HVAC replacement when experts disagree

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jco
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HVAC replacement when experts disagree

Post by jco »

[The OP has posted an update here (Page 2) --admin LadyGeek]

We need to replace our A/C (and may replace the furnace too). My problem is that I'm getting very different sizing quotes about what size A/C and furnace we should get. All the way from a 2.5 Ton A/C to a 4 Ton A/C.

The companies all say they are using Manual J to do the calculations. The one who spent the most time doing the calculations recommended a 2.5 Ton A/C. The one who spent the least amount of time recommends 4 tons. Others are in between. What I can say is that our current 3.5 Ton A/C would run all day and barely make the house comfortable. That has me thinking a slightly bigger A/C is the way to go. I've also noticed that they're recommending smaller furnace units, but our current furnace has generally made the house very comfortable. We don't have any real concerns there.

Is it possible some companies are trying to sell me a smaller A/C and furnace so the quote looks more competitive? The quotes for a bigger A/C and same-sized furnace is slightly more, but could be the better choice if it actually makes us comfortable!

Edit: Since some people are asking for some additional info:

Our house is a 20+-year-old two-story house with a basement. Total sq ft: 2900, with the main level and second story being about 1900 sq ft and the basement having 1000 sq ft.

I don't think the insulation is bad. R-30 in the attic ceilings, R-20 in the exterior walls. I've seen the attic insulation, so I think it's legitimate. We have a fair number of windows in our house, and some of them are on the bigger side, but it would never be characterized as a house "with a lot of windows". Each room pretty much as at least one window, and one bathroom has two.

We do not have vaulted ceilings (in general) but the entryway is vaulted. But the floor area is about 12x10' where it's vaulted, so I doubt it makes a huge difference in determining equipment sizing.

Both the furnace and A/C are original to the house (to the best of my knowledge). The furnace is original to the house. The A/C is not, and was poorly installed. The furnace has always worked fine. The A/C has always been pretty disappointing in the 3ish years we've lived in the home.

Based on UALflyer's questions below:

Have any contractors checked the refrigerant pressure to ensure that it's not low?
- No, since we want to replace the system. The fan motor is definitely bad. And R22 is so expensive. So they're focusing on replacement. Even an independent HVAC servicer that doesn't do installs suggested I get it replaced.

Have any contractors looked at the supply and return vents and confirmed that the pressure is balanced?
- I don't think so. A couple have commented on removing the return vents up the ceiling in our upstairs bedrooms to improve cooling in the summer. They have said that the supply vents are not designed well and that there is probably a lot of turbuluence. These professionals suggested installed scoops and dividers to better direct the airflow.

What tonnage are the ducts sized for?
- No idea, but our furnace is 110,000 BTU with a 5-ton blower (learned this today). So probably pretty big.
- Apparently, it sized for a 2.5 ton A/C (the main supply is 24x8, which technically supports a 2.75 A/C). This supports 1100 CFM. However, there are two 17x8 supply branches, than in theory, pull 750 CFM. So the main supply line seems undersized.
- The return is 24x10, which is effectively a 4 ton return.

Do the ducts have any air leaks?
- It's possible, but most of them are inside the walls, so it's pretty tough to tell.
- There are definitely some small leaks. This can be improved in the basement/crawl space.

Are they well insulated?
- Some are taped at the joints. I don't think they've been sealed with the pastey stuff (my HVAC-technician friend called it "pookie").
- They are not. But this could be improved in some places.

Is your furnace in the attic or in the basement?
- Basement

Has the indoor coil ever been cleaned?
- Yes, in the past few years.

What type of air filter do you use (how thick and what MERV rating) and how often do you replace it?
- 1" with a MERV of 4, which is about as flimsy as I could find. I usually change them every 2 or 3 months. My nest reminds me to change them.

Update:

We ended up going with a 3-ton, two-stage heat pump and 90K two-stage 80% AFUE furnace. The contractor provided a detailed manual J based on our desired settings. The heat pump comes with a pretty substantial rebate, so the system will cost about $10,000 after rebates. They're also performing a lot of ductwork improvement to make sure the air flows as well as possible, as well as adding some hot air returns in the upstairs (basically, cutting a return at the top of the wall to get better hot air removal during the summer). And it comes with a 10-year parts and labor warranty at a price similar to what other contractors were offering with a 2 or 3-year labor warranty. We'll have to do twice-a-year maintenance, but we should be doing that anyway.
Last edited by jco on Mon May 03, 2021 5:01 pm, edited 6 times in total.
Teague
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Re: HVAC replacement when experts disagree

Post by Teague »

Was your current unit adequate back when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
Semper Augustus
mervinj7
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Re: HVAC replacement when experts disagree

Post by mervinj7 »

Teague wrote: Mon Apr 26, 2021 11:47 am Was your current unit adequate back when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
Is that still true with two stage condensers? That is, even if you sightly oversize, can it run at the lower setting longer?

OP, are all the quotes for similar types of units (e.g one stage, two stage, heat pump).
Ddd7651
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Re: HVAC replacement when experts disagree

Post by Ddd7651 »

jco wrote: Mon Apr 26, 2021 11:31 am We need to replace our A/C (and may replace the furnace too). My problem is that I'm getting very different sizing quotes about what size A/C and furnace we should get. All the way from a 2.5 Ton A/C to a 4 Ton A/C.

The companies all say they are using Manual J to do the calculations. The one who spent the most time doing the calculations recommended a 2.5 Ton A/C. The one who spent the least amount of time recommends 4 tons. Others are in between. What I can say is that our current 3.5 Ton A/C would run all day and barely make the house comfortable. That has me thinking a slightly bigger A/C is the way to go. I've also noticed that they're recommending smaller furnace units, but our current furnace has generally made the house very comfortable. We don't have any real concerns there.

Is it possible some companies are trying to sell me a smaller A/C and furnace so the quote looks more competitive? The quotes for a bigger A/C and same-sized furnace is slightly more, but could be the better choice if it actually makes us comfortable!
We recently replaced our ac. My gut says that if you already feel that a 3.5 barely keeps up, a 4 ton wins out. I did discover many new expensive units have fans that are variable rate vs a high medium low. It pushes air at precisely the right amount so it doesn't "kick on and off" as much. You don't have 2-3 degree fluctuations anymore, but perhaps a 1 degree fluctuation and no more hot/cold spots as it simply speeds up just a tad or slows down and running more doesn't always equal a higher electric bill the way they move air around.

That's all I can add in helping make a decision. I don't see how a 2.5 tons unit can keep up when a 3.5 unit that barely kept up but I'm not a pro.

I'll also add that I looked at the amount they were charging me to upgrade from an 2 year labor plan to a 10 year labor plan. I instead put the equivalent money in an investment account earmarking for the "warranty services" that I struggle with buying. Playing the odds I can double it in 7-10 years and have more than paying out of pocket would be. If it makes it 10 years, vacation time.
Topic Author
jco
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Re: HVAC replacement when experts disagree

Post by jco »

Teague wrote: Mon Apr 26, 2021 11:47 am What size is your current unit and was it adequate when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
We currently have a 41,900 BTU 3.5 Ton 10 SEER Single Phase unit. It has never worked well, running all day to make it feel barely tolerable. But it was pretty old, so maybe it was just inefficient. We've only lived in the house 3 years, so I can't comment on how well it worked originally.

We're in Colorado, so not super humid.
Big Dog
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Re: HVAC replacement when experts disagree

Post by Big Dog »

how large is your hosue? Lotsa windows? Cathedral ceilings? Open floor plan? Did they show you the Manual J numbers/assumptions?

Did the HVAC guys check the ducting thoroughly? (I had three guys look at my returns and say they were fine, but then I pulled off the cover and showed them that the cavity returns were blocked underneath. Once we opened another return, the air flow increased nearly two fold.)
cyclist
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Re: HVAC replacement when experts disagree

Post by cyclist »

When we replaced ours, our contractor recommended a significantly smaller unit than what was there - but he refused to do the installation until we improved our attic insulation.

It’s worked beautifully.

Cyclist
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Watty
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Re: HVAC replacement when experts disagree

Post by Watty »

There are lots and lots of other variables but if you know of any neighbors with similar houses that have recently replaced their AC then you could ask them what size they used and if it is working well. I would take that with a huge grain of salt but it could at least give you another frame of reference.
tibbitts
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Re: HVAC replacement when experts disagree

Post by tibbitts »

All the estimates I got (7) agreed on the size, and all were wrong. Go big, at least with >=2 stage systems.
Silverado
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Re: HVAC replacement when experts disagree

Post by Silverado »

tibbitts wrote: Mon Apr 26, 2021 12:28 pm All the estimates I got (7) agreed on the size, and all were wrong. Go big, at least with >=2 stage systems.
I also always say 'go big'. You’ll hear comments about short cycling and other things that go bump in the night. However, cooling has been a big topic in my life as we have owned or rented houses in numerous states (and countries for that matter) and have made cooling a topic of a lot of conversations. And never have I heard someone say 'my AC is too large'. However, I have heard many times, and said it several times 'my AC does not cool enough'.

I will grant that installation is a large factor, maybe the largest, and I would love to be around when my system was originally installed so I could unkink, unblock, etc. the runs. Not so easy later though. So wield the hammer you can.

If your last system was not satisfactory, no way would I ever accept a replacement that was not at least a half ton larger.
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hand
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Re: HVAC replacement when experts disagree

Post by hand »

I almost always say "go small" and happily forced my preferred contractor to size down - system runs more consistently and better handles humidity which markedly improves comfort.

Given the empirical evidence that your current 3.5 ton struggles to keep up, I would have a really hard time going smaller unless the contractor who spent the most time (and may very well be the most diligent or knowledgeable) can provide a compelling explanation on what other variables will be addressed (return duct work, insulation, leaks, etc.) to allow a smaller unit to meet your needs.

Contractors have little incentive to undersize - 2.5 ton is likely the proper manual J answer, you just need to figure out why that answer does not align with your experience in your house (perhaps you prefer colder than typical, perhaps you have exceptionally poor insulation, perhaps you have a bad leak or a dramatically underperforming older system).
alfaspider
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Re: HVAC replacement when experts disagree

Post by alfaspider »

Silverado wrote: Mon Apr 26, 2021 1:08 pm
tibbitts wrote: Mon Apr 26, 2021 12:28 pm All the estimates I got (7) agreed on the size, and all were wrong. Go big, at least with >=2 stage systems.
I also always say 'go big'. You’ll hear comments about short cycling and other things that go bump in the night. However, cooling has been a big topic in my life as we have owned or rented houses in numerous states (and countries for that matter) and have made cooling a topic of a lot of conversations. And never have I heard someone say 'my AC is too large'. However, I have heard many times, and said it several times 'my AC does not cool enough'.

I will grant that installation is a large factor, maybe the largest, and I would love to be around when my system was originally installed so I could unkink, unblock, etc. the runs. Not so easy later though. So wield the hammer you can.

If your last system was not satisfactory, no way would I ever accept a replacement that was not at least a half ton larger.
The problem with the "by the book" calc is it tends not to calculate for extreme situations. My central AC is too large by the book, and it is for about 80% of a/c season. But during those hottest days of the year, it's running nearly constantly during the afternoon to keep up. I'll accept a bit more humidity when it's 90* to be comfortable when it's 104*.
Topic Author
jco
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Re: HVAC replacement when experts disagree

Post by jco »

hand wrote: Mon Apr 26, 2021 1:22 pm Contractors have little incentive to undersize - 2.5 ton is likely the proper manual J answer, you just need to figure out why that answer does not align with your experience in your house (perhaps you prefer colder than typical, perhaps you have exceptionally poor insulation, perhaps you have a bad leak or a dramatically underperforming older system).
I've wondered this also. I've seen the insulation in the attics and the exterior walls. It all seems to be kosher. In fact, we have several INTERIOR walls that are insulated! I don't think that would have a negative effect, but who knows.

But the leak and underperforming system are definitely possibilities.
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lthenderson
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Re: HVAC replacement when experts disagree

Post by lthenderson »

jco wrote: Mon Apr 26, 2021 11:31 am We need to replace our A/C (and may replace the furnace too). My problem is that I'm getting very different sizing quotes about what size A/C and furnace we should get. All the way from a 2.5 Ton A/C to a 4 Ton A/C.

The companies all say they are using Manual J to do the calculations.
The problem I see is that the accredited Manual J software packages are quite complex and there are lots of things that must be answered that can be quite subjectively like insulation levels, and as a result, produce widely varying results. It just takes a few wrong clicks to get a drastically different answer. You have a real data point with your old unit. That is what I would base my new unit size.
UALflyer
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Re: HVAC replacement when experts disagree

Post by UALflyer »

Silverado wrote: Mon Apr 26, 2021 1:08 pm I also always say 'go big'. You’ll hear comments about short cycling and other things that go bump in the night. However, cooling has been a big topic in my life as we have owned or rented houses in numerous states (and countries for that matter) and have made cooling a topic of a lot of conversations. And never have I heard someone say 'my AC is too large'. However, I have heard many times, and said it several times 'my AC does not cool enough'.

I will grant that installation is a large factor, maybe the largest, and I would love to be around when my system was originally installed so I could unkink, unblock, etc. the runs. Not so easy later though. So wield the hammer you can.

If your last system was not satisfactory, no way would I ever accept a replacement that was not at least a half ton larger.
People don't say "my AC is too large," as saying this requires them to be aware of the cause of their discomfort. People do complain about various a/c issues all the time, however, and a number of complaints can be caused by oversizing the unit.

Have any competent contractors actually taken a look at the ducts? If you put in a larger unit and the ducts are undersized for it, static pressure will increase, which will cause excessive noise as air moves through them (it'll also cause additional wear and tear of the unit, which will cause breakdowns; it can also cause your indoor coil to freeze up regularly). On the other hand, if you stick a smaller unit in, which causes the ducts to be oversized, you'll be reducing air flow, which will cause some rooms to receive insufficient airflow and to feel uncomfortable. Likewise, if there's an issue with your returns (too small, not enough of them, wrong location, etc...), not enough air will be pulled from certain rooms into the unit, which can also create noise (and cause additional wear and tear) and cause rooms to feel stuffy.

So, when people say "my AC does not cool enough," in many cases it'll be less about insufficient cooling capacity and more about insufficient airflow or insufficient moisture removal. In those situations, upsizing the unit can actually make the situation worse. So, you simply can't just blindly stick a larger unit in... well, you can, and incompetent contractors do this all the time, which in a lot of cases ends up only making the problem worse.
Normchad
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Re: HVAC replacement when experts disagree

Post by Normchad »

jco wrote: Mon Apr 26, 2021 11:57 am
Teague wrote: Mon Apr 26, 2021 11:47 am What size is your current unit and was it adequate when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
We currently have a 41,900 BTU 3.5 Ton 10 SEER Single Phase unit. It has never worked well, running all day to make it feel barely tolerable. But it was pretty old, so maybe it was just inefficient. We've only lived in the house 3 years, so I can't comment on how well it worked originally.

We're in Colorado, so not super humid.
If 3.5 tons isn’t adequate, you better pick something at least that big...... you’re just about guaranteed go be unhappy with anything smaller...... in your shoes, I’d go 4 tons.

Separately, you might inquire about having an energy audit done of your home. Maybe something odd is going on.....

Is your home uniformly uncomfortable? Or is it just a couple rooms that you can’t get right? If it’s just a couple rooms, ask the AC guys what they can do to fix that. It could be leaky ducts. Oppress it could be undersized ducts to those rooms, or even just inadequate air returns.
UALflyer
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Re: HVAC replacement when experts disagree

Post by UALflyer »

Normchad wrote: Mon Apr 26, 2021 7:34 pm
jco wrote: Mon Apr 26, 2021 11:57 am
Teague wrote: Mon Apr 26, 2021 11:47 am What size is your current unit and was it adequate when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
We currently have a 41,900 BTU 3.5 Ton 10 SEER Single Phase unit. It has never worked well, running all day to make it feel barely tolerable. But it was pretty old, so maybe it was just inefficient. We've only lived in the house 3 years, so I can't comment on how well it worked originally.

We're in Colorado, so not super humid.
If 3.5 tons isn’t adequate, you better pick something at least that big......
It's more complicated than that. If the existing unit has a leak and the refrigerant pressure is low, the temperature differential would be lower, so it would, in fact, be taking it much, much longer to achieve the desired temperature. The same is true if there are airflow restrictions, or breaks in the ductwork, etc...
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Re: HVAC replacement when experts disagree

Post by WS1 »

1rl9DS5gl2
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Re: HVAC replacement when experts disagree

Post by 1rl9DS5gl2 »

I was in the HVAC industry for over 30 years. In my experience it's much more likely that a contractor will recommend a larger unit so as to insure he does not have to risk the "it isn't doing the job even though it runs all the time" complaint, rather than a smaller unit to lower the cost.
Galt guy
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Re: HVAC replacement when experts disagree

Post by Galt guy »

We have a 15-year old 4 ton unit that was getting low on refrigerant. HVAC company topped it off last year and it worked much better, although I’m prepared to bite the bullet and replace the system if it leaks any more. Maybe you’re low on refrigerant?
Coburn
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Re: HVAC replacement when experts disagree

Post by Coburn »

Lot of details which the OP hasn't shared, but assuming poor insulation, low refrigerant, etc...have been already been addressed and if no change with duct-work on up-sizing , # of ducts, etc...yeah, I'd be looking at 4-ton (at least) on the replacement, too.

A 2-stage is nice, but merits may be unduly emphasized...too little info.
congruentsea
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Re: HVAC replacement when experts disagree

Post by congruentsea »

Last year I spent a few hours on a forum equivalent to the Bogleheads of HVAC. Similar personalities to here. My biggest takeaway short of needed repair/replace etc was to avoid the 1" pleated filter. The higher the filtration the more restrictive on the system. The experts agreed, get a hoghair filter and replace it frequently. I did get better cooling from that simple tip.
illumination
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Re: HVAC replacement when experts disagree

Post by illumination »

A "quick and dirty" way to check would be to see if your current unit is giving you a 15-20 degree drop. You take the air temp before the unit on the intake and then after on a register, as close as you can get in terms of distance. If you're getting around that range of drop, before and after, it probably means the unit is otherwise working correctly. But something like a refrigerant level being off even just a bit can make a big difference.

I'm in the camp that a slightly oversized unit is better than one that is undersized. People complain more about a unit not being able to "keep up" cooling down a room than they will dehumidification. But 2.5 to 4 tons is such a huge range, you really need to get a better read on things.

If the unit from the beginning has never felt like it could properly cool the house down, I'm inclined to say go up a size.
jumppilot
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Re: HVAC replacement when experts disagree

Post by jumppilot »

congruentsea wrote: Tue Apr 27, 2021 3:45 pm Last year I spent a few hours on a forum equivalent to the Bogleheads of HVAC. Similar personalities to here. My biggest takeaway short of needed repair/replace etc was to avoid the 1" pleated filter. The higher the filtration the more restrictive on the system. The experts agreed, get a hoghair filter and replace it frequently. I did get better cooling from that simple tip.
I recently had a 4 inch filter retrofit. According to the internet, you only need to change those every 6 months.

However, the filter box is so tight (no airflow unless it’s through the filter!) I found I have to replace them every 2 months to get adequate heating and cooling. In fact, my furnace would short cycle in the winter due to the reduced airflow causing an over temp situation.

So yeah, replace your filters often.
congruentsea
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Re: HVAC replacement when experts disagree

Post by congruentsea »

jumppilot wrote: Tue Apr 27, 2021 4:35 pm
congruentsea wrote: Tue Apr 27, 2021 3:45 pm Last year I spent a few hours on a forum equivalent to the Bogleheads of HVAC. Similar personalities to here. My biggest takeaway short of needed repair/replace etc was to avoid the 1" pleated filter. The higher the filtration the more restrictive on the system. The experts agreed, get a hoghair filter and replace it frequently. I did get better cooling from that simple tip.
I recently had a 4 inch filter retrofit. According to the internet, you only need to change those every 6 months.

However, the filter box is so tight (no airflow unless it’s through the filter!) I found I have to replace them every 2 months to get adequate heating and cooling. In fact, my furnace would short cycle in the winter due to the reduced airflow causing an over temp situation.

So yeah, replace your filters often.
the 4" is far better than the 1". It is not intuitive unfortunately. It has significant more filtering surface area so provides less restriction. A 1" high efficiency is a nightmare when it gets dirty.
mgensler
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Re: HVAC replacement when experts disagree

Post by mgensler »

We're in the process of changing out our hvac to mini-split hear pumps. I did the manual j myself. You have to measure all of the room dimensions, windows, doors, insulation, etc. It's very comprehensive. The software also has a lot of built in cushions so you can actually go smaller.than what manual j is recommending.

I would go with the contractor that spent the most time running the calculations. If you're nervous about it, ask them for a copy of the manual j and pay a mechanical engineer an hour or two to review it with you.
jumppilot
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Re: HVAC replacement when experts disagree

Post by jumppilot »

congruentsea wrote: Tue Apr 27, 2021 4:48 pm
jumppilot wrote: Tue Apr 27, 2021 4:35 pm
congruentsea wrote: Tue Apr 27, 2021 3:45 pm Last year I spent a few hours on a forum equivalent to the Bogleheads of HVAC. Similar personalities to here. My biggest takeaway short of needed repair/replace etc was to avoid the 1" pleated filter. The higher the filtration the more restrictive on the system. The experts agreed, get a hoghair filter and replace it frequently. I did get better cooling from that simple tip.
I recently had a 4 inch filter retrofit. According to the internet, you only need to change those every 6 months.

However, the filter box is so tight (no airflow unless it’s through the filter!) I found I have to replace them every 2 months to get adequate heating and cooling. In fact, my furnace would short cycle in the winter due to the reduced airflow causing an over temp situation.

So yeah, replace your filters often.
the 4" is far better than the 1". It is not intuitive unfortunately. It has significant more filtering surface area so provides less restriction. A 1" high efficiency is a nightmare when it gets dirty.

I switched to the lowest MERV rating for that reason.
jackbeagle
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Re: HVAC replacement when experts disagree

Post by jackbeagle »

Don't accept excuses from people whom you are paying, or suggestions which go against your gut.

What many people think to be an under/oversized unit, is actually a less-than-ideal fan speed. Maybe a 3.5 ton unit would have performed great, if the blower speed were turned up. Perhaps then, it wouldn't run constantly, even in the summer. A performing unit, even under extreme conditions, should still have enough "headroom" to cycle on/off in the most oppressive part of the year. The most typical pattern I see with my Nest history report is about 15 min on, 15 min off in the dead of summer. My house is not anywhere NEAR being new, and the insulation has never been upgraded. On the hottest days, my system run time doesn't exceed 12 hours (half the day).

If your unit is running 18 hours + a day, and running 4+ hour stretches without cycling off, I would call it undersized, or your fan is basically only whispering air out of your vents. You should be able to feel the air a good 4-5 feet away from the vent, but it shouldn't be so fast as to dry your eyes out from across the room. This is a time when you put your gut feelings ahead of the tech's advice. They know the systems they sell, but they haven't lived in your house, nor do they know you like to keep it 68 degrees when it's 98 outside. A central AC should be able to do that. It's all excuses if they tell you it can't.

Statements such as "you can only expect 20 degrees cooler than it is outside" - wrong, Jimbo. That's the temperature difference you can expect between return and your vents. By nature of a home working sort of like a car with the a/c turned on recirculation, it should CONTINUALLY cool the air until the cooling capacity of the system is reached.
talzara
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Re: HVAC replacement when experts disagree

Post by talzara »

alfaspider wrote: Mon Apr 26, 2021 1:23 pm The problem with the "by the book" calc is it tends not to calculate for extreme situations. My central AC is too large by the book, and it is for about 80% of a/c season. But during those hottest days of the year, it's running nearly constantly during the afternoon to keep up. I'll accept a bit more humidity when it's 90* to be comfortable when it's 104*.
Actually, "by the book" only calculates for extreme situations.

The Manual J design temperature is the the 99th percentile temperature. Manual J includes a built-in oversizing factor of 15%, and Manual S rounds up to the next half-ton. If you have 2.7 tons of calculated load, you end up with a 3.5-ton air conditioner: 130% of load. Since this includes internal loads, the delta T could be 140% or 150%.

Air conditioners are designed to run at 100% of capacity. It's hardest on the components when the compressor starts up. The air conditioner will last longer if it cycles less.
talzara
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Re: HVAC replacement when experts disagree

Post by talzara »

jco wrote: Mon Apr 26, 2021 11:31 am Is it possible some companies are trying to sell me a smaller A/C and furnace so the quote looks more competitive? The quotes for a bigger A/C and same-sized furnace is slightly more, but could be the better choice if it actually makes us comfortable!
No. There is a relatively small difference between the different equipment sizes, and the installation costs are the same.

HVAC contractors like to sell the largest equipment they can. In state that don't require Manual J, it's hard to find a contractor that will do it. In states that require a Manual J, contractors know how to change the numbers to make it come out bigger.

It is likely that the 2.5 ton HVAC contractor did the calculation correctly, and everyone else did it wrong. That doesn't mean the contractor is actually correct. It just means that it is much more likely than the other way around. HVAC contractors always want to increase equipment size, so if someone doesn't do it, then there must be a reason.

3.5 tons is a lot of cooling for Colorado, unless you live in a really hot place like LaJunta. If it is not cooling adequately, it is likely that the equipment is malfunctioning. How big is the house?
talzara
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Re: HVAC replacement when experts disagree

Post by talzara »

jco wrote: Mon Apr 26, 2021 11:57 am We currently have a 41,900 BTU 3.5 Ton 10 SEER Single Phase unit. It has never worked well, running all day to make it feel barely tolerable. But it was pretty old, so maybe it was just inefficient. We've only lived in the house 3 years, so I can't comment on how well it worked originally.

We're in Colorado, so not super humid.
13 SEER has been required since 2006. A 10 SEER unit is at least 15 years old.

According to Manual J, no city in Colorado needs dehumidification except LaJunta.

Since you don't have to dehumidify, there are fewer consequences if you oversize your air conditioner. It may be worse at dehumidifciation, but you don't need it to dehumidify. An oversized system will be less efficient, but you're starting at 10 SEER.
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hand
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Re: HVAC replacement when experts disagree

Post by hand »

jackbeagle wrote: Tue Apr 27, 2021 6:01 pm On the hottest days, my system run time doesn't exceed 12 hours (half the day).
Isn't this the definition of oversized? Why *wouldn't* you want it to run 90+ percent of the time (and only less than 100% to allow for the possibility of future heating load increases)?
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Re: HVAC replacement when experts disagree

Post by aerofreaky11 »

Do yourself a favor and get a blower test on your home for air leaks. Just seeing insulation isn't sufficient. There's also a big difference between a properly air sealed house and what appears to be a well insulated house.

Certain style of homes such as capes have real air sealing issues.
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Re: HVAC replacement when experts disagree

Post by tibbitts »

hand wrote: Wed Apr 28, 2021 6:58 am
jackbeagle wrote: Tue Apr 27, 2021 6:01 pm On the hottest days, my system run time doesn't exceed 12 hours (half the day).
Isn't this the definition of oversized? Why *wouldn't* you want it to run 90+ percent of the time (and only less than 100% to allow for the possibility of future heating load increases)?
Nobody wants to be that close. For example where I live you want to turn off your unit completely during the hottest hours of the day since cost is based on the load on the power grid. You're looking at paying 2-3x as much to run the unit during some hours than others. So typically you'll have no air conditioning for at least four hours. After that four hours you want your unit to get the temperature back down, fast. Similarly if you're just away for a day or two you want that temperature brought back to where you want it without much delay.
UnLearnYourself
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Re: HVAC replacement when experts disagree

Post by UnLearnYourself »

mervinj7 wrote: Mon Apr 26, 2021 11:54 am
Teague wrote: Mon Apr 26, 2021 11:47 am Was your current unit adequate back when it was working well?

How humid is your location? The more humidity the more important it is to not oversize.
Is that still true with two stage condensers? That is, even if you sightly oversize, can it run at the lower setting longer?

OP, are all the quotes for similar types of units (e.g one stage, two stage, heat pump).
Be careful not to oversize your AC or heat pump. I know it sounds counterintuitive but the risk you run is that the unit may think it reaches its temp set point before bringing the humidity down, and the feel in the room will not be cool the way you wanted it.

I had 2 rooms in my house where minisplits/heat pumps were installed. We slightly oversized them initially because we wanted the extra winter heat to seap from the rooms into the halls and service the whole floor as a result. That worked great - until summer. I would wake up every night in a clammy room. It would read that it reached the temp but it remained super humid - thus not comfortable at all.

We had our HVAC guy replace the indoor units with smaller BTU heads and now the spaces cool just right.

When in doubt get more quotes. It might feel ridiculous but my advice OP is to keep getting quotes until you know the right one.
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Re: HVAC replacement when experts disagree

Post by jco »

talzara wrote: Tue Apr 27, 2021 9:54 pm
jco wrote: Mon Apr 26, 2021 11:31 am 3.5 tons is a lot of cooling for Colorado, unless you live in a really hot place like LaJunta. If it is not cooling adequately, it is likely that the equipment is malfunctioning. How big is the house?
About 2900 sq ft. 1900 above ground (1st and second story) and 1000 in the basement.
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Re: HVAC replacement when experts disagree

Post by jco »

aerofreaky11 wrote: Wed Apr 28, 2021 7:57 am Do yourself a favor and get a blower test on your home for air leaks. Just seeing insulation isn't sufficient. There's also a big difference between a properly air sealed house and what appears to be a well insulated house.

Certain style of homes such as capes have real air sealing issues.
This is an interesting idea.
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Re: HVAC replacement when experts disagree

Post by jco »

UnLearnYourself wrote: Wed Apr 28, 2021 8:21 am When in doubt get more quotes. It might feel ridiculous but my advice OP is to keep getting quotes until you know the right one.
Thanks. I actually started this yesterday. I should have about 10 quotes when finished. I also like getting an impression of the different companies: are they giving me a generic sales pitch or are they really trying to figure out my equipment needs?
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Re: HVAC replacement when experts disagree

Post by motorider »

I'm also in CO in a slightly smaller house (2600sqft) spread over 3 floors including a finished basement. My house was built 12yr ago and is an "Energy Star" rated house so the insulation is R30+ in the attic and R21 in the walls. My A/C is a 2.5t, 14seer, single stage and it works just fine but we also rarely run it cooler than 76F in the house.

Follow the science. There are a lot of good comments on efficiency, duct sizes and leakage. This isn't rocket science, it's thermodynamics. Invest in a full-house HVAC evaluation that includes taking CFM and delta T measurements at each register. Transfer grills matter if you have a common cold air return. Also do an energy audit on the house. This sets your baseline and will identify any improvements that need to be done before you install a nice, expensive new system. Don't put a band-air on a gushing wound.

HVAC entails so many components that it's worth taking a step back and understanding the current state of the entire system (insulation, leakage, ducts, etc.) before replacing the most important part. Well worth the $$$ to get good info upfront so you can make an informed decision.
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Re: HVAC replacement when experts disagree

Post by jackbeagle »

tibbitts wrote: Wed Apr 28, 2021 8:17 am
hand wrote: Wed Apr 28, 2021 6:58 am
jackbeagle wrote: Tue Apr 27, 2021 6:01 pm On the hottest days, my system run time doesn't exceed 12 hours (half the day).
Isn't this the definition of oversized? Why *wouldn't* you want it to run 90+ percent of the time (and only less than 100% to allow for the possibility of future heating load increases)?
Nobody wants to be that close. For example where I live you want to turn off your unit completely during the hottest hours of the day since cost is based on the load on the power grid. You're looking at paying 2-3x as much to run the unit during some hours than others. So typically you'll have no air conditioning for at least four hours. After that four hours you want your unit to get the temperature back down, fast. Similarly if you're just away for a day or two you want that temperature brought back to where you want it without much delay.
Forgot about the peak rates in some areas. I should take a step back and acknowledge this is a huge factor in some areas. My area (climate zone 3) is essentially a flat $0.10/kWh year round. There is a 4% seasonal adjustment for 6 months out of the year, but I don't think it quite moves the rate up to the next cent. Our power bills here don't quite touch $150/mo. mid-summer. A few colleagues in California said the same, except that THEIR POWER BILL INCLUDED ABSOLUTELY ZERO A/C USE. That was strictly lighting and appliances. I was blown away.
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Re: HVAC replacement when experts disagree

Post by UALflyer »

jco wrote: Mon Apr 26, 2021 11:31 amEdit: Since some people are asking for some additional info:

Our house is a 20+-year-old two-story house with a basement. Total sq ft: 2900, with the main level and second story being about 1900 sq ft and the basement having 1000 sq ft.

I don't think the insulation is bad. R-38 in the attic ceilings, R-20 in the exterior walls. I've seen the attic insulation, so I think it's legitimate. We have a fair number of windows in our house, and some of them are on the bigger side, but it would never be characterized as a house "with a lot of windows". Each room pretty much as at least one window, and one bathroom has two.

We do not have vaulted ceilings (in general) but the entryway is vaulted. But the floor area is about 12x10' where it's vaulted, so I doubt it makes a huge difference in determining equipment sizing.

Both the furnace and A/C are original to the house (to the best of my knowledge). The furnace has always worked fine. The A/C has always been pretty disappointing in the 3ish years we've lived in the home.
Thanks for adding additional details. Can you provide more info on what the issue is with the a/c? Does it struggle to get to your setpoint throughout the summer or only during the hottest days? What is your setpoint? Do you have hot and cold areas in your house? If so, is the complaint that some of the upstairs room are hot while the downstairs room are cold?

Have any contractors checked the refrigerant pressure to ensure that it's not low? Have any contractors looked at the supply and return vents and confirmed that the pressure is balanced? What tonnage are the ducts sized for? Do the ducts have any air leaks? Are they well insulated? Is your furnace in the attic or in the basement? Has the indoor coil ever been cleaned? What type of air filter do you use (how thick and what MERV rating) and how often do you replace it?
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Re: HVAC replacement when experts disagree

Post by tibbitts »

jackbeagle wrote: Wed Apr 28, 2021 12:34 pm
tibbitts wrote: Wed Apr 28, 2021 8:17 am
hand wrote: Wed Apr 28, 2021 6:58 am
jackbeagle wrote: Tue Apr 27, 2021 6:01 pm On the hottest days, my system run time doesn't exceed 12 hours (half the day).
Isn't this the definition of oversized? Why *wouldn't* you want it to run 90+ percent of the time (and only less than 100% to allow for the possibility of future heating load increases)?
Nobody wants to be that close. For example where I live you want to turn off your unit completely during the hottest hours of the day since cost is based on the load on the power grid. You're looking at paying 2-3x as much to run the unit during some hours than others. So typically you'll have no air conditioning for at least four hours. After that four hours you want your unit to get the temperature back down, fast. Similarly if you're just away for a day or two you want that temperature brought back to where you want it without much delay.
Forgot about the peak rates in some areas. I should take a step back and acknowledge this is a huge factor in some areas. My area (climate zone 3) is essentially a flat $0.10/kWh year round. There is a 4% seasonal adjustment for 6 months out of the year, but I don't think it quite moves the rate up to the next cent. Our power bills here don't quite touch $150/mo. mid-summer. A few colleagues in California said the same, except that THEIR POWER BILL INCLUDED ABSOLUTELY ZERO A/C USE. That was strictly lighting and appliances. I was blown away.
My rate goes up by over 2.5x during peak hours of the day. If you have the cooling power to knock the temperature down a little below where you might have it before the hottest part of the day, then can recover quickly after, you can save considerably and still be somewhat comfortable. But if you have a unit that runs 100% just to keep up on the hottest days (like mine), it's not comfortable doing that (although I try to do it anyway.)
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Re: HVAC replacement when experts disagree

Post by alfaspider »

talzara wrote: Tue Apr 27, 2021 9:35 pm
alfaspider wrote: Mon Apr 26, 2021 1:23 pm The problem with the "by the book" calc is it tends not to calculate for extreme situations. My central AC is too large by the book, and it is for about 80% of a/c season. But during those hottest days of the year, it's running nearly constantly during the afternoon to keep up. I'll accept a bit more humidity when it's 90* to be comfortable when it's 104*.
Actually, "by the book" only calculates for extreme situations.

The Manual J design temperature is the the 99th percentile temperature. Manual J includes a built-in oversizing factor of 15%, and Manual S rounds up to the next half-ton. If you have 2.7 tons of calculated load, you end up with a 3.5-ton air conditioner: 130% of load. Since this includes internal loads, the delta T could be 140% or 150%.

Air conditioners are designed to run at 100% of capacity. It's hardest on the components when the compressor starts up. The air conditioner will last longer if it cycles less.
That's the problem though. Sometimes you get 99.99th percentile temperature. Plus, they are often based off inaccurate assumptions of what 99th percentile is. All I know is I've seen many an "oversized" a/c unit fail to keep up on extremely hot days.
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Re: HVAC replacement when experts disagree

Post by talzara »

UnLearnYourself wrote: Wed Apr 28, 2021 8:21 am Be careful not to oversize your AC or heat pump. I know it sounds counterintuitive but the risk you run is that the unit may think it reaches its temp set point before bringing the humidity down, and the feel in the room will not be cool the way you wanted it.
The intuition is that the first few minutes of runtime give you no dehumidification at all. The coil can only dehumidify when it gets colder than the dewpoint, so it has to get down to temperature first. The longer it runs, the less time is wasted on cooling the air without dehumidifying.

It's not quite linear because larger air conditioners also cool down the coil faster, but not fast enough to dehumidify as well.

However, the OP lives in Colorado. The table in Manual J says that no dehumidification is needed in Colorado except near LaJunta. An oversized air conditioner will still work, but it'll be less efficient.
talzara
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Re: HVAC replacement when experts disagree

Post by talzara »

jco wrote: Wed Apr 28, 2021 8:31 am About 2900 sq ft. 1900 above ground (1st and second story) and 1000 in the basement.
3.5 tons is a lot for 2,900 square feet in Colorado if you're not living near LaJunta, especially when only 1,900 feet are above ground. The basement gets much less heat gain because most of it is in the ground.

If you can't cool your house to 75 degrees at 3.5 tons, then something is wrong with the air conditioner, the duct system, or the building envelope.

The residential HVAC industry likes to sell equipment. They do not like to fix problems.
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Re: HVAC replacement when experts disagree

Post by UALflyer »

jco wrote: Mon Apr 26, 2021 11:31 amBased on UALflyer's questions below:

Have any contractors checked the refrigerant pressure to ensure that it's not low?
- No, since we want to replace the system. The fan motor is definitely bad. And R22 is so expensive. So they're focusing on replacement. Even an independent HVAC servicer that doesn't do installs suggested I get it replaced.
I understand that, but am trying to figure out the reason that you've been dissatisfied with the a/c performance. If, for instance, the R22 pressure has been low for a while, it would explain the reason that it hasn't been cooling well.
Have any contractors looked at the supply and return vents and confirmed that the pressure is balanced?
- I don't think so. A couple have commented on removing the return vents up the ceiling in our upstairs bedrooms to improve cooling in the summer. They have said that the supply vents are not designed well and that there is probably a lot of turbuluence. These professionals suggested installed scoops and dividers to better direct the airflow.
Do get them to focus on the ductwork and see if you can get more useful info out of them. If there's an issue with the ductwork, adding a/c tonnage may not help and could actually hurt.

You can't come up with a good solution without knowing the exact source of your problem.
What tonnage are the ducts sized for?
- No idea, but our furnace is 110,000 BTU with a 5-ton blower (learned this today). So probably pretty big.
I wouldn't assume anything and get contractors to look at the ducts and figure it out, which should only take 5 minutes. You can't even talk about the tonnage without knowing what the ductwork is designed to handle.
Is your furnace in the attic or in the basement?
- Basement
This is good and bad. It's good because basements are naturally much cooler, so the a/c doesn't have to struggle with attic temperatures. It's bad because you've got a single a/c cooling both floors, and the fact that the furnace is in the basement means that the upstairs supply is located pretty far from the unit. This probably means that the upstairs rooms are getting less airflow and higher temperatures than the downstairs rooms, all while the upstairs rooms are already naturally warmer (heat rises).
What type of air filter do you use (how thick and what MERV rating) and how often do you replace it?
- 1" with a MERV of 4, which is about as flimsy as I could find. I usually change them every 2 or 3 months. My nest reminds me to change them.
Good, so it's not that.
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Re: HVAC replacement when experts disagree

Post by talzara »

alfaspider wrote: Wed Apr 28, 2021 1:18 pm That's the problem though. Sometimes you get 99.99th percentile temperature. Plus, they are often based off inaccurate assumptions of what 99th percentile is. All I know is I've seen many an "oversized" a/c unit fail to keep up on extremely hot days.
The 99th percentile in Manual J is based on Weather Service data over 30 years. HVAC contractors often increase the temperature to sell larger equipment. Nobody ever decreases it.

If an oversized air conditioner can't keep up on extremely hot days, then something is wrong with the air conditioner, the duct system, or the building envelope.

The residential HVAC industry likes to sell new equipment. They don't like to solve problems. It takes too long and requires too much math, and they get paid less.
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Re: HVAC replacement when experts disagree

Post by UALflyer »

talzara wrote: Wed Apr 28, 2021 1:59 pm
alfaspider wrote: Wed Apr 28, 2021 1:18 pm That's the problem though. Sometimes you get 99.99th percentile temperature. Plus, they are often based off inaccurate assumptions of what 99th percentile is. All I know is I've seen many an "oversized" a/c unit fail to keep up on extremely hot days.
The 99th percentile in Manual J is based on Weather Service data over 30 years. HVAC contractors often increase the temperature to sell larger equipment. Nobody ever decreases it.

If an oversized air conditioner can't keep up on extremely hot days, then something is wrong with the air conditioner, the duct system, or the building envelope.

The residential HVAC industry likes to sell new equipment. They don't like to solve problems. It takes too long and requires too much math, and they get paid less.
I agree with everything that you said, but just to clarify, if the outdoor temperature goes above the design temperature, the a/c will lose ground, so you won't be able to get to the setpoint. With a properly designed system, this should be a rather infrequent occurrence.

Oversizing a unit just so it doesn't lose ground less than 1% of the time means that you'll be a lot less comfortable than you could be be the rest of the time, which is a much bigger problem. I know it's not intuitive, but whenever in doubt, it's a lot better to slightly undersize a system than to oversize it.
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Re: HVAC replacement when experts disagree

Post by plumberboy »

From the information you gave I would go with the 3.5 or 4.0 ton 2 stage unit and a 2 stage furnace with a variable speed blower.You will not be sorry for going with the 2 stage unit and variable speed blower. I speak from personal experience. When the a/c operates in the 1st stage it only uses 1/2 the power it uses in the second stage. In the 1st stage it will run longer and quieter but that is what you want to help reduce humidity. If you want it to cool down faster you can adjust the length of time the unit is in the 1st stage before it activates the 2nd stage. 2 stage units cost a bit more but are worth every cent. Another thing, the high Merv ratings on filters sounds good but they are detrimental to the operation of the system. They are to dense and don't allow enough return air to be drawn into the system. A Merv 5 or 7 is the highest I use. Just as an aside, Do not run your blower continuously when the A/C is being used.There is always some water in the drain pan and it will actually put moisture back into the air when the compressor is not on.
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Re: HVAC replacement when experts disagree

Post by tibbitts »

talzara wrote: Wed Apr 28, 2021 1:59 pm
alfaspider wrote: Wed Apr 28, 2021 1:18 pm That's the problem though. Sometimes you get 99.99th percentile temperature. Plus, they are often based off inaccurate assumptions of what 99th percentile is. All I know is I've seen many an "oversized" a/c unit fail to keep up on extremely hot days.
The 99th percentile in Manual J is based on Weather Service data over 30 years. HVAC contractors often increase the temperature to sell larger equipment. Nobody ever decreases it.

If an oversized air conditioner can't keep up on extremely hot days, then something is wrong with the air conditioner, the duct system, or the building envelope.

The residential HVAC industry likes to sell new equipment. They don't like to solve problems. It takes too long and requires too much math, and they get paid less.
Sometimes solving all the other "little" problems that add up to making the "correct" amount of cooling just barely adequate costs a lot more than the next size hvac and the extra cost to run it for whatever the life of it is.
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