Hot house: a central air conditioning query
July 18, 2024 12:31 PM Subscribe
My in-laws live in a house built in the 1980s in an area known for hot, muggy summers. They've lived there for almost 30 years. The house has central air conditioning and the first floor is usually very cool and comfortable. But upstairs it's like there's no air conditioning at all (at least 10-15 degrees hotter than the rest of the house without much cool air coming through the upstairs vents).
I grew up with swamp coolers, but I thought that the whole point of central air was that it would cool the whole house?
I know that hot air rises so the upstairs is likely to be warmer but I thought that having a thermostat upstairs and downstairs was supposed to cool the upstairs too? To cope with how hot it is upstairs, my in-laws have purchased those portable AC units that vent out the windows for all of the bedrooms. They seem to think this is totally reasonable and not a sign that something in the air conditioner might need some work.
My in-laws think it is totally normal to have central air that doesn't actually cool the upstairs. It seems nuts to me to have an extra air conditioner in nearly every room in a house with central air - I have never seen this before.
What do we think here: normal or a sign that something in the HVAC system needs a look?
I grew up with swamp coolers, but I thought that the whole point of central air was that it would cool the whole house?
I know that hot air rises so the upstairs is likely to be warmer but I thought that having a thermostat upstairs and downstairs was supposed to cool the upstairs too? To cope with how hot it is upstairs, my in-laws have purchased those portable AC units that vent out the windows for all of the bedrooms. They seem to think this is totally reasonable and not a sign that something in the air conditioner might need some work.
My in-laws think it is totally normal to have central air that doesn't actually cool the upstairs. It seems nuts to me to have an extra air conditioner in nearly every room in a house with central air - I have never seen this before.
What do we think here: normal or a sign that something in the HVAC system needs a look?
Best answer: Live in a house built in the 1950s which has added central air. It reaches one vent on the second floor. The duct system is not 'zoned' and unbalanced and possibly designed by someone with no physics knowledge, so air trying to get upstairs has too far to go to be effective. Many ducted systems are poorly designed and this is probably the cause.
posted by cobaltnine at 12:43 PM on July 18 [16 favorites]
posted by cobaltnine at 12:43 PM on July 18 [16 favorites]
I have a two story house too. The AC cools the house until the thermostat reaches the measured temperature. The thermostat is usually on the main floor and the main floor is guaranteed to be cooler than the second floor, because cold air sinks and hot air rises. What I've noticed in the summer is that my wife and I's east facing bedroom is in line with the thermostat in the evening. My daughters' west facing bedrooms are being solar heated by the windows in the evening. Opening our master bedroom door and their bedroom doors have helped to allow the air to circulate.
I did purchase some furnace vent fan boosters to pull up more cold air in the summer months. I did think there was some benefit, but I can't quantify it fully.
I do think our master east facing bedroom is hotter in the mornings, but it's usually not as noticeable since we're awake and about in the mornings.
I've been thinking that modern homes with large windows is a mistake as windows aren't insulation efficient as much as walls are.
posted by DetriusXii at 12:50 PM on July 18
I did purchase some furnace vent fan boosters to pull up more cold air in the summer months. I did think there was some benefit, but I can't quantify it fully.
I do think our master east facing bedroom is hotter in the mornings, but it's usually not as noticeable since we're awake and about in the mornings.
I've been thinking that modern homes with large windows is a mistake as windows aren't insulation efficient as much as walls are.
posted by DetriusXii at 12:50 PM on July 18
Best answer: A competent HVAC person can do a survey of the system and re-balance it . This should help matters somewhat, without getting all fancy about re-zoning etc. It's a fairly common request, as things can change over time (balance dampers move out of position, the usage patterns of the house change, etc.)
posted by aramaic at 12:54 PM on July 18 [3 favorites]
posted by aramaic at 12:54 PM on July 18 [3 favorites]
Yeah, we moved into this house in 2011 and I was super excited because it was my first home with central air conditioning and, like your in-laws, it works absolutely great downstairs, but it's the worst upstairs. Our master bedroom is upstairs and if the thermostat is set to 72, it can easily be 84 in the bedroom.
I think cobaltnine has it.
One thing we did recently is have a reputable company come out and put aeroseal into the ducting. This is a kind of chemical glue that closes gaps in the HVAC ducting to improve efficiency. They did a before and after pressure test on the system to show truly how much leakage there was prior to performing the seal, and it was pretty staggering. It's helped a little bit with the problem, but is not a miracle cure.
I also think that, as you point, hot air rises, and central air conditioning, as good as it is, can have difficulty fighting that fundamental fact.
I feel like the real answer is those cool mini-split zone air conditioners that are big in, like, Japan and Europe. I'm not sure why I think I know this, but I feel like at some point I learned that they're the real way to tackle this problem usefully.
posted by kbanas at 12:54 PM on July 18 [1 favorite]
I think cobaltnine has it.
One thing we did recently is have a reputable company come out and put aeroseal into the ducting. This is a kind of chemical glue that closes gaps in the HVAC ducting to improve efficiency. They did a before and after pressure test on the system to show truly how much leakage there was prior to performing the seal, and it was pretty staggering. It's helped a little bit with the problem, but is not a miracle cure.
I also think that, as you point, hot air rises, and central air conditioning, as good as it is, can have difficulty fighting that fundamental fact.
I feel like the real answer is those cool mini-split zone air conditioners that are big in, like, Japan and Europe. I'm not sure why I think I know this, but I feel like at some point I learned that they're the real way to tackle this problem usefully.
posted by kbanas at 12:54 PM on July 18 [1 favorite]
Heat rises and the thermostat is on the first floor, so the AC stops running once the downstairs is cool. Assuming that cold air does blow out of the registers upstairs, upgrade the thermostat to a smart one that has remote sensors so it can tell what the temperature is up there. (I chose the Honeywell T9 ; there are also Ecobee, Sensi, and Nest. All are pretty easy to install yourself assuming your furnace has a "common wire" to provide power and doesn't rely on sending commands. The Trane stats have that issue.) The trick is that the fan will run longer to make sure air is circulated through the house enough to make the temperature even. This might result in the fan running nearly constantly on the hottest days and it still might not keep up entirely, but it does improve things a lot. The sensors can also be set to detect which rooms have people in them and prioritize the temperature in those rooms.
If you don't want to upgrade the stat, then see if the existing one has an option to have the fan run constantly, which is a brute force way of doing the same thing. Some also have a "circulate" mode, which guarantees that the fan will run some proportion of the time (the default is typically 33%).
We had a similar problem in our house despite having a smart thermostat. A secondary symptom was that the AC simply couldn't keep up in the afternoon when the sun comes in on the back side of the house (we developed a habit of chilling the house in the morning so it would stay cool enough in the afternoon).
It turned out that the system's blower was woefully undersized for the amount of ducting we had, and was caked with dust to boot. Since the AC compressor was also on its last legs, we replaced the whole HVAC system less than a month ago, including the thermostat. It was expensive but it did solve the problem. The installer for the new HVAC also determined we were losing at least 25% of our cooling capacity to leaks in the ductwork in the basement, and caulked those. So it may be worth having the system checked out by a professional, especially if it is 10+ years old, or is original to the house (builders tend to cut corners).
posted by kindall at 1:12 PM on July 18
If you don't want to upgrade the stat, then see if the existing one has an option to have the fan run constantly, which is a brute force way of doing the same thing. Some also have a "circulate" mode, which guarantees that the fan will run some proportion of the time (the default is typically 33%).
We had a similar problem in our house despite having a smart thermostat. A secondary symptom was that the AC simply couldn't keep up in the afternoon when the sun comes in on the back side of the house (we developed a habit of chilling the house in the morning so it would stay cool enough in the afternoon).
It turned out that the system's blower was woefully undersized for the amount of ducting we had, and was caked with dust to boot. Since the AC compressor was also on its last legs, we replaced the whole HVAC system less than a month ago, including the thermostat. It was expensive but it did solve the problem. The installer for the new HVAC also determined we were losing at least 25% of our cooling capacity to leaks in the ductwork in the basement, and caulked those. So it may be worth having the system checked out by a professional, especially if it is 10+ years old, or is original to the house (builders tend to cut corners).
posted by kindall at 1:12 PM on July 18
Best answer: What's the insulation situation in the house? When we moved into our current home, we realized the insulation was highly insufficient to heating and cooling. The first year, it was too hot in the summer and too cold in the winter. New insulation went in and things are much, much better.
posted by cooker girl at 1:12 PM on July 18 [2 favorites]
posted by cooker girl at 1:12 PM on July 18 [2 favorites]
Best answer: Also agree with cobaltnine.
Earlier this week I was trying to figure out if I could adjust dampers to get more cool air into an upstairs bedroom vent. So I went down to the basement, and discovered it was 60 degrees down there because two "courtesy" heating vents down in the utility space were blasting out freezing air! Closed them up, and suddenly there's more AC available for actual living spaces.
The damper settings that work OK in heating season may not make sense during cooling season.
posted by ReferenceDesk at 1:14 PM on July 18 [1 favorite]
Earlier this week I was trying to figure out if I could adjust dampers to get more cool air into an upstairs bedroom vent. So I went down to the basement, and discovered it was 60 degrees down there because two "courtesy" heating vents down in the utility space were blasting out freezing air! Closed them up, and suddenly there's more AC available for actual living spaces.
The damper settings that work OK in heating season may not make sense during cooling season.
posted by ReferenceDesk at 1:14 PM on July 18 [1 favorite]
Best answer: The easiest thing they can do today is set the fan on their thermostat to On instead of Auto so that it runs all the time. This will probably not completely solve the problem but it will definitely help. It's also not that big of an energy drain.
Slightly more complicated would be checking the filter; if it's really clogged the whole system is going to work poorly and they can probably DIY this.
I doubt either of these will fully solve the problem, though, so if it were my house I'd also have an HVAC tech out to look at the system. The answer could be something really expensive, if cobaltnine is correct and the ducts are fundamentally mis-designed, or it could be a pretty cheap and easy rebalancing. Getting a smarter thermostat with remote temperature sensors could be another, relatively cheap, part of the solution.
posted by Aizkolari at 1:18 PM on July 18 [1 favorite]
Slightly more complicated would be checking the filter; if it's really clogged the whole system is going to work poorly and they can probably DIY this.
I doubt either of these will fully solve the problem, though, so if it were my house I'd also have an HVAC tech out to look at the system. The answer could be something really expensive, if cobaltnine is correct and the ducts are fundamentally mis-designed, or it could be a pretty cheap and easy rebalancing. Getting a smarter thermostat with remote temperature sensors could be another, relatively cheap, part of the solution.
posted by Aizkolari at 1:18 PM on July 18 [1 favorite]
Best answer: House built in the 80s likely has ducts that are properly sized for AC (before the 1970s or so can be a crap shoot); forced air requires less airflow than AC, so duct sizing is often a problem in older houses. It MAY be a damper balancing issue as someone mentioned, but my first question if I showed up to this service call would be “has it ever cooled the upstairs properly?” If the answer to that is yes, then there’s likely a mechanical problem. If the answer to that is no, then there are two different things to look at: 1. Holistic view of the system. Is it sized correctly, is it balanced properly, is the ductwork sufficient? And then 2. What color is the roof? Black/dark? What kind of insulation is there, both in the walls and between the roof and living space. It may be more of an insulation problem than an HVAC problem, and I’d bet pretty good money, based on my experience, that if it hasn’t ever really cooled the upstairs, it’s more of an insulation problem.
There are so many variables here that it’s probably best to get an energy audit done (see if there is a government program or utility company that does them; an HVAC company will more likely than not just try to sell you something). If an energy audit comes back positively, and you have sufficient insulation, then it’s time to call an HVAC company, but be careful with that too, as there is a plague of private equity buying out residential companies across the country and enshittifying them. Do lots of research on an HVAC company before talking with them. Sooooooo so many (even ones that advertise as “family/locally owned”) are in fact owned by private equity vultures.
posted by HVACDC_Bag at 1:20 PM on July 18 [3 favorites]
There are so many variables here that it’s probably best to get an energy audit done (see if there is a government program or utility company that does them; an HVAC company will more likely than not just try to sell you something). If an energy audit comes back positively, and you have sufficient insulation, then it’s time to call an HVAC company, but be careful with that too, as there is a plague of private equity buying out residential companies across the country and enshittifying them. Do lots of research on an HVAC company before talking with them. Sooooooo so many (even ones that advertise as “family/locally owned”) are in fact owned by private equity vultures.
posted by HVACDC_Bag at 1:20 PM on July 18 [3 favorites]
Definitely bring someone out to take a look, although not all problems can be solved with balancing or tune-ups. They should also be getting the A/C looked at every season to keep it running its best.
I live in a 100+ year old home that used to have radiator heat, and was retrofitted with central air and forced air heat fairly recently (before we bought it) by idiot flippers who had no idea what they were doing. Our second floor is actually a converted attic, with no space between the roof and the ceiling other than the depth of the rafters and it gets HOT up there. The sun just beats down on the roof and heats up the whole floor, and the terrible retrofitted ductwork barely moves any air at all.
We ended up going the ductless mini-split route after few years of portable and window A/C units. It's glorious! But it's expensive and given your parents' house is newer and not full of retrofits and conversions, in theory they should have A/C that works upstairs.
posted by misskaz at 1:26 PM on July 18
I live in a 100+ year old home that used to have radiator heat, and was retrofitted with central air and forced air heat fairly recently (before we bought it) by idiot flippers who had no idea what they were doing. Our second floor is actually a converted attic, with no space between the roof and the ceiling other than the depth of the rafters and it gets HOT up there. The sun just beats down on the roof and heats up the whole floor, and the terrible retrofitted ductwork barely moves any air at all.
We ended up going the ductless mini-split route after few years of portable and window A/C units. It's glorious! But it's expensive and given your parents' house is newer and not full of retrofits and conversions, in theory they should have A/C that works upstairs.
posted by misskaz at 1:26 PM on July 18
A similar was just asked. Attic fans can be very effective. Plant deciduous trees on the south side to give shade in summer, allow sun in winter; good trees add to a house's value. They take time to grow, so plant now. Use shades, blinds or good curtains to keep hot sun out.
Despite the energy crisis of the 70s, many, many homes are poorly insulated, and insulation makes a big difference.
And have an HVAC person come out and assess. No sense using lots of extra electricity; it may be a fixable problem.
posted by theora55 at 2:04 PM on July 18 [1 favorite]
Despite the energy crisis of the 70s, many, many homes are poorly insulated, and insulation makes a big difference.
And have an HVAC person come out and assess. No sense using lots of extra electricity; it may be a fixable problem.
posted by theora55 at 2:04 PM on July 18 [1 favorite]
I just cover up the downstairs vents with magnetic sheets (you could also tape them up) to force the air upstairs. Just closing the dampers didn't do much. That's an extremely cheap and easy approach that you could test before investing more time and money.
posted by Mr.Know-it-some at 2:11 PM on July 18 [3 favorites]
posted by Mr.Know-it-some at 2:11 PM on July 18 [3 favorites]
I came to suggest checking to see if they have small attic fans that might need replacing. By attic fan, what I'm talking about is a low-power automatic fan that pulls hot air from the attic space and blows it out to the world. This helps keep air moving and circulating.
There are also the previously mentioned (and in the other thread) giant attic exhaust fans that suck air from outside through any available space and pull air through the house and up into the attic. They're pretty loud, but lemme tell you, being able to rapid cycle the air in your house is pretty great. We run ours at night before bed and very early in the morning, when it's still cool outside. Big energy saver.
posted by ApathyGirl at 2:31 PM on July 18
There are also the previously mentioned (and in the other thread) giant attic exhaust fans that suck air from outside through any available space and pull air through the house and up into the attic. They're pretty loud, but lemme tell you, being able to rapid cycle the air in your house is pretty great. We run ours at night before bed and very early in the morning, when it's still cool outside. Big energy saver.
posted by ApathyGirl at 2:31 PM on July 18
It seems nuts to me to have an extra air conditioner in nearly every room in a house with central air - I have never seen this before.
It's actually very common. Blocking off ducts is often a bad idea, due to leakage, so be careful with that. If your ducts are mostly in a hot attic, be extra careful. Dampers on your AC (so blocked off or reduced right at the unit) 2nd AC systems, one upstairs, one down is very common.
More insulation is a very good idea. The amount of recommended insulation has increased quite a bit from the past. If you have enough insulation, things like attic fans and attic ventilation don't make much difference.
The problem with window A/Cs is that they block windows, so you get kind of stuck with them, and can't open windows on nice days. Ductless minisplits is the best, luxury option.
Most of these will increase your AC bills.
posted by The_Vegetables at 2:42 PM on July 18
It's actually very common. Blocking off ducts is often a bad idea, due to leakage, so be careful with that. If your ducts are mostly in a hot attic, be extra careful. Dampers on your AC (so blocked off or reduced right at the unit) 2nd AC systems, one upstairs, one down is very common.
More insulation is a very good idea. The amount of recommended insulation has increased quite a bit from the past. If you have enough insulation, things like attic fans and attic ventilation don't make much difference.
The problem with window A/Cs is that they block windows, so you get kind of stuck with them, and can't open windows on nice days. Ductless minisplits is the best, luxury option.
Most of these will increase your AC bills.
posted by The_Vegetables at 2:42 PM on July 18
So how is the heat upstairs venting? There needs to be some way for the heat to escape the house. It's much easier to vent the hot air entirely out of the house than to try to force it back down again and get it to get sucked through the air conditioner to chill it. Heat wants to rise. The solution is to let it, and it will suck the cold air upstairs as it does.
When that house was built, if it was built halfway decently, there should have been a way to do it. But the house was built in the 80's so maybe they failed to install vents. They were worrying about heating bills back then, after the seventies energy crisis. A lot of buildings got built working from the assumption that air conditioning was a luxury and if it were that bad you could always go to the mall, but that high heating bills were a home owner's nightmare.
posted by Jane the Brown at 4:06 PM on July 18 [1 favorite]
When that house was built, if it was built halfway decently, there should have been a way to do it. But the house was built in the 80's so maybe they failed to install vents. They were worrying about heating bills back then, after the seventies energy crisis. A lot of buildings got built working from the assumption that air conditioning was a luxury and if it were that bad you could always go to the mall, but that high heating bills were a home owner's nightmare.
posted by Jane the Brown at 4:06 PM on July 18 [1 favorite]
Best answer: Are there any return vents upstairs? Ideally the system should be drawing its air from the hottest sections of the house. I've been a lot of houses where the returns are in the wrong place or not adjusted properly. In my hottest months I close almost all the returns on the first floor.
posted by JoeZydeco at 5:48 PM on July 18
posted by JoeZydeco at 5:48 PM on July 18
My current house also is warm upstairs despite central air. My very competent hvac guy looked at it and said that the ducting wasn't adequate for the house (multiple additions) - it would cost a fortune to re-duct and be very disruptive. Options were to add a minisplit upstairs or fans. We have a ceiling fan in our bedroom and it helps a lot. I'm not ready to spring for a mini-split. I suppose another option might be a window air conditioner to supplement the central air.
posted by leslies at 7:10 PM on July 18 [1 favorite]
posted by leslies at 7:10 PM on July 18 [1 favorite]
Our house was built in the 60s and my understanding is that the ductwork taking the air to the upstairs are too small. That they could be replaced with larger ones but that “nobody pays [our HVAC guy] to tear apart their walls.” But that some of them could be replaced if we were ever to remodel our kitchen or whatnot.
He did give us some out of the box alternatives like trying to run a new trunk of ductwork up to the attic and then pipe it down into the ceilings of the bedrooms, but ultimately it comes down to money. If you’re going to spend a few thousand dollars to do something like that, it’ll never pay for itself vs an extra $50 or $100 on your electric bill for 4 months out of the year.
I do want to point out that I have read that you aren’t supposed to cover any vents, you can increase the pressure in other areas, disrupt your airflow and potentially cause mildew if it’s a humid environment. I would get a couple companies to come out and talk through different ideas.
posted by cali59 at 11:41 AM on July 19
He did give us some out of the box alternatives like trying to run a new trunk of ductwork up to the attic and then pipe it down into the ceilings of the bedrooms, but ultimately it comes down to money. If you’re going to spend a few thousand dollars to do something like that, it’ll never pay for itself vs an extra $50 or $100 on your electric bill for 4 months out of the year.
I do want to point out that I have read that you aren’t supposed to cover any vents, you can increase the pressure in other areas, disrupt your airflow and potentially cause mildew if it’s a humid environment. I would get a couple companies to come out and talk through different ideas.
posted by cali59 at 11:41 AM on July 19
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posted by anastasiav at 12:36 PM on July 18 [6 favorites]