HVAC decision help please

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runner540
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Joined: Sun Feb 26, 2017 4:43 pm

HVAC decision help please

Post by runner540 »

Current system is 15 years old. It does need to be replaced. 2nd story is more difficult to cool each summer. Ducting is part of the answer, to redirect the order the air flows to which rooms.
Question is with what?
1 system with 2 zones
2 systems? (Seems like we on the threshold of 1 big or 2 small units being appropriate)

Have quotes from a reliable provider for:
5 Ton Carrier Infinity Greenspeed Horizontal Gas system

5 Ton Carrier Two Stage Seer2 Horizontal Gas System

5 Ton Carrier Infinity 5 Stage Horizontal Gas system

House specs:
1960s house in DFW (hot summers 🥵
2 stories
2800 Sq ft
Gas for furnace

Other things to help:
Energy audit (how to find a qualified person?)
Improve or redo attic insulation - what questions to ask, who to do it
Thank you!!
Buzzman
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Location: North AL

Re: HVAC decision help please

Post by Buzzman »

My last house was two story and 2800 sq ft. We had two HVAC units one for downstairs and one for upstairs. The bonus room over the garage was limited in how much insulation could be in the attic and had the garage underneath so we added a window AC unit to help that room in the summer. The added benefit of a second unit is you still get some heating and cooling even if one unit is down.

Personally, I would go with two units if you can given limits of current ductwork, wiring, architecture, etc.

Typically you can just blow in more insulation on top of what you have. Whether that is cost effective is a harder question to answer.
steelhorse
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Re: HVAC decision help please

Post by steelhorse »

If you want to give a boost to your existing system while making a decision, have your outside condenser coils cleaned (especially if it's never been done). You can call an HVAC company to do it, or check out a YouTube video and diy, it's not that hard to do. I did mine a few years ago and it really made a huge difference. Even if they look clean, it's surprising the amount of dirt coating them and interfering with cooling capacity. Good luck.
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id0ntkn0wjack
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Re: HVAC decision help please

Post by id0ntkn0wjack »

An industry expert wrote this post: https://www.energyvanguard.com/blog/5-t ... ht-answer/ which would lead me to believe that a 5-ton system is never the right answer.

I highly recommend having a Home Energy Audit conducted by a certified rater. They can diagnose why your 2nd floor is hard to cool (as you mentioned, likely in need of air sealing & insulation) as well as provide you with a roadmap for the best way to spend your money to reduce energy use and to improve comfort.

RESNET is a primary certifying body for Home Energy Raters. It doesn't look like the have a rater finder on their site, but you would likely be able to find a person nearby using a google search: https://www.resnet.us/raters/hers-raters/

The Inflation Reduction Act offers tax credits/rebates for Home Energy Audits, insluation updgrades, as well as heat pump conversions. https://homes.rewiringamerica.org/calculator
NYCaviator
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Re: HVAC decision help please

Post by NYCaviator »

runner540 wrote: Sun Jul 07, 2024 5:04 am Current system is 15 years old. It does need to be replaced. 2nd story is more difficult to cool each summer. Ducting is part of the answer, to redirect the order the air flows to which rooms.
Before you blame your duct work, it could be the air circulation.

We have a very tall house (3 finished stories with high ceilings) and used to have a traditional HVAC system that turned on and off based on the thermostat. In the summer, the basement level was always freezing cold and the top level was always hot. We recently switched to a heat pump and the blower fan runs almost continuously at variable speeds. The air gets circulated constantly so now there is almost no temperature difference between the three floors. It's been a great change. Traditional systems turn on and cool the house quickly and then turn off. So the cold air doesn't get circulated well and you will inevitably have hot and cold spots.

If you don't want to go the heat pump route, I think some of the newer systems with variable speeds can achieve the same thing. Just ask your HVAC people about a blower that can run continuously. We initially looked at doing separate systems for upstairs and down, but it would have involved a lot of drywall cutting and re-doing of duct work, which would have cost a lot and been a major headache.

I'd definitely recommend getting your attic re-insulated if it isn't well insulated. That can make a huge difference in your energy bills. And many cities/states are offering big rebates to do energy efficiency upgrades, so check on that.

Also, if you don't want to go the two-system route, check out mini-splits for your upstairs sleeping areas. I know several people who have done that and are very happy. It's a lot easier to install, they are very efficient, and everyone can set the temp at what they want for their individual bedroom.
tibbitts
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Re: HVAC decision help please

Post by tibbitts »

People are talking about insulation but my experience has been that with many houses - especially multi-story - you're looking at only a few inches between the shingles and the finished inside ceiling. There's only so much insulation that can squeeze in there and it's not that easy to replace or even add to it.
texas lawdog
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Re: HVAC decision help please

Post by texas lawdog »

When I lived in the DFW area, I had a company called Energy Attic come and clean out all the old rockwool insulation and replace with 21" of R-60 insulation. The night after the old insulation was sucked out I could see every space, gap, and hole in the attic which couldn't be seen previously. They did a good job of sealing all the gaps and replacing the insulation. We also discussed attic ventilation, radiant barrier, etc. but decided the extra cost wasn't worth the investment.

The above would maybe solve half your issue with the other half being a new split system. I'd make sure you have enough vents in the bedrooms where people are sleeping to ensure adequate air flow. Summers seem to be getting hotter and comfort was top of mind for me when living in the area but had no problem with keeping the house cold during the summer.
InvisibleAerobar
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Re: HVAC decision help please

Post by InvisibleAerobar »

NYCaviator wrote: Sun Jul 07, 2024 9:23 am If you don't want to go the heat pump route, I think some of the newer systems with variable speeds can achieve the same thing. Just ask your HVAC people about a blower that can run continuously. We initially looked at doing separate systems for upstairs and down, but it would have involved a lot of drywall cutting and re-doing of duct work, which would have cost a lot and been a major headache.

I'd definitely recommend getting your attic re-insulated if it isn't well insulated. That can make a huge difference in your energy bills. And many cities/states are offering big rebates to do energy efficiency upgrades, so check on that.

Also, if you don't want to go the two-system route, check out mini-splits for your upstairs sleeping areas. I know several people who have done that and are very happy. It's a lot easier to install, they are very efficient, and everyone can set the temp at what they want for their individual bedroom.
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
tibbitts
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Re: HVAC decision help please

Post by tibbitts »

InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
I have that configuration and it is capable of continuous fan operation; I have no idea if it would help in your situation. I don't run the fan continuously. Maybe someone else will know how much continuous operation will reduce the lifetime of the blower motor. If you d-i-y the variable speed blower motors can cost many hundreds of dollars to replace; thousands of dollars if you don't.
THY4373
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Re: HVAC decision help please

Post by THY4373 »

InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
No a single stage AC is basically 100% on or 100% off. Other than the hottest days of the year it will cycle on and off (or over cool the house if you running continuously). Inverter or variable speed AC/heat pump systems can cycle their cooling and heating (if a heat pump) up and down from anywhere from around 40-60% to 100% output. Incidentally when running below maximum capacity they are even more efficient. A two speed AC/heat pump is a middle ground though generally speaking they do not go as low as a variable speed so you have say 75% and 100% output. Now in a single speed system you can run the blower motor all the time to circulate the inside air but from what I have read that tends to circulate some of the removed moisture in the air back into the house as it evaporates of the a-coil when the AC is off.

FWIW I went with a two inverter models from an Asian manufacture because they have been building them a lot longer than the US manufactures. So far I much prefer the inverter systems for their more even heating and cooling and their relative silence (the outside units are outside a couple of windows including one of my bedroom windows (it is on the ground floor).
THY4373
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Re: HVAC decision help please

Post by THY4373 »

Buzzman wrote: Sun Jul 07, 2024 7:36 am Personally, I would go with two units if you can given limits of current ductwork, wiring, architecture, etc.
+1 the last two houses I have owned have had separate systems for the two living levels and I much prefer that. Even my current smallish but well built 2200 square foot house has two. The ability to maintain temperatures even different temperature (to some extent) on different floors is nice. Does cost more though.
InvisibleAerobar
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Re: HVAC decision help please

Post by InvisibleAerobar »

THY4373 wrote: Sun Jul 07, 2024 10:41 am
InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
No a single stage AC is basically 100% on or 100% off. Other than the hottest days of the year it will cycle on and off (or over cool the house if you running continuously). Inverter or variable speed AC/heat pump systems can cycle their cooling and heating (if a heat pump) up and down from anywhere from around 40-60% to 100% output. Incidentally when running below maximum capacity they are even more efficient. A two speed AC/heat pump is a middle ground though generally speaking they do not go as low as a variable speed so you have say 75% and 100% output. Now in a single speed system you can run the blower motor all the time to circulate the inside air but from what I have read that tends to circulate some of the removed moisture in the air back into the house as it evaporates of the a-coil when the AC is off.

FWIW I went with a two inverter models from an Asian manufacture because they have been building them a lot longer than the US manufactures. So far I much prefer the inverter systems for their more even heating and cooling and their relative silence (the outside units are outside a couple of windows including one of my bedroom windows (it is on the ground floor).
tibbitts wrote: Sun Jul 07, 2024 10:35 am
InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
I have that configuration and it is capable of continuous fan operation; I have no idea if it would help in your situation. I don't run the fan continuously. Maybe someone else will know how much continuous operation will reduce the lifetime of the blower motor. If you d-i-y the variable speed blower motors can cost many hundreds of dollars to replace; thousands of dollars if you don't.
Thanks for the helpful info. I'm in a somewhat similar situation as OP, albeit less urgent, as the above-story area is less by 500 sq ft, the climate is generally cooler (Wisc.), and the HVAC system should have 7-10 years of service left. But even still, the upstairs BRs get quite toasty.

I'm doing the "loss reduction" stuff now (air sealing, a topic on which I received numerous helpful suggestions earlier this week), but I'm contemplating doing a mini-split system next year (though heat-pump would be more preferable).
Big Dog
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Re: HVAC decision help please

Post by Big Dog »

look for HVAC contractors that do a lot of duct work. Ducting can be messy/dirty work and lower profit, so contractors assume the builder's design is fine (to make it easier for them). We had a HVAC firm come in and crawled thru the attic where they found crushed soft ducting which fed the 2nd fl. Replaced it all, and voila, the back BR on the 2nd floor which is the furthest away from the blower is only 2 degrees different temp than downstairs. (it used to be 5-6 degree differential). Saved us from installing a mini-split on the back of the house.

Ask for a Manual J where they will actually measure the air flow per register and the returns. (Contractors hate this bcos it takes a couple of hours.)

https://www.acca.org/standards/technica ... s/manual-j
Lucky72
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Re: HVAC decision help please

Post by Lucky72 »

We’re putting in a new HVAC system in a couple of weeks. Our home is 4k square feet with basement, main level and second story. We have 17 foot ceiling on the main level that are open to the second story. This makes heating and cooling difficult. The basement is always freezing cold in the summer because the current single stage, single zone AC runs constantly to keep the second floor cooler. A reputable installer recommended adding a second zone for the second floor along with a thermostat for upstairs. We’re going with that plan in addition to the new HVAC system.

As a data point we went with Lennox Signature series 99 with modular burners and variable speed blower for the furnace and Lennox elite series 23 with variable speed compressor with heat pump upgrade. We’re also installing a Lennox bypass whole house humidifier for the winter. The total price including tax is $24k and we get $2450 in rebates from the power and gas companies. The system also qualifies for federal tax credits.
tibbitts
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Re: HVAC decision help please

Post by tibbitts »

THY4373 wrote: Sun Jul 07, 2024 10:41 am
InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
No a single stage AC is basically 100% on or 100% off.
The compressor is either on or off. But in the case of a "non-communicating" system as this probably would be, as far as I know (hopefully someone will correct me if I'm wrong) the blower motor is entirely - except as noted below - thermostat-controlled and if the thermostat tells the fan to run continuously, it will. My thermostat is the same one left over from my 2-cool/2-heat system, which I had before migrating to 1-cool/2-heat. I didn't change any settings; it's jumpered such that the blower sees the same signal for 1st or 2nd-stage cooling.

With my newer 2-stage furnace blower motor vs. the previous model, the blower itself does have some internally-configured ramping independent of the thermostat. So while the single-stage compressor is either on/off, the fan ramping does provide a somewhat similar end-user experience as the two-stage compressor did. But that only applies when the compressor is operating.
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id0ntkn0wjack
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Re: HVAC decision help please

Post by id0ntkn0wjack »

InvisibleAerobar wrote: Sun Jul 07, 2024 10:50 am
I'm doing the "loss reduction" stuff now (air sealing, a topic on which I received numerous helpful suggestions earlier this week), but I'm contemplating doing a mini-split system next year (though heat-pump would be more preferable).
Just to clarify: A mini-split is a heat pump.

Also, work w/ Focus on Energy for immediate discounts: https://focusonenergy.com/residential/h ... nd-cooling
InvisibleAerobar
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Re: HVAC decision help please

Post by InvisibleAerobar »

id0ntkn0wjack wrote: Sun Jul 07, 2024 11:08 am
InvisibleAerobar wrote: Sun Jul 07, 2024 10:50 am
I'm doing the "loss reduction" stuff now (air sealing, a topic on which I received numerous helpful suggestions earlier this week), but I'm contemplating doing a mini-split system next year (though heat-pump would be more preferable).
Just to clarify: A mini-split is a heat pump.

Also, work w/ Focus on Energy for immediate discounts: https://focusonenergy.com/residential/h ... nd-cooling
Thanks for the correction. Lots to learn.
InvisibleAerobar
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Re: HVAC decision help please

Post by InvisibleAerobar »

runner540 wrote: Sun Jul 07, 2024 5:04 am

Other things to help:
Energy audit (how to find a qualified person?)
Improve or redo attic insulation - what questions to ask, who to do it
Thank you!!
In my state (Wisc.), there's a webpage set up as a concerted effort by various utility companies in the state, and the webpage provides details on which utilities participates in rebate programs, what improvements qualify for rebate (audit, insulation, and air seal all qualify in my state), and a list of Trade Allies (contractors) who can implement the improvements.

I'm not sure how much vetting is performed, but it's a start.

Also, fwiw, the audit that qualifies for a rebated generally costs $150 more, which is the exact amount of the rebate. I'm not quite clear as to what additional advantages are conferred.

Perhaps Texas has something similar? I found out about the program in Wisc. from the website of my local electric utility.

ETA: this particular webpage is the one @id0ntkn0wjack helpfully linked upthread.
mkc
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Re: HVAC decision help please

Post by mkc »

runner540 wrote: Sun Jul 07, 2024 5:04 am
Energy audit (how to find a qualified person?)
It's been years, but our electric co-operative in Texas did energy audits at no cost to the consumer. That included FLIR imaging of walls and HVAC ducting in the attic (as long as temperature deltas were amenable to seeing issues - 70-75F inside and out and you're not going to see much.

You might see if your electric provider offers this service.
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runner540
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Re: HVAC decision help please

Post by runner540 »

Lucky72 wrote: Sun Jul 07, 2024 11:02 am We’re putting in a new HVAC system in a couple of weeks. Our home is 4k square feet with basement, main level and second story. We have 17 foot ceiling on the main level that are open to the second story. This makes heating and cooling difficult. The basement is always freezing cold in the summer because the current single stage, single zone AC runs constantly to keep the second floor cooler. A reputable installer recommended adding a second zone for the second floor along with a thermostat for upstairs. We’re going with that plan in addition to the new HVAC system.

As a data point we went with Lennox Signature series 99 with modular burners and variable speed blower for the furnace and Lennox elite series 23 with variable speed compressor with heat pump upgrade. We’re also installing a Lennox bypass whole house humidifier for the winter. The total price including tax is $24k and we get $2450 in rebates from the power and gas companies. The system also qualifies for federal tax credits.
Thanks for this! We already have a dual zone system that works. It’s really just our primary bedroom that is too hot in winter and too hot in summer.
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runner540
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Re: HVAC decision help please

Post by runner540 »

mkc wrote: Sun Jul 07, 2024 1:39 pm
runner540 wrote: Sun Jul 07, 2024 5:04 am
Energy audit (how to find a qualified person?)
It's been years, but our electric co-operative in Texas did energy audits at no cost to the consumer. That included FLIR imaging of walls and HVAC ducting in the attic (as long as temperature deltas were amenable to seeing issues - 70-75F inside and out and you're not going to see much.

You might see if your electric provider offers this service.
Thanks, summer is definitely a time to see temperature deltas!
NYCaviator
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Re: HVAC decision help please

Post by NYCaviator »

InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am
NYCaviator wrote: Sun Jul 07, 2024 9:23 am If you don't want to go the heat pump route, I think some of the newer systems with variable speeds can achieve the same thing. Just ask your HVAC people about a blower that can run continuously. We initially looked at doing separate systems for upstairs and down, but it would have involved a lot of drywall cutting and re-doing of duct work, which would have cost a lot and been a major headache.

I'd definitely recommend getting your attic re-insulated if it isn't well insulated. That can make a huge difference in your energy bills. And many cities/states are offering big rebates to do energy efficiency upgrades, so check on that.

Also, if you don't want to go the two-system route, check out mini-splits for your upstairs sleeping areas. I know several people who have done that and are very happy. It's a lot easier to install, they are very efficient, and everyone can set the temp at what they want for their individual bedroom.
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
I don't think the number of furnace/AC stages has anything to do with the fan being able to run constantly. What you want is a variable speed blower in your air handler. You can run a normal blower fan constantly, but if it's not made to do that it'll wear out prematurely.

The multi-stage furnace and AC simply allow the units to scale up or down the heating/cooling power to keep the house at a more consistent temp. The problem with a single stage furnace or A/C is that it's all or nothing. The temp hits the set point and it either blasts hot air or cold air until the temp cools/heats and then turns off. The temp changes, and it does it all over again. The multi-stage/variable systems allow it to modulate heating and cooling so it's more consistent.
Dufus
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Re: HVAC decision help please

Post by Dufus »

https://hvac-talk.com/vbb/ is a BBS dedicated to HVAC
mortfree
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Re: HVAC decision help please

Post by mortfree »

Lucky72 wrote: Sun Jul 07, 2024 11:02 am We’re putting in a new HVAC system in a couple of weeks. Our home is 4k square feet with basement, main level and second story. We have 17 foot ceiling on the main level that are open to the second story. This makes heating and cooling difficult. The basement is always freezing cold in the summer because the current single stage, single zone AC runs constantly to keep the second floor cooler. A reputable installer recommended adding a second zone for the second floor along with a thermostat for upstairs. We’re going with that plan in addition to the new HVAC system.

As a data point we went with Lennox Signature series 99 with modular burners and variable speed blower for the furnace and Lennox elite series 23 with variable speed compressor with heat pump upgrade. We’re also installing a Lennox bypass whole house humidifier for the winter. The total price including tax is $24k and we get $2450 in rebates from the power and gas companies. The system also qualifies for federal tax credits.
Where will the thermostat be located on the 2nd floor?

I have a two zone system that was original to the house. I replaced the standard round dial thermostats with nest thermostats and the remote temperature sensors which I placed in a bedroom on the second floor. This allows the system to maintain cooler temps more easily than if it was trying to satisfy the 2nd floor thermostat that is open to the first floor. Reverse in the winter where the thermostat is the sensor to satisfy the temperature. However at night in the winter I switch to the remote sensor to make the bedroom the right temperature.
Closer to 50 than 40
livesoft
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Re: HVAC decision help please

Post by livesoft »

I wonder if you (the OP) have used little bluetooth thermometers to record and plot temperatures in various locations in your house. We have found that invaluable.

Out 2-story home is only a little bigger than the OPs and we have 2 separate HVAC systems for downstairs and upstairs. The separate thermostats are 1991 vintage, but have some intelligence. The HVAC units are nothing special either: just single stage, not a variable fan. We live in southeast Texas, so hot in summer and humid. Here's a plot from just now of what the upstairs temps have been this afternoon:

Image

I conclude that the AC turned on 7 times between 15:05 and 17:50 today and ran about 4 minutes each time to cool the upstairs abou 0.5 deg F.

I think more modern systems might keep an even tighter range on the temperature, but a 0.5 to 1 degree swing is OK with me. So I conclude I don't need a fancy schmancy AC system. OTOH, maybe it is a fancy system?
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runner540
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Re: HVAC decision help please

Post by runner540 »

livesoft wrote: Sun Jul 07, 2024 5:58 pm I wonder if you (the OP) have used little bluetooth thermometers to record and plot temperatures in various locations in your house. We have found that invaluable.

Out 2-story home is only a little bigger than the OPs and we have 2 separate HVAC systems for downstairs and upstairs. The separate thermostats are 1991 vintage, but have some intelligence. The HVAC units are nothing special either: just single stage, not a variable fan. We live in southeast Texas, so hot in summer and humid. Here's a plot from just now of what the upstairs temps have been this afternoon:

Image

I conclude that the AC turned on 7 times between 15:05 and 17:50 today and ran about 4 minutes each time to cool the upstairs abou 0.5 deg F.

I think more modern systems might keep an even tighter range on the temperature, but a 0.5 to 1 degree swing is OK with me. So I conclude I don't need a fancy schmancy AC system. OTOH, maybe it is a fancy system?
Thank you, livesoft! I’ll look into those sensors to get more hard data for the installers.

Basically, the primary upstairs BR (where the upstairs zone thermostat is located) is 78-80 in the afternoon and evening, on the hotter days (95+ F) which is approx 3 months of the summer. The hallway and other upstairs rooms are cooler but still not quite at the desired temps.
In the winter, the primary BR can be 75 when the thermostat is set to 67…so it’s getting too much heat and not enough AC.
tibbitts
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Re: HVAC decision help please

Post by tibbitts »

NYCaviator wrote: Sun Jul 07, 2024 4:55 pm
InvisibleAerobar wrote: Sun Jul 07, 2024 10:21 am
NYCaviator wrote: Sun Jul 07, 2024 9:23 am If you don't want to go the heat pump route, I think some of the newer systems with variable speeds can achieve the same thing. Just ask your HVAC people about a blower that can run continuously. We initially looked at doing separate systems for upstairs and down, but it would have involved a lot of drywall cutting and re-doing of duct work, which would have cost a lot and been a major headache.

I'd definitely recommend getting your attic re-insulated if it isn't well insulated. That can make a huge difference in your energy bills. And many cities/states are offering big rebates to do energy efficiency upgrades, so check on that.

Also, if you don't want to go the two-system route, check out mini-splits for your upstairs sleeping areas. I know several people who have done that and are very happy. It's a lot easier to install, they are very efficient, and everyone can set the temp at what they want for their individual bedroom.
Would you happen to know if a dual-stage furnace paired to a single-stage AC condenser would accomplish the continuous circulation you mentioned?
I don't think the number of furnace/AC stages has anything to do with the fan being able to run constantly. What you want is a variable speed blower in your air handler. You can run a normal blower fan constantly, but if it's not made to do that it'll wear out prematurely.

The multi-stage furnace and AC simply allow the units to scale up or down the heating/cooling power to keep the house at a more consistent temp. The problem with a single stage furnace or A/C is that it's all or nothing. The temp hits the set point and it either blasts hot air or cold air until the temp cools/heats and then turns off. The temp changes, and it does it all over again. The multi-stage/variable systems allow it to modulate heating and cooling so it's more consistent.
As far as I know a two-stage furnace or air conditioner will have the same variable-speed blower motor that a variable-speed air conditioner will have; whether it's actually designed for continuous operation without periodic replacement I don't know. My experience has been that replacement can be expensive (even just for parts) relative to the older single-speed blower motors.
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runner540
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Re: HVAC decision help please

Post by runner540 »

texas lawdog wrote: Sun Jul 07, 2024 10:00 am When I lived in the DFW area, I had a company called Energy Attic come and clean out all the old rockwool insulation and replace with 21" of R-60 insulation. The night after the old insulation was sucked out I could see every space, gap, and hole in the attic which couldn't be seen previously. They did a good job of sealing all the gaps and replacing the insulation. We also discussed attic ventilation, radiant barrier, etc. but decided the extra cost wasn't worth the investment.

The above would maybe solve half your issue with the other half being a new split system. I'd make sure you have enough vents in the bedrooms where people are sleeping to ensure adequate air flow. Summers seem to be getting hotter and comfort was top of mind for me when living in the area but had no problem with keeping the house cold during the summer.
Thank you Texas lawdog! Reached out to this company.
texas lawdog
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Re: HVAC decision help please

Post by texas lawdog »

runner540 wrote: Mon Jul 08, 2024 3:03 pm
texas lawdog wrote: Sun Jul 07, 2024 10:00 am When I lived in the DFW area, I had a company called Energy Attic come and clean out all the old rockwool insulation and replace with 21" of R-60 insulation. The night after the old insulation was sucked out I could see every space, gap, and hole in the attic which couldn't be seen previously. They did a good job of sealing all the gaps and replacing the insulation. We also discussed attic ventilation, radiant barrier, etc. but decided the extra cost wasn't worth the investment.

The above would maybe solve half your issue with the other half being a new split system. I'd make sure you have enough vents in the bedrooms where people are sleeping to ensure adequate air flow. Summers seem to be getting hotter and comfort was top of mind for me when living in the area but had no problem with keeping the house cold during the summer.
Thank you Texas lawdog! Reached out to this company.
Good to hear. I remember it took most of the day for them to fill up my 1800 sq ft house and think something was missed in the translation because it was much deeper than 21" in spots and looked like big snow drifts up there in the attic.
talzara
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Re: HVAC decision help please

Post by talzara »

livesoft wrote: Sun Jul 07, 2024 5:58 pm Out 2-story home is only a little bigger than the OPs and we have 2 separate HVAC systems for downstairs and upstairs. The separate thermostats are 1991 vintage, but have some intelligence. The HVAC units are nothing special either: just single stage, not a variable fan. We live in southeast Texas, so hot in summer and humid. Here's a plot from just now of what the upstairs temps have been this afternoon:

...

I conclude that the AC turned on 7 times between 15:05 and 17:50 today and ran about 4 minutes each time to cool the upstairs abou 0.5 deg F.

I think more modern systems might keep an even tighter range on the temperature, but a 0.5 to 1 degree swing is OK with me. So I conclude I don't need a fancy schmancy AC system. OTOH, maybe it is a fancy system?
This is an example of why a single-stage system is not enough.

Your system is maintaining 54% relative humidity. It should be maintaining 50% relative humidity. This is because it's short-cycling, running for only 4 minutes at a time. Air is not passing over the evaporator coil for long enough to remove the humidity.

On July 7, the temperature in southeastern Texas was in the low 80s for most of the afternoon and only briefly rose to the upper 80s. That's why your air conditioner was short-cycling. There wasn't enough sensible load to run the air conditioner for 10 minutes at a time.

A two-stage air conditioner would have 7-minute runtimes in first stage, and it would maintain a lower relative humidity. It would still be short-cycling, but not as badly.

A variable-speed air conditioner would have 20-minute runtimes at 25% load, and it would maintain an even lower relative humidity.
z0r
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Re: HVAC decision help please

Post by z0r »

talzara wrote: Tue Jul 09, 2024 12:01 pm
livesoft wrote: Sun Jul 07, 2024 5:58 pm Out 2-story home is only a little bigger than the OPs and we have 2 separate HVAC systems for downstairs and upstairs. The separate thermostats are 1991 vintage, but have some intelligence. The HVAC units are nothing special either: just single stage, not a variable fan. We live in southeast Texas, so hot in summer and humid. Here's a plot from just now of what the upstairs temps have been this afternoon:

...

I conclude that the AC turned on 7 times between 15:05 and 17:50 today and ran about 4 minutes each time to cool the upstairs abou 0.5 deg F.

I think more modern systems might keep an even tighter range on the temperature, but a 0.5 to 1 degree swing is OK with me. So I conclude I don't need a fancy schmancy AC system. OTOH, maybe it is a fancy system?
This is an example of why a single-stage system is not enough.

Your system is maintaining 54% relative humidity. It should be maintaining 50% relative humidity. This is because it's short-cycling, running for only 4 minutes at a time. Air is not passing over the evaporator coil for long enough to remove the humidity.

On July 7, the temperature in southeastern Texas was in the low 80s for most of the afternoon and only briefly rose to the upper 80s. That's why your air conditioner was short-cycling. There wasn't enough sensible load to run the air conditioner for 10 minutes at a time.

A two-stage air conditioner would have 7-minute runtimes in first stage, and it would maintain a lower relative humidity. It would still be short-cycling, but not as badly.

A variable-speed air conditioner would have 20-minute runtimes at 25% load, and it would maintain an even lower relative humidity.
50% is good but 54% is unacceptable? can you elaborate? is 51% acceptable? what about 52%?
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Re: HVAC decision help please

Post by talzara »

z0r wrote: Tue Jul 09, 2024 1:14 pm 50% is good but 54% is unacceptable? can you elaborate? is 51% acceptable? what about 52%?
Manual J specifies indoor conditions of 50% relative humidity at 75°F.

The temperature was acceptable because the sensor was on the second floor, so the first floor was at 75°F. However, the humidity doesn't change by floor, so the system was failing to maintain humidity.

It is not always possible for an air conditioner to maintain the Manual J conditions. For example, if it's 70°F outside, then the air conditioner cannot remove humidity because it's not running. That's when you need a dehumidifier.

However, the temperature in southeastern Texas on the afternoon of July 7 was in the 80s. It was possible for an air conditioner to maintain 50% relative humidity, but that system was short-cycling. It was only running for 4 minutes at a time!
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Re: HVAC decision help please

Post by z0r »

talzara wrote: Tue Jul 09, 2024 1:31 pm
z0r wrote: Tue Jul 09, 2024 1:14 pm 50% is good but 54% is unacceptable? can you elaborate? is 51% acceptable? what about 52%?
Manual J specifies indoor conditions of 50% relative humidity at 75°F.

The temperature was acceptable because the sensor was on the second floor, so the first floor was at 75°F. However, the humidity doesn't change by floor, so the system was failing to maintain humidity.

It is not always possible for an air conditioner to maintain the Manual J conditions. For example, if it's 70°F outside, then the air conditioner cannot remove humidity because it's not running. That's when you need a dehumidifier.

However, the temperature in southeastern Texas on the afternoon of July 7 was in the 80s. It was possible for an air conditioner to maintain 50% relative humidity, but that system was short-cycling. It was only running for 4 minutes at a time!
I don't think this is right. are you saying that the acca load calc guide specifies that humidity should be exactly 50%? that sounds more like a number used for sizing decisions, not some kind of acceptable limit

dehumidification with air conditioners changes with only a few variables: 1. cfm/ton 2. it helps a little to run long enough that the evaporator coil can get loaded up and start dripping 3. post run fan operation is bad (evaporating remaining water off the coil) 4. sucking humid air into ducts while running if they're in hostile areas, this gets worse with increased runtime. staged and variable systems win two of these, lose one, and push one, so it's possible (every house being different) for a single stage unit to actually be best, if your ducts are especially leaky

if you like hitting a lower humidity target then the best way is to turn down your fan speed, rather than spring for a new system

as you note, if humidity is really important to you, getting a dehumidifier is a good idea
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Re: HVAC decision help please

Post by livesoft »

I'm laughing at 50% vs 54% and short cycling.

For one thing why do you even believe the 54% relative humidity number in the first place? The measuring device has not been calibrated nor tested versus a known standard.

But here is something cool (pun!): Our power has been off for a while because of the hurricane. Power was just restored about an hour ago and the AC units are definitely not short cycling. LOL!

Image
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talzara
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Re: HVAC decision help please

Post by talzara »

z0r wrote: Tue Jul 09, 2024 8:35 pm I don't think this is right. are you saying that the acca load calc guide specifies that humidity should be exactly 50%? that sounds more like a number used for sizing decisions, not some kind of acceptable limit
No, but it was possible to maintain 50% relative humidity on the afternoon of July 7 in southeast Texas.

It wasn't an easy day to dehumidify because it wasn't that hot, but it wasn't impossible either.
z0r wrote: Tue Jul 09, 2024 8:35 pm dehumidification with air conditioners changes with only a few variables: 1. cfm/ton 2. it helps a little to run long enough that the evaporator coil can get loaded up and start dripping 3. post run fan operation is bad (evaporating remaining water off the coil) 4. sucking humid air into ducts while running if they're in hostile areas, this gets worse with increased runtime. staged and variable systems win two of these, lose one, and push one, so it's possible (every house being different) for a single stage unit to actually be best, if your ducts are especially leaky

if you like hitting a lower humidity target then the best way is to turn down your fan speed, rather than spring for a new system
Single-stage air conditioners with variable-speed blowers can usually adjust the airflow by about 10%.

If you have a little too much humidity and long runtimes, 10% might be enough.

This unit was only running for 4 minutes at a time. Reducing the fan speed by 10% won't solve the problem.
talzara
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Re: HVAC decision help please

Post by talzara »

livesoft wrote: Tue Jul 09, 2024 9:29 pm I'm laughing at 50% vs 54% and short cycling.

For one thing why do you even believe the 54% relative humidity number in the first place? The measuring device has not been calibrated nor tested versus a known standard.
If you don't believe the data that you're posting, then why are you posting it? How do you know the sensor isn't off in the other direction?

One way to calibrate the sensor is to to take it outside and compare it to a weather station. Those are calibrated at the factory. You can also compare the data from several weather stations to each other.
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Re: HVAC decision help please

Post by livesoft »

Why should I care that my AC was only running 4 minutes at a time? Do you believe that it shortens the life of the unit? We replaced a system after 29 years, so I don't believe the life of the system was affected.

As for why I posted the data: I posted because we are quite comfortable in our house with an apparent simple system and might not even notice a multi-stage variable this or that. I also posted to show the OP that one can get cheap measuring / recording devices.
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talzara
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Re: HVAC decision help please

Post by talzara »

livesoft wrote: Tue Jul 09, 2024 9:49 pm Why should I care that my AC was only running 4 minutes at a time? Do you believe that it shortens the life of the unit? We replaced a system after 29 years, so I don't believe the life of the system was affected.
No, I believe that the 4-minute runtime caused the humidity to be higher in your house.
livesoft wrote: Tue Jul 09, 2024 9:49 pm As for why I posted the data: I posted because we are quite comfortable in our house with an apparent simple system and might not even notice a multi-stage variable this or that. I also posted to show the OP that one can get cheap measuring / recording devices.
You questioned your own data because the sensor hasn't been calibrated. If you can't believe the data, then how does that help the OP decide to buy a single-stage unit? Why should the OP buy "cheap measuring / recording devices" if they give you bad data?

I can't put my own sensors in your house. I can only look at the data that you have posted.

If you can believe the data, then it shows that the OP was right to be looking at two-stage and variable-speed air conditioners. Sometimes the single-stage unit isn't enough, such as the afternoon of July 7 in southeastern Texas. Sometimes you need a two-stage or variable-speed unit.
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Re: HVAC decision help please

Post by livesoft »

Sorry, I was adding to my previous reply when we lost power and internet, so I will add here:

Another reason I posted was to elicit any criticisms that anyone might have, so I think I was successful in that regard. :)

And another reason: I wanted to see if anyone else had such graphs for their own systems and would post them.
talzara wrote: Tue Jul 09, 2024 10:22 pmI can't put my own sensors in your house. I can only look at the data that you have posted.

If you can believe the data, then it shows that the OP was right to be looking at two-stage and variable-speed air conditioners. Sometimes the single-stage unit isn't enough, such as the afternoon of July 7 in southeastern Texas. Sometimes you need a two-stage or variable-speed unit.
Please post the charts from your "own sensors" from your own house. :)

But the criticism/response gave me the idea to take these little bluetooth thermometers/hygrometers and put them in a neighbor's house to see what the results might be. I will chat with a neighbor and maybe find one with a fancier HVAC system and update this thread. Of course, I would encourage anyone else that makes such measurements to post charts.

And you know the morning of July 7 before it rained, the outside temps in SE Texas were in the upper 90s (I have one of these devices outside, too).

For me personally, while perhaps my single-stage units are not keeping the temp and humidity inside my house at laboratory quality +- 0.1 degree or 50% +- 0.5% humidity, I can say that I don't even notice that my perceived comfort level is any different than where I worked with in such lab rooms where it was the case that temp control was critical. So do I "need" a two-stage or variable-speed unit? I am not convinced.

[I will take my devices back to my old job and get some measurements and revisit this thread. And maybe I will add charts from when the furnaces are running next winter. Thanks for that idea as well.]

Thanks!
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zlandar
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Re: HVAC decision help please

Post by zlandar »

My two story house with a walkout basement has an AC/gas furnace unit for each floor (3 total). The top floor barely runs from Oct-Apr (27 hours of cooling, 15 hours of heat). From May-Sep it runs a lot. In June alone it ran 300 hours. I live in GA and it gets hot in the summer.

Depending on your sq footage and layout of the 2nd floor having two separate units may be better. One big unit may cause uneven heating and cooling between floors during the year.

All three are single-stage units. If the unit is properly sized single stage works fine. They are the cheapest, less parts to break, and cheapest to repair. If I was replacing a unit now I would consider a one or two stage depending on price. I am staying away from the variable units because of the additional cost, more parts that can break, and high cost of repair.
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Re: HVAC decision help please

Post by THY4373 »

I will have to post some graphs but I will say the one criticism I have of Mitsubishi variable speed units is they don't control humidity as well as my old single speed systems (the average humidity is running about 5% higher now). I actually do not notice this from a comfort level I am actually more comfortable (but my monitoring devices clearly show the swing and higher average) but when they are running low and slow they are clearly not removing as much latent heat as when they are going full out. So basically you see a cycle where humidity raises over night and through the morning and then around midday as they ramp up in comes down and then starts to rise again in the evening as they ramp down. This is usually around a 5% swing but on the hottest day this year I saw a 10% swing mostly because they were running hard longer.
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Re: HVAC decision help please

Post by tibbitts »

About this 4-minute cycling don't thermostats have a configuration to prevent short-cycling? I had actually thought the hvac itself should have that but I don't know how I'd implement the electronics for short-cycling that results from power failures without sticking a battery in the hvac itself vs. just having one in the thermostat. Does anybody know if any units do this, or how? How do compressors not get damaged when short-cycling due to power issues?
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Re: HVAC decision help please

Post by livesoft »

tibbitts wrote: Wed Jul 10, 2024 9:00 am About this 4-minute cycling don't thermostats have a configuration to prevent short-cycling?
Maybe some thermostats do, but I don't consider a 4-minute cycle a bad thing. Maybe one needs to insulate their house better if one is concerned or perhaps move the house to a different climate where it isn't so hot and humid. Or have fewer people and animals creating humidity inside the house.

I would think that one would want the temperature to cool as quickly as possible. And same for humidity: Reduce humidity as quickly as possible. In particular, I can see spikes in humidity on a device whenever I take a shower and the AC removes that tiny spike nicely and quickly. At least I am not running my large tropical aquarium anymore. :twisted:
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Re: HVAC decision help please

Post by firebirdparts »

Big fan of separate units in a 2 story house. FWIW. Ductwork modifications are going to be pricey if you find somebody to do it, and so it's probably not financially "good". There could be a "zone" alternative that's less difficult to retrofit, but that just depends on where you need to get into the ductwork to add zone controls. There's also a version of this where you have two returns, and you can use only the upstairs return in summer, and use the downstairs return in winter.
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Re: HVAC decision help please

Post by firebirdparts »

tibbitts wrote: Wed Jul 10, 2024 9:00 am About this 4-minute cycling don't thermostats have a configuration to prevent short-cycling? I had actually thought the hvac itself should have that but I don't know how I'd implement the electronics for short-cycling that results from power failures without sticking a battery in the hvac itself vs. just having one in the thermostat. Does anybody know if any units do this, or how? How do compressors not get damaged when short-cycling due to power issues?
This question gets into the history of thermostats, which is all washed away by time. Traditionally, thermostats had an elegant configuration to prevent overshot. That is, to prevent long cycling. This was called the heat anticipator and there is an analogous anticipator for air conditioning as well which was a bit odd. This was used on good thermostats 50 years ago.

Thermostats today, being digital, have different less elegant ways of doing that, but they now can incorporate dumb timers. They typically have a 5 minute block on the compressor to allow the head pressure to bleed down. You may be able to turn this off if you want to. I'm sure they vary.

Not planning to do any cycle time virtue signaling.
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Re: HVAC decision help please

Post by talzara »

livesoft wrote: Wed Jul 10, 2024 5:34 am Please post the charts from your "own sensors" from your own house. :)
I don't live in Texas.

The OP lives in the Dallas area. You live in southeastern Texas, which is a similar climate. That's why we're talking about your data, not mine.
livesoft wrote: Wed Jul 10, 2024 5:34 am For me personally, while perhaps my single-stage units are not keeping the temp and humidity inside my house at laboratory quality +- 0.1 degree or 50% +- 0.5% humidity, I can say that I don't even notice that my perceived comfort level is any different than where I worked with in such lab rooms where it was the case that temp control was critical. So do I "need" a two-stage or variable-speed unit? I am not convinced.
If you feel comfortable, then you don't need to change equipment.

However, HVAC systems are supposed to be designed for most people's comfort. For example, some people may feel comfortable at 80°F, but most people don't. That's why Manual J specifies 75°F. If an HVAC system maintains 75°F, then there won't be a lot of complaints.
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Re: HVAC decision help please

Post by talzara »

THY4373 wrote: Wed Jul 10, 2024 8:38 am I will have to post some graphs but I will say the one criticism I have of Mitsubishi variable speed units is they don't control humidity as well as my old single speed systems (the average humidity is running about 5% higher now). I actually do not notice this from a comfort level I am actually more comfortable (but my monitoring devices clearly show the swing and higher average) but when they are running low and slow they are clearly not removing as much latent heat as when they are going full out. So basically you see a cycle where humidity raises over night and through the morning and then around midday as they ramp up in comes down and then starts to rise again in the evening as they ramp down. This is usually around a 5% swing but on the hottest day this year I saw a 10% swing mostly because they were running hard longer.
An inverter heat pump can remove more humidity, or it can remove less. It's controlled by the computer.

Some mini-splits have a dehumidify mode. The computer will run a different program that keeps the coil cooler to remove more humidity.
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Re: HVAC decision help please

Post by chuckwalla »

Is it the norm to just replace the entire HVAC system every 10-15 years nowadays?
Is it more cost effective than repairing it when something breaks?
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Re: HVAC decision help please

Post by talzara »

tibbitts wrote: Wed Jul 10, 2024 9:00 am About this 4-minute cycling don't thermostats have a configuration to prevent short-cycling? I had actually thought the hvac itself should have that but I don't know how I'd implement the electronics for short-cycling that results from power failures without sticking a battery in the hvac itself vs. just having one in the thermostat. Does anybody know if any units do this, or how? How do compressors not get damaged when short-cycling due to power issues?
It's the off-on cycle that damages the air conditioner, not the on-off cycle.

Running an air conditioner for 4 minutes doesn't damage it. Turning it off and turning it back on 4 minutes later will increase wear-and-tear.

That's why I said the 4-minute runtime is limiting the humidity control, but I didn't say it was reducing the equipment life. It was turning off after 4 minutes, not turning on after 4 minutes.
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Re: HVAC decision help please

Post by livesoft »

chuckwalla wrote: Wed Jul 10, 2024 3:26 pm Is it the norm to just replace the entire HVAC system every 10-15 years nowadays?
Is it more cost effective than repairing it when something breaks?
It certainly has not been my norm. :)
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