby ralph124cf » Mon Mar 06, 2017 4:25 pm

Sizing a heat pump for a basement is hard, because the floor is seldom insulated, and your major heat loss is probably thru the floor into the earth.

Ralph

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Mon Mar 06, 2017 4:35 pm

ralph124cf wrote:Sizing a heat pump for a basement is hard, because the floor is seldom insulated, and your major heat loss is probably thru the floor into the earth.

Ralph


Yup! But I need to start somewhere. With load being low and insulated walls and ceiling, I think I'm down to a 9K or 12K inverter
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Mon Mar 06, 2017 7:34 pm

Size is not based on square footage, except as a very rough ballpark. Size is based on net heat loss. Properly sizing a system involves determining effective U-factors for the walls, floor, ceiling (in this case, the ceiling would be irrelevant, and windows, as well as air leakage, and determining heat loss on a worst case day for that. This can be based on known installed insulation, or on tables of standard values for given construction types.

I wish I'd bookmarked some of the resources I've dug up on this in the past, but I'm afraid I don't have much specific details to offer at the moment.

My gut sense is if you just bought a 15,000 BTU system based on the rule of thumb somebody gave you, you'll probably be, but inverter driven ductless systems in particular are fairly versatile and will probably do fine being oversized. If you want to be sure, and were planning on having it professionally installed anyways, get 2-3 installers out to scope out the install and provide quotes.

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Mon Mar 06, 2017 7:50 pm

iamlucky13 wrote:Size is not based on square footage, except as a very rough ballpark. Size is based on net heat loss. Properly sizing a system involves determining effective U-factors for the walls, floor, ceiling (in this case, the ceiling would be irrelevant, and windows, as well as air leakage, and determining heat loss on a worst case day for that. This can be based on known installed insulation, or on tables of standard values for given construction types.

I wish I'd bookmarked some of the resources I've dug up on this in the past, but I'm afraid I don't have much specific details to offer at the moment.

My gut sense is if you just bought a 15,000 BTU system based on the rule of thumb somebody gave you, you'll probably be, but inverter driven ductless systems in particular are fairly versatile and will probably do fine being oversized. If you want to be sure, and were planning on having it professionally installed anyways, get 2-3 installers out to scope out the install and provide quotes.

Ya got three quotes and theee different recommendations: 9K, 12K, & 18K

Middle ground is to go 12K.

Online load calculators are also all over the map.
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Tue Mar 07, 2017 1:07 am

airahcaz wrote:Ya got three quotes and theee different recommendations: 9K, 12K, & 18K

Middle ground is to go 12K.

Online load calculators are also all over the map.


Did they provide any explanation why they chose those sizes? This is a large expense. They shouldn't be leaving you guessing and combing the internet trying to understand why they deserve your money. Even if the answer is, "I've installed a bunch of systems this size in basements like yours and never have any complaints," that's something.

As the space is currently staying in the mid-50's, clearly you are either in a very mild climate or the space already gets a lot of heat from the main floors. In either case, it sounds credible that a 9000 BTU heat pump could do the job, but if none of them can tell you what their outside design temperature and heat loss calculation was, I'd shy away from the smallest system.

Needing 18,000 BTU would surprise me - my house has a 24,000 BTU ducted (that means I lose some heat to my crawlspace) which adequately heats 1800 square feet, moderately insulated, down to about 35 degrees outside temperature. Somewhere below that the regular electric heat has to come on to keep the house comfortable, or I use my wood stove.

Valuethinker
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Re: Sizing a ductless min-split for basement

Postby Valuethinker » Tue Mar 07, 2017 9:39 am

airahcaz wrote:How may BTU ductless mini-split is needed to heat a well insulated 1000 sq. foot basement? (say temp in basement averages mid to high 50's now, and would like to get it to 70?)

Some have said 15 btu per 1000 sq ft which makes it a 15,000 btu unit, others 30?


There is apparently an HVAC discussion forum, and although the mods prohibit commercial recommendations (I believe), the people there are extremely helpful.

To answer the question properly you need an estimate of heat loss through the fabric.

Wellfleet
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Re: Sizing a ductless min-split for basement

Postby Wellfleet » Tue Mar 07, 2017 9:42 am

I would keep looking until an installer can mention the Manual J and that they will be doing one to size correctly.

I had 2 "recommended" installers make wild guesses about what I needed and one that ran the Manual J before making their recommendation.

They were hesitant to share it with me because they said other homeowners just steal the results and pass it along to the other bidders. It turned out that their taking the time to do the Manual J resulted in the best price and least number of units (1) for my needs. Primarily cooling and supplemental heating for first floor.

Guess who won the business?

Valuethinker
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Re: Sizing a ductless min-split for basement

Postby Valuethinker » Tue Mar 07, 2017 10:42 am

Wellfleet wrote:I would keep looking until an installer can mention the Manual J and that they will be doing one to size correctly.

I had 2 "recommended" installers make wild guesses about what I needed and one that ran the Manual J before making their recommendation.

They were hesitant to share it with me because they said other homeowners just steal the results and pass it along to the other bidders. It turned out that their taking the time to do the Manual J resulted in the best price and least number of units (1) for my needs. Primarily cooling and supplemental heating for first floor.

Guess who won the business?


And it's one area where an *oversizing* is bad as well as an undersizing. Too many on/off cycles?

mortfree
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Re: Sizing a ductless min-split for basement

Postby mortfree » Tue Mar 07, 2017 10:45 am

get an electric fireplace or space heater...

I had someone come out for a mini-split and he basically sized the biggest one and most expensive. He took zero measurements in my basement. think it was a fujitsu model and just over $4,000. he was going for the pay day. I only needed heat and the unit included A/C as well.

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Tue Mar 07, 2017 10:55 am

Valuethinker wrote:
Wellfleet wrote:I would keep looking until an installer can mention the Manual J and that they will be doing one to size correctly.

I had 2 "recommended" installers make wild guesses about what I needed and one that ran the Manual J before making their recommendation.

They were hesitant to share it with me because they said other homeowners just steal the results and pass it along to the other bidders. It turned out that their taking the time to do the Manual J resulted in the best price and least number of units (1) for my needs. Primarily cooling and supplemental heating for first floor.

Guess who won the business?


And it's one area where an *oversizing* is bad as well as an undersizing. Too many on/off cycles?


Don't think so. Supposedly these inverters modulate themselves accordingly.
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Wed Mar 08, 2017 12:06 am

Valuethinker wrote:And it's one area where an *oversizing* is bad as well as an undersizing. Too many on/off cycles?


Maybe a few extra cycles, but more certainly, the cycles will be shorter, with less time spent at their ideal operating point, resulting in reduced efficiency.

In general, I'd rather have a slightly oversized unit than an undersized one that can't keep the place warm.

mortfree wrote:get an electric fireplace or space heater...


If only occasionally using the basement spaces, so keeping the thermostat turned down is acceptable, that's not a bad option. Baseboards will perform much better, though. A typical electric fireplace or space heat is around 4000 BTU, so you'd need a couple of them.

If using the space regularly, there is a good chance the energy savings will pay back the higher initial cost. Assessing the payback is another reason why heat loss calculations can be helpful.

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Wed Mar 08, 2017 12:38 am

So one contractor is stating that a higher btu unit will have a heat pump that operates in lower outdoor temperatures, and below a certain outdoor temperature a smaller unit will produce less heat?

"The output mostly depends on the outdoor temperature, but as you can see it does differ with indoor temperatures as well but very minimal.

It isn't a calculation of how much load is needed like it would be in manual J.

For instance if you take a look at the chart in the red box. It shows you the output for the 24 degrees farenheit outdoor temperature with indoor temperature of 64 degrees will only give you 9.51K BTU on the Art Cool Gallery unit. Which is not enough for your need of 12K you mentioned when the temperature goes that low."
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Wed Mar 08, 2017 2:14 am

All heat pumps have reduced output at lower temperatures. There's not a certain outdoor temperature where it happens. It's a continuous curve.

They are sized to a balance point - the temperature where the heat pump heat output matches the calculated heat loss. When temperatures are lower than that, the unit will not keep the space at the design indoor temperature, so it either needs to be rare enough you can live with it, or you need a backup heat source - ducted heat pumps are usually paired with electric furnace strips for cold days. A ductless heat pump could be backed up by a space heater, or in houses where it replaces a baseboard heater, the baseboards are often kept in place. Or it could be sized all the way down to the lowest expected temperature.

So ideally the installer picks a desired balance point, calculates the heat loss at that temperature, and specifies a heat pump that is adequate.

If one installer designs to a lower outdoor temperature, he will naturally pick a higher capacity unit. Or stated more like your contractor said, a larger heat pump will be adequate in lower temperatures.

Or he'll just pick one he's pretty sure based on experience is big enough like the contractors you had quote seem to be doing, counting on the fact that inverter-driven models work well at partial load (some sources indicate perhaps even work at their most efficient at partial load) to mitigate oversizing.

Valuethinker
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Re: Sizing a ductless min-split for basement

Postby Valuethinker » Wed Mar 08, 2017 4:41 am

iamlucky13 wrote:
Valuethinker wrote:And it's one area where an *oversizing* is bad as well as an undersizing. Too many on/off cycles?


Maybe a few extra cycles, but more certainly, the cycles will be shorter, with less time spent at their ideal operating point, resulting in reduced efficiency.

In general, I'd rather have a slightly oversized unit than an undersized one that can't keep the place warm.


Agreed. However comfort if you are too far oversized? Too warm during the on cycles, too cold during the off?


mortfree wrote:get an electric fireplace or space heater...


If only occasionally using the basement spaces, so keeping the thermostat turned down is acceptable, that's not a bad option. Baseboards will perform much better, though. A typical electric fireplace or space heat is around 4000 BTU, so you'd need a couple of them.

If using the space regularly, there is a good chance the energy savings will pay back the higher initial cost. Assessing the payback is another reason why heat loss calculations can be helpful.
[/quote]

Getting it right first time is preferable to "back up" systems, unless one is in a situation where the latter are used only during the annual cold spell (say 1-2 weeks a winter). When I lived in North America, the average winter temperature was probably around 30 degrees F, but you had a couple of months when it could drop to 10 degrees F.

pshonore
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Re: Sizing a ductless min-split for basement

Postby pshonore » Wed Mar 08, 2017 9:24 am

Does a typical mini-split have a thermostatic control?

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Wed Mar 08, 2017 9:27 am

pshonore wrote:Does a typical mini-split have a thermostatic control?



Absolutely
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

pshonore
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Re: Sizing a ductless min-split for basement

Postby pshonore » Wed Mar 08, 2017 9:50 am

airahcaz wrote:
pshonore wrote:Does a typical mini-split have a thermostatic control?



Absolutely
And how does it work? My Mitsubishi has four settings, HEAT, COOL, DRY (no temp set; really a dehumidifier) and AUTO. To my knowledge, the the first three settings are not controlled by a thermostat (the unit will not start and stop automatically). To quote the manual, "During Auto mode, the unit changes mode (Cool <--> Heat) when the room temp is 4 degrees away from the set temp for more than 15 minutes". We use it primarily for Cooling in Summer so never use Auto. Perhaps we're doing something wrong?

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Wed Mar 08, 2017 1:18 pm

Valuethinker wrote:
iamlucky13 wrote:In general, I'd rather have a slightly oversized unit than an undersized one that can't keep the place warm.


Agreed. However comfort if you are too far oversized? Too warm during the on cycles, too cold during the off?


Right. Significantly oversized systems can overshoot and then have long periods of waiting to hit the on-temperature again.

Getting it right first time is preferable to "back up" systems, unless one is in a situation where the latter are used only during the annual cold spell (say 1-2 weeks a winter). When I lived in North America, the average winter temperature was probably around 30 degrees F, but you had a couple of months when it could drop to 10 degrees F.


Yes, that's the kind of scenario I'm talking about - something like the 95th percentile or even less frequent low temperatures.

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Wed Mar 08, 2017 1:27 pm

iamlucky13 wrote:All heat pumps have reduced output at lower temperatures. There's not a certain outdoor temperature where it happens. It's a continuous curve.

They are sized to a balance point - the temperature where the heat pump heat output matches the calculated heat loss. When temperatures are lower than that, the unit will not keep the space at the design indoor temperature, so it either needs to be rare enough you can live with it, or you need a backup heat source - ducted heat pumps are usually paired with electric furnace strips for cold days. A ductless heat pump could be backed up by a space heater, or in houses where it replaces a baseboard heater, the baseboards are often kept in place. Or it could be sized all the way down to the lowest expected temperature.

So ideally the installer picks a desired balance point, calculates the heat loss at that temperature, and specifies a heat pump that is adequate.

If one installer designs to a lower outdoor temperature, he will naturally pick a higher capacity unit. Or stated more like your contractor said, a larger heat pump will be adequate in lower temperatures.

Or he'll just pick one he's pretty sure based on experience is big enough like the contractors you had quote seem to be doing, counting on the fact that inverter-driven models work well at partial load (some sources indicate perhaps even work at their most efficient at partial load) to mitigate oversizing.


Loved this response from you iamlucky13, so thank you.

As for most recent response from installer: seems in-line, and I'll just use a space heater for those rare cases that the temp outside stays at single digits or teens for multiple days. Here is what he said:

It means you will get full 12K BTU from the machine when outdoor temperatures are at around 30-40 degrees. Below that the units starts to lose BTU. With the chart I gave you, you can see at 14 degrees you only get 8.8K BTU.


9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

iamlucky13
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Re: Sizing a ductless min-split for basement

Postby iamlucky13 » Wed Mar 08, 2017 1:49 pm

airahcaz wrote:9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?


Good to go as in enough to keep the space at your set temperature at 14 degrees? I don't know without a sizing a calculation.

Good to go as in the heat pump will continue to function and even on the coldest days still provide some of the heat your basement needs, yes.

The bottom line here is if I were buying, I'd have more confidence in the hardware choice if it was based on a load calculation, but if for some reason none of the installers I'm working with are willing or able to do those calculations, but do have their own experience to fall back on, I'd be willing to proceed. If I were thinking of going with the guy offering the 9000 BTU unit, I'd want to know how confident he is that the smallest unit is sufficient.
Last edited by iamlucky13 on Wed Mar 08, 2017 2:03 pm, edited 1 time in total.

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Wed Mar 08, 2017 1:53 pm

iamlucky13 wrote:
airahcaz wrote:9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?


Good to go as in enough to keep the space at your set temperature at 14 degrees? I don't know without a sizing a calculation.

Good to go as in the heat pump will continue to function and at least keep the basement significantly warmer than it otherwise would be, yes.

The bottom line here is if I were buying, I'd have more confidence in the hardware choice if it was based on a load calculation, but if for some reason none of the installers I'm working with are willing or able to do those calculations, but do have their own experience to fall back on, I'd be willing to proceed. If I were thinking of going with the guy offering the 9000 BTU unit, I'd want to know how confident he is that the smallest unit is sufficient.


Yes good to go as in the heat pump will keep going and produce whatever heat it can to increase the interior temperature, even at teens outside.

I am leaning towards the 12K and call it a day - those extremely cold days are an anomaly in my area, and can be supplemented with a portable heater as needed as you've mentioned.
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

Valuethinker
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Re: Sizing a ductless min-split for basement

Postby Valuethinker » Thu Mar 09, 2017 5:24 am

airahcaz wrote:
It means you will get full 12K BTU from the machine when outdoor temperatures are at around 30-40 degrees. Below that the units starts to lose BTU. With the chart I gave you, you can see at 14 degrees you only get 8.8K BTU.


9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?


OK what worries me is no Manual J.

What he says re performance is totally on the level. Basically, as the gap between input and output temperature grows, efficiency falls. It can never fall below Coefficient of Performance of 1.0 (1 kwhr moves 1 kwhr of heat) because that's the efficiency of an electric bar (Air Source Heat Pumps, and this is what this is, usually have electric bar backup). So yes, as the gap grows, your efficiency (and hence your output) will fall. The Japanese have made amazing strides with their "splits" but (I am given to understand that) if you faced regularly less than 0 F out (say -10 F, -20 F) you'd need a geothermal unit (Ground Source Heat Pump) to be efficient.

The issue for you is "well insulated". I worry that you are over specced. However as long as the degree of overcapacity is not too high, that's not going to be a big issue- particularly if the new ones "modulate" (i.e. they can flex down their heat output).

The general problem with temperature in commercial buildings is they only have 1 setting (or so it seems). So when people walk into a 64 degree room and feel cold, they set it to 80 degrees to "heat up faster".

(warning: Rant Model Enabled) - Which ignores basic physics. And I've had this conversation with highly intelligent people, and they don't get it. You set the temperature to where you want to *get to* not to *overshoot*. It doesn't get their any faster if you do the latter.

Studies from Systems Dynamics researchers show people just don't get this, which is why basic physics is often ignored in public policy discussions. Another Dan Ariely style "predictably irrational".

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 6:58 am

Valuethinker wrote:
airahcaz wrote:
It means you will get full 12K BTU from the machine when outdoor temperatures are at around 30-40 degrees. Below that the units starts to lose BTU. With the chart I gave you, you can see at 14 degrees you only get 8.8K BTU.


9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?


OK what worries me is no Manual J.

What he says re performance is totally on the level. Basically, as the gap between input and output temperature grows, efficiency falls. It can never fall below Coefficient of Performance of 1.0 (1 kwhr moves 1 kwhr of heat) because that's the efficiency of an electric bar (Air Source Heat Pumps, and this is what this is, usually have electric bar backup). So yes, as the gap grows, your efficiency (and hence your output) will fall. The Japanese have made amazing strides with their "splits" but (I am given to understand that) if you faced regularly less than 0 F out (say -10 F, -20 F) you'd need a geothermal unit (Ground Source Heat Pump) to be efficient.

The issue for you is "well insulated". I worry that you are over specced. However as long as the degree of overcapacity is not too high, that's not going to be a big issue- particularly if the new ones "modulate" (i.e. they can flex down their heat output).

The general problem with temperature in commercial buildings is they only have 1 setting (or so it seems). So when people walk into a 64 degree room and feel cold, they set it to 80 degrees to "heat up faster".

(warning: Rant Model Enabled) - Which ignores basic physics. And I've had this conversation with highly intelligent people, and they don't get it. You set the temperature to where you want to *get to* not to *overshoot*. It doesn't get their any faster if you do the latter.

Studies from Systems Dynamics researchers show people just don't get this, which is why basic physics is often ignored in public policy discussions. Another Dan Ariely style "predictably irrational".


Rant appreciated. I've had the same conversation with my own family that setting the temperature higher doesn't mean it gets warmer faster lol, so "please just keep it at 71 and auto".
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 7:15 am

Valuethinker wrote:
airahcaz wrote:
It means you will get full 12K BTU from the machine when outdoor temperatures are at around 30-40 degrees. Below that the units starts to lose BTU. With the chart I gave you, you can see at 14 degrees you only get 8.8K BTU.


9K btu at 14 degrees outside should be still good to go right?! It may just run at high capacity and for an extended period of time, but that should be ok and a homeowner should expect to pay higher utilities when it is really cold outside anyway. If it went to 0 btu heat, then I suppose it'd be more concerning?


OK what worries me is no Manual J.

What he says re performance is totally on the level. Basically, as the gap between input and output temperature grows, efficiency falls. It can never fall below Coefficient of Performance of 1.0 (1 kwhr moves 1 kwhr of heat) because that's the efficiency of an electric bar (Air Source Heat Pumps, and this is what this is, usually have electric bar backup). So yes, as the gap grows, your efficiency (and hence your output) will fall. The Japanese have made amazing strides with their "splits" but (I am given to understand that) if you faced regularly less than 0 F out (say -10 F, -20 F) you'd need a geothermal unit (Ground Source Heat Pump) to be efficient.

The issue for you is "well insulated". I worry that you are over specced. However as long as the degree of overcapacity is not too high, that's not going to be a big issue- particularly if the new ones "modulate" (i.e. they can flex down their heat output).

The general problem with temperature in commercial buildings is they only have 1 setting (or so it seems). So when people walk into a 64 degree room and feel cold, they set it to 80 degrees to "heat up faster".

(warning: Rant Model Enabled) - Which ignores basic physics. And I've had this conversation with highly intelligent people, and they don't get it. You set the temperature to where you want to *get to* not to *overshoot*. It doesn't get their any faster if you do the latter.

Studies from Systems Dynamics researchers show people just don't get this, which is why basic physics is often ignored in public policy discussions. Another Dan Ariely style "predictably irrational".


NJ average Winter low seems to be in the 20's
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Re: Sizing a ductless min-split for basement

Postby just frank » Thu Mar 09, 2017 7:19 am

I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 7:21 am

just frank wrote:I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.


Unfinished and walls will be 2" of R5 so total R10 and rim joists will be 3.5" of closed cell spray foam.
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Re: Sizing a ductless min-split for basement

Postby Valuethinker » Thu Mar 09, 2017 7:25 am

airahcaz wrote:
just frank wrote:I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.


Unfinished and walls will be 2" of R5 so total R10 and rim joists will be 3.5" of closed cell spray foam.


We have an uninsulated wood floor (sits on beams sitting on a brick foundation, no basement, ie typical Victorian dwelling) and I really feel that cold floor.

Are there no online calculators which would give you an estimate?

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Re: Sizing a ductless min-split for basement

Postby just frank » Thu Mar 09, 2017 12:02 pm

airahcaz wrote:
just frank wrote:I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.


Unfinished and walls will be 2" of R5 so total R10 and rim joists will be 3.5" of closed cell spray foam.


The gen'l formula for conductive heat transfer is Area(sqft)*DeltaT(°F)/R_value(US units, as above) = Heat loss(BTU/hour)

With rim sealing, you can probably neglect air leakage losses.

Example, if above ground wall area is 400 square feet, R-10 and design temp is 20°F, this is 400*50/10 = 2000 BTU/h. For the walls.

Add in windows/doors using the same formula and different R-values.

Slab losses are harder to quantify....generally, they are all around the perimeter...the earth under the slab comes up to setpoint and the heat loss goes away. I would assume a loss area to be a couple square feet per foot of slab perimeter, e.g. a 100' perimeter has losses similar to 200 sq ft. I would then assume R-3-ish and a Delta-T of only 20°F (since the ground temp is prob 50°F) This gives you another 1000 BTU/h or so.

Bottom line...a R-10 insulated, airsealed basement in NJ does not need a minisplit, it probably only needs 3000 kBTU/h or less at design temp. It needs a branch supply and return off your central HVAC.

If you want to install a mini...tie it in to you main living space.

Insulate and seal the basement first....after a few days to heat the slab/earth, it will run 3-5°F below the setpoint upstairs, rather than 50°F as now.

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 12:10 pm

just frank wrote:
airahcaz wrote:
just frank wrote:I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.


Unfinished and walls will be 2" of R5 so total R10 and rim joists will be 3.5" of closed cell spray foam.


The gen'l formula for conductive heat transfer is Area(sqft)*DeltaT(°F)/R_value(US units, as above) = Heat loss(BTU/hour)

With rim sealing, you can probably neglect air leakage losses.

Example, if above ground wall area is 200 square feet, R-10 and design temp is 20°F, this is 200*50/10 = 1000 BTU/h. For the walls.

Add in windows/doors using the same formula and different R-values.

Slab losses are harder to quantify....generally, they are all around the perimeter...the earth under the slab comes up to setpoint and the heat loss goes away. I would assume a loss area to be a couple square feet per foot of slab perimeter, e.g. a 100' perimeter has losses similar to 200 sq ft. I would then assume R-3-ish and a Delta-T of only 20°F (since the ground temp is prob 50°F) This gives you another 1000 BTU/h or so.

Bottom line...a R-10 insulated, airsealed basement in NJ does not need a minisplit, it probably only needs 3000 kBTU/h or less at design temp. It needs a branch supply and return off your central HVAC.

If you want to install a mini...tie it in to you main living space.

Insulate and seal the basement first....after a few days to heat the slab/earth, it will run 3-5°F below the setpoint upstairs, rather than 50°F as now.


This is amazing. So many thanks. But now you've really got me, do I go with the 9K or 12K?

Assume a 60x40 area with 6' below grade and 2' above grade (rim joists). That is larger than my area but should be good to get a rough idea.
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Re: Sizing a ductless min-split for basement

Postby hand » Thu Mar 09, 2017 12:41 pm

just frank wrote:
airahcaz wrote:
just frank wrote:I'd still say its impossible to estimate over the 'net....are the walls insulated or not, to what R-value. Are the rims and sill sealed, insulated? Is the slab insulated, or not, R-value? The first two are more important than the last one.


Unfinished and walls will be 2" of R5 so total R10 and rim joists will be 3.5" of closed cell spray foam.


The gen'l formula for conductive heat transfer is Area(sqft)*DeltaT(°F)/R_value(US units, as above) = Heat loss(BTU/hour)

With rim sealing, you can probably neglect air leakage losses.

Example, if above ground wall area is 400 square feet, R-10 and design temp is 20°F, this is 400*50/10 = 2000 BTU/h. For the walls.

Add in windows/doors using the same formula and different R-values.

Slab losses are harder to quantify....generally, they are all around the perimeter...the earth under the slab comes up to setpoint and the heat loss goes away. I would assume a loss area to be a couple square feet per foot of slab perimeter, e.g. a 100' perimeter has losses similar to 200 sq ft. I would then assume R-3-ish and a Delta-T of only 20°F (since the ground temp is prob 50°F) This gives you another 1000 BTU/h or so.

Bottom line...a R-10 insulated, airsealed basement in NJ does not need a minisplit, it probably only needs 3000 kBTU/h or less at design temp. It needs a branch supply and return off your central HVAC.

If you want to install a mini...tie it in to you main living space.

Insulate and seal the basement first....after a few days to heat the slab/earth, it will run 3-5°F below the setpoint upstairs, rather than 50°F as now.


This is great information and most of it makes intuitive sense to me based on my experience with an R10, air sealed basement in a similar climate. In the winter, my ~1000 sq ft basement is toasty with just the waste heat from the basement installed furnace + limited output from two poorly functioning basement registers.

While this is fine for the winter when the whole house needs heat, there is a period of time in the spring where it is in the 70s / 80s outside, and no heat required in the house, but the basement is in the low 60s and uncomfortably cool due to the ground / foundation temperature. My experience is that some heat additional to the main furnace would be welcome to keep the basement comfortable when the furnace is not needed for the rest of the house.

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Re: Sizing a ductless min-split for basement

Postby Wellfleet » Thu Mar 09, 2017 1:30 pm

We just had one put in yesterday. Don't forget about the condensate drain location. It will need to gravity flow to drain or outside or be pumped.

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Re: Sizing a ductless min-split for basement

Postby BHUser27 » Thu Mar 09, 2017 2:03 pm

pshonore wrote:
airahcaz wrote:
pshonore wrote:Does a typical mini-split have a thermostatic control?

Absolutely
And how does it work? My Mitsubishi has four settings, HEAT, COOL, DRY (no temp set; really a dehumidifier) and AUTO. To my knowledge, the the first three settings are not controlled by a thermostat (the unit will not start and stop automatically). To quote the manual, "During Auto mode, the unit changes mode (Cool <--> Heat) when the room temp is 4 degrees away from the set temp for more than 15 minutes". We use it primarily for Cooling in Summer so never use Auto. Perhaps we're doing something wrong?


pshonore -

I'm not sure I understand your question. HEAT/COOL/DRY/AUTO are operating Modes. If set to HEAT, the unit will only operate in heat mode to reach the set point temperature. If set to COOL it will only operate in cool mode to reach the set point temperature. If set to AUTO it will toggle between heat or cool mode as needed to hold the set point temperature. In all modes you can still set the desired temperature same as any thermostat. I am *sure* your Mitsu mini-split lets you change temperature with the up/down arrows on the remote regardless of the Mode it is in.

See page 7 of this... http://www.mitsubishielectric.ca/en/hvac/PDF/m-series/MSZ-A09-12-15-17NA.pdf

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Re: Sizing a ductless min-split for basement

Postby Ged » Thu Mar 09, 2017 2:08 pm

airahcaz wrote:
Valuethinker wrote:
Wellfleet wrote:I would keep looking until an installer can mention the Manual J and that they will be doing one to size correctly.

I had 2 "recommended" installers make wild guesses about what I needed and one that ran the Manual J before making their recommendation.

They were hesitant to share it with me because they said other homeowners just steal the results and pass it along to the other bidders. It turned out that their taking the time to do the Manual J resulted in the best price and least number of units (1) for my needs. Primarily cooling and supplemental heating for first floor.

Guess who won the business?


And it's one area where an *oversizing* is bad as well as an undersizing. Too many on/off cycles?


Don't think so. Supposedly these inverters modulate themselves accordingly.


Inverters generally have an operating range. So suppose you have one with a range of around 4:1 or so. That means if you have a 15K capacity unit it will operate with few or no on-off cycles at 5K load. However if you have a 25K unit and a 4K load you will still get on-off cycles because the minimum output is 6K.

So you still want to get the sizing right for best efficiency, and of course minimum up front cost.

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Re: Sizing a ductless min-split for basement

Postby pshonore » Thu Mar 09, 2017 2:15 pm

BHUser27 wrote:
pshonore wrote:
airahcaz wrote:
pshonore wrote:Does a typical mini-split have a thermostatic control?

Absolutely
And how does it work? My Mitsubishi has four settings, HEAT, COOL, DRY (no temp set; really a dehumidifier) and AUTO. To my knowledge, the the first three settings are not controlled by a thermostat (the unit will not start and stop automatically). To quote the manual, "During Auto mode, the unit changes mode (Cool <--> Heat) when the room temp is 4 degrees away from the set temp for more than 15 minutes". We use it primarily for Cooling in Summer so never use Auto. Perhaps we're doing something wrong?


pshonore -

I'm not sure I understand your question. HEAT/COOL/DRY/AUTO are operating Modes. If set to HEAT, the unit will only operate in heat mode to reach the set point temperature. If set to COOL it will only operate in cool mode to reach the set point temperature. If set to AUTO it will toggle between heat or cool mode as needed to hold the set point temperature. In all modes you can still set the desired temperature same as any thermostat. I am *sure* your Mitsu mini-split lets you change temperature with the up/down arrows on the remote regardless of the Mode it is in.

See page 7 of this... http://www.mitsubishielectric.ca/en/hvac/PDF/m-series/MSZ-A09-12-15-17NA.pdf
Absolutely, but where is the measurement taken that turns the unit on or off? On the remote? on the unit itself just below the ceiling? In the summer we usually set on it 72 - 74 for cooling, but it seems to run continuously (but uses hardly any electricity) and the room is certainly comfortable.

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 2:20 pm

Wellfleet wrote:We just had one put in yesterday. Don't forget about the condensate drain location. It will need to gravity flow to drain or outside or be pumped.


Which, what size, square footage, and basement or other level?
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Re: Sizing a ductless min-split for basement

Postby just frank » Thu Mar 09, 2017 2:29 pm

airahcaz wrote:
This is amazing. So many thanks. But now you've really got me, do I go with the 9K or 12K?

Assume a 60x40 area with 6' below grade and 2' above grade (rim joists). That is larger than my area but should be good to get a rough idea.


Big basement.

OK. LEt's say the ground freezes down to 2' so thats 4' at about 20°F, and a perimeter of 200' is 800 sqft at R-10. That is 800*50/10 = 4000 BTU/h

Let's say the bottom two ft of the wall are at 50°F, 400*20/10 = 800 BTU/h

Let's say the slab edge loss is a 2' wide strip 200' long and R-3 to 50°F it would be 400*20/3 = 2666.

If you add it up, you get 7500 BTU/h conduction at 20°F outside. This load would be well buffered by the underground thermal mass. If the temp fell for one day to 0°F, it would NOT increase proportionally (to (70/50)*7500 = 10,500). It would take days at that temp for the load to creep up, but the mini output would fall during weather that cold.

If the 9K put out 9K at 20°F, you'd be fine with a little margin, and it would throttle down as needed and not cycle too much.

If it would bug you if the basement got a little chilly during a cold snap (like 3 days in single digits) every few years, then go up one size. It will prob be slightly more efficient running at lower throttle.

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Re: Sizing a ductless min-split for basement

Postby Wellfleet » Thu Mar 09, 2017 2:38 pm

We had a Fujitsu AOU15RLFFH floor unit installed in our kitchen/dining room, first floor with condenser and drains through walkout basement.

We installed primarily for cooling and shoulder season heating.

Took a few minutes of discussion with installer once we had finished basement ceiling opened up to figure out where condensate would go.

http://www.fujitsugeneral.com/us/reside ... 15rlf.html

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 2:41 pm

Wellfleet wrote:We had a Fujitsu AOU15RLFFH floor unit installed in our kitchen/dining room, first floor with condenser and drains through walkout basement.

We installed primarily for cooling and shoulder season heating.

Took a few minutes of discussion with installer once we had finished basement ceiling opened up to figure out where condensate would go.

http://www.fujitsugeneral.com/us/reside ... 15rlf.html


Wow, heard of these but never knew anyone who actually, installed - how did you determine to go with a floor unit vs. wall?
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 2:42 pm

just frank wrote:
airahcaz wrote:
This is amazing. So many thanks. But now you've really got me, do I go with the 9K or 12K?

Assume a 60x40 area with 6' below grade and 2' above grade (rim joists). That is larger than my area but should be good to get a rough idea.


Big basement.

OK. LEt's say the ground freezes down to 2' so thats 4' at about 20°F, and a perimeter of 200' is 800 sqft at R-10. That is 800*50/10 = 4000 BTU/h

Let's say the bottom two ft of the wall are at 50°F, 400*20/10 = 800 BTU/h

Let's say the slab edge loss is a 2' wide strip 200' long and R-3 to 50°F it would be 400*20/3 = 2666.

If you add it up, you get 7500 BTU/h conduction at 20°F outside. This load would be well buffered by the underground thermal mass. If the temp fell for one day to 0°F, it would NOT increase proportionally (to (70/50)*7500 = 10,500). It would take days at that temp for the load to creep up, but the mini output would fall during weather that cold.

If the 9K put out 9K at 20°F, you'd be fine with a little margin, and it would throttle down as needed and not cycle too much.

If it would bug you if the basement got a little chilly during a cold snap (like 3 days in single digits) every few years, then go up one size. It will prob be slightly more efficient running at lower throttle.


Ya, cost difference between the 9 and 12K are insignificant, so really want to go with what's appropriate - given this, would you do 9 or 12? :)
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Re: Sizing a ductless min-split for basement

Postby just frank » Thu Mar 09, 2017 2:46 pm

airahcaz wrote:
Ya, cost difference between the 9 and 12K are insignificant, so really want to go with what's appropriate - given this, would you do 9 or 12? :)


If the cost difference doesn't matter....get the 12. You will get cold enough sometimes to really limit its BTU output...and then the 12 will put out more than the 9.

I personally wouldn't worry much if the bigger unit cycles a bit under low load, it would be 'soft start'. And it would run quieter.
Last edited by just frank on Thu Mar 09, 2017 2:48 pm, edited 1 time in total.

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Re: Sizing a ductless min-split for basement

Postby Wellfleet » Thu Mar 09, 2017 2:47 pm

airahcaz wrote:Wow, heard of these but never knew anyone who actually, installed - how did you determine to go with a floor unit vs. wall?


Mostly the look. Our family's supreme allied commander (not me) did not like the wall mounted look in a high traffic room.

Mitsubishi offers one too but no installer had experience with it.

Secondly, this website has a lot of good information and one of their basic findings is that mounting the wall unit closer to the floor gets lower return temps which makes for better operation. http://www.greenbuildingadvisor.com/

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Re: Sizing a ductless min-split for basement

Postby BHUser27 » Thu Mar 09, 2017 2:58 pm

pshonore wrote:Absolutely, but where is the measurement taken that turns the unit on or off? On the remote? on the unit itself just below the ceiling? In the summer we usually set on it 72 - 74 for cooling, but it seems to run continuously (but uses hardly any electricity) and the room is certainly comfortable.

OK I understand. The temperature sensor is built into the wall unit. You can buy a remote temperature sensor for some makes/models.

As far as running continuously, this isn't necessarily a bad thing for a mini-split/heat-pump. The mini is most efficient when it can chug along and just keep up with the heat loss of the room/house. This is why sizing is important. They run/cycle differently than common forced-air HVAC systems by design.

From... http://www.greenbuildingadvisor.com/articles/dept/musings/rules-thumb-ductless-minisplits
"These units are designed to modulate, and operate at higher efficiencies when they run continuously than when they are turned off and on."

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Thu Mar 09, 2017 3:23 pm

BHUser27 wrote:
pshonore wrote:Absolutely, but where is the measurement taken that turns the unit on or off? On the remote? on the unit itself just below the ceiling? In the summer we usually set on it 72 - 74 for cooling, but it seems to run continuously (but uses hardly any electricity) and the room is certainly comfortable.

OK I understand. The temperature sensor is built into the wall unit. You can buy a remote temperature sensor for some makes/models.

As far as running continuously, this isn't necessarily a bad thing for a mini-split/heat-pump. The mini is most efficient when it can chug along and just keep up with the heat loss of the room/house. This is why sizing is important. They run/cycle differently than common forced-air HVAC systems by design.

From... http://www.greenbuildingadvisor.com/articles/dept/musings/rules-thumb-ductless-minisplits
"These units are designed to modulate, and operate at higher efficiencies when they run continuously than when they are turned off and on."


Thanks for the article:

Ueno and Loomis wrote, “One homeowner complained of temperature unevenness; when the data were examined, it was clear that they operated their MSHP [minisplit heat pump] in an ‘on-off’ manner, rather than using a fixed setpoint. This resulted in wide swings in interior temperature (between 60°F and 70°F+). The electricity use showed many hours with the MSHP running at maximum capacity, followed by periods with the unit shut off. When operated in this manner, the MSHP is heating at its least efficient (maximum output) state. Electricity consumption was a high consumption outlier; when compared with simulations, it was the worst-performing house (heating [energy] use 57% higher than simulation).”
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Re: Sizing a ductless min-split for basement

Postby qiora » Fri Mar 10, 2017 12:11 pm

I have been considering installing mini-splits in my house in LI, and have done a lot of research. My feeling is that for a "well-insulated" 1000 sq ft basement in NJ, assuming its open floor layout with no air distribution issue, even a 9000 BTU/h mini-split may be oversized.

If you have time to research and want to get the sizing right, I also recommend going through the greenbuildingadvisor website for info/personal education on mini-splits and insulation. So you decide for yourself. I will provide some useful articles below. You may post your specific case on their Q&A section, and receive sound professional advice.

Here are what I learnt from my research that may apply to your case:
1) I would not rely on HVAC contractors to accurately estimate heating/cooling loads for sizing. I have similar experience when asking for quotes. Sizing and upfront install cost from several contractors are all over the place. Most contractors size equipment by rule-of-thumb only (say 500 sq ft per ton - one ton = 12,000 BTU/h), which is extremely conservative for a "well-insulated" basement in NJ.

2) Its easy to rough-estimate your whole-house heating load using the fuel-use based method as outlined in the article below.

http://www.greenbuildingadvisor.com/blo ... ut-old-new

For estimating heating load of just one room,

http://www.greenbuildingadvisor.com/blo ... ion-part-1

For estimating cooling loads, it requires Manual J. But it is very much a garbage-in-garbage-out analysis. If you ask a HVAC contractor to do it, it will be a waste of time unless you know how to sanity-check their results.

3) In general, if you airseal/insulate rim joists, and at least insulate the above-grade portion of your basement, your peak load at 99% design outdoor temp (+15F in your area?) should be small enough to be covered by a 9000 BTU/h mini-split. But more importantly, you need to check your part load at 47F (shoulder seasons - spring and fall) against the minimum output of the equipment at 47F reported in typical product catalog. If the minimum output is much higher than your part load, your unit's efficiency will suffer, and eventually short cycle to death during shoulder seasons. Details are outlined in article below (also check their comment section at the bottom).

http://www.greenbuildingadvisor.com/art ... -minisplit

This is where I lost all contractors who think there is no harm to over-sizing. Yes, they will say mini-splits are smart enough to "modulate" to different outdoor temp, but only to a point.

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Re: Sizing a ductless min-split for basement

Postby airahcaz » Fri Mar 10, 2017 12:24 pm

qiora wrote:I have been considering installing mini-splits in my house in LI, and have done a lot of research. My feeling is that for a "well-insulated" 1000 sq ft basement in NJ, assuming its open floor layout with no air distribution issue, even a 9000 BTU/h mini-split may be oversized.

If you have time to research and want to get the sizing right, I also recommend going through the greenbuildingadvisor website for info/personal education on mini-splits and insulation. So you decide for yourself. I will provide some useful articles below. You may post your specific case on their Q&A section, and receive sound professional advice.

Here are what I learnt from my research that may apply to your case:
1) I would not rely on HVAC contractors to accurately estimate heating/cooling loads for sizing. I have similar experience when asking for quotes. Sizing and upfront install cost from several contractors are all over the place. Most contractors size equipment by rule-of-thumb only (say 500 sq ft per ton - one ton = 12,000 BTU/h), which is extremely conservative for a "well-insulated" basement in NJ.

2) Its easy to rough-estimate your whole-house heating load using the fuel-use based method as outlined in the article below.

http://www.greenbuildingadvisor.com/blo ... ut-old-new

For estimating heating load of just one room,

http://www.greenbuildingadvisor.com/blo ... ion-part-1

For estimating cooling loads, it requires Manual J. But it is very much a garbage-in-garbage-out analysis. If you ask a HVAC contractor to do it, it will be a waste of time unless you know how to sanity-check their results.

3) In general, if you airseal/insulate rim joists, and at least insulate the above-grade portion of your basement, your peak load at 99% design outdoor temp (+15F in your area?) should be small enough to be covered by a 9000 BTU/h mini-split. But more importantly, you need to check your part load at 47F (shoulder seasons - spring and fall) against the minimum output of the equipment at 47F reported in typical product catalog. If the minimum output is much higher than your part load, your unit's efficiency will suffer, and eventually short cycle to death during shoulder seasons. Details are outlined in article below (also check their comment section at the bottom).

http://www.greenbuildingadvisor.com/art ... -minisplit

This is where I lost all contractors who think there is no harm to over-sizing. Yes, they will say mini-splits are smart enough to "modulate" to different outdoor temp, but only to a point.


Thanks! Will definitely read through all this and report back, but what is meant by this so I'm clear when I go through the exercise:

(+15F in your area?)
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)

qiora
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Re: Sizing a ductless min-split for basement

Postby qiora » Fri Mar 10, 2017 12:47 pm

+15F is the 99% design outdoor temperature for winter in Long Island. I assume its the same in NJ. This is the outdoor temp that a location stays above for 99% of all hours in a year, based on 30-year average. People typically size HVAC equipment using 99% design outdoor temp. Feel free to find out what your actual design temp is based on your location.

http://www.greenbuildingadvisor.com/blo ... e-99-and-1

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just frank
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Location: Philly Metro

Re: Sizing a ductless min-split for basement

Postby just frank » Fri Mar 10, 2017 9:54 pm

The important point here is whether the basement is 1000 sq ft, where the 9kBTU is oversized, or 2400 sq ft, in which case it is probably just adequate.

Perhaps the OP could provide the actual dimensions?

airahcaz
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Re: Sizing a ductless min-split for basement

Postby airahcaz » Fri Mar 10, 2017 9:57 pm

just frank wrote:The important point here is whether the basement is 1000 sq ft, where the 9kBTU is oversized, or 2400 sq ft, in which case it is probably just adequate.

Perhaps the OP could provide the actual dimensions?


Hi -

I am deciding between the LG Art Cool Gallery 9000K BTU LA090HVP or their 12000 BTU LA120HVP for our basement, mostly because we like the picture option, trust the brand, and have been told by various HVAC folks that 9 or 12K will suffice, but honestly have discounted all those that have said my basement requires 18-24K min. Doubt that, because:

The living space of the basement that I predominantly wish to heat is approx. 32' x 27', or say 900 sq. ft.

Currently unfinished, but 6' of the below grade walls will have 2" of R5 rigid foam, so total R10, and rim joists (approx. 2' of above ground) will be 3.5" of closed cell spray foam, so basement will lean towards well insulated.

Basement averages mid to high 50's now, and would like to get it to 68/70.

I'm in Northern NJ where my Heating 99% Dry Bulb is 10 degrees, so DeltaT is 60 we can assume.

Any help appreciated and thanks in advance.
1) Invest you must 2) Time is your friend 3) Impulse is your enemy 4) Basic arithmetic works 5) Stick to simplicity 6) Stay the course. (Plagiarized, but worth stealing)


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