You are not my HVAC tech...but I don't trust my HVAC tech
July 12, 2020 1:08 PM Subscribe
We had central AC added last summer. It sucks. I am trying to understand if our expectations are reasonable, and what next steps (if any) might be to improve the cooling of our home.
During this heat wave (but also in more reasonable summer weather) the central AC really struggles to keep up. Most days, only cooling to say 76 degrees (we are trying for low 70s). Even in cooler temperatures, I don't think the house could ever be chilled to say 65 degrees. J
I have had the company that installed the system out twice (once soon after we got it, once last week). They say the house and duct work are to blame. I feel like that's not the whole story and am wondering if we were sold an undersized unit (if so, how could we tell?)
Details on the house: 100 year old double brick 2 story + finished basement. The basement gets really cold, and all vents there are closed in effort to force air upwards. The main floor (which is open concept) can get down to mid 70s, the 2nd floor whose rooms have 1 vent each, is always significantly hotter. The furnace is 15 years old, in decent shape (we have no trouble with heat in the winter, even deep freezes), and the furnace filter is changed regularly. The outside compressor of the AC is clean and doesn't seem to have developed any ice. In general, the air that comes out of the floor vents is cool but not cold. If a door to the house is open for more than a few minutes, the system really struggles to get back to the set temperature.
Other info - the layout of rooms upstairs is not great for adding a mini-split (we did think about it). We have window coverings drawn in the upstairs all day, it doesn't seem to help much.
Questions: is our expectation of being able to achieve 70 degrees reasonable ? Is there merit in having another company come and look at the system? Would replacing the furnace help?
Adding window units seems to be the one viable option, but it feels like defeat...
During this heat wave (but also in more reasonable summer weather) the central AC really struggles to keep up. Most days, only cooling to say 76 degrees (we are trying for low 70s). Even in cooler temperatures, I don't think the house could ever be chilled to say 65 degrees. J
I have had the company that installed the system out twice (once soon after we got it, once last week). They say the house and duct work are to blame. I feel like that's not the whole story and am wondering if we were sold an undersized unit (if so, how could we tell?)
Details on the house: 100 year old double brick 2 story + finished basement. The basement gets really cold, and all vents there are closed in effort to force air upwards. The main floor (which is open concept) can get down to mid 70s, the 2nd floor whose rooms have 1 vent each, is always significantly hotter. The furnace is 15 years old, in decent shape (we have no trouble with heat in the winter, even deep freezes), and the furnace filter is changed regularly. The outside compressor of the AC is clean and doesn't seem to have developed any ice. In general, the air that comes out of the floor vents is cool but not cold. If a door to the house is open for more than a few minutes, the system really struggles to get back to the set temperature.
Other info - the layout of rooms upstairs is not great for adding a mini-split (we did think about it). We have window coverings drawn in the upstairs all day, it doesn't seem to help much.
Questions: is our expectation of being able to achieve 70 degrees reasonable ? Is there merit in having another company come and look at the system? Would replacing the furnace help?
Adding window units seems to be the one viable option, but it feels like defeat...
HVAC aside, a house that is 100 years old is probably a leaky air and poor insulation nightmare. I'd make sure your windows and doors are sealing properly as well as look for any cracks, openings, vents, or other places hot air is getting in.
The other thing to look at is your attic insulation. Hot sun beating down on a dark roof without adequate insulation equals a really, really warm second floor.
One last thing to consider - since your original furnace install didn't take AC into account, it may not be up to the task - but that is definitely a question for an HVAC professional.
At this point, the best thing you can do is talk to 2+ HVAC companies about this and see what the consensus is. They usually don't have a fee to come take a look at give you a rough quote on the work they want to do.
posted by _DB_ at 1:36 PM on July 12, 2020 [6 favorites]
The other thing to look at is your attic insulation. Hot sun beating down on a dark roof without adequate insulation equals a really, really warm second floor.
One last thing to consider - since your original furnace install didn't take AC into account, it may not be up to the task - but that is definitely a question for an HVAC professional.
At this point, the best thing you can do is talk to 2+ HVAC companies about this and see what the consensus is. They usually don't have a fee to come take a look at give you a rough quote on the work they want to do.
posted by _DB_ at 1:36 PM on July 12, 2020 [6 favorites]
Also suggesting that the system may have been mis-sized. One thing we discovered is that AC systems are sized to be able to reduce the temperature a certain number of degrees compared to the outside air. Our system (which was done as part of a remodel) was sized to lower the temperature 15* based on the size of the house. When it is 98* outside, 83* inside was not what we wanted. We ended up replacing it.
Also, hot air rises and cold air falls, so if the duct work is not tight, it may be less of a problem in the winter where the air tends to go where you want it. Roof insulation and use of an attic vent can also make a difference.
posted by metahawk at 1:39 PM on July 12, 2020
Also, hot air rises and cold air falls, so if the duct work is not tight, it may be less of a problem in the winter where the air tends to go where you want it. Roof insulation and use of an attic vent can also make a difference.
posted by metahawk at 1:39 PM on July 12, 2020
What’s the temperature difference between the main floor and upstairs during the day? My house is of a similar age and I definitely expect the upstairs to be warmer when I’m running the a/c. Also, would you really want to chill the house to 65? Or would you like it a few degrees cooler upstairs? Have you done an energy audit in your house?
Is it cooling off below 70 or so at night? It takes a bit more work, but if it’s cooling off at night, I turn off the a/c, open the windows, and try to cool the upstairs off as much as possible using box fans (sometimes one is aimed out). My goal is to get as much of that warm air out at night. Then I close the windows in the morning and turn the a/c back on. Overhead fans in the upstairs can also help.
posted by bluedaisy at 2:03 PM on July 12, 2020 [2 favorites]
Is it cooling off below 70 or so at night? It takes a bit more work, but if it’s cooling off at night, I turn off the a/c, open the windows, and try to cool the upstairs off as much as possible using box fans (sometimes one is aimed out). My goal is to get as much of that warm air out at night. Then I close the windows in the morning and turn the a/c back on. Overhead fans in the upstairs can also help.
posted by bluedaisy at 2:03 PM on July 12, 2020 [2 favorites]
I'm far from an expert, but we're dealing with an AC system that doesn't work fantastically at our house too, so I've done a bit of research. I have three ideas:
- First, I think it's very possible your system is undersized. I'd have two or three other HVAC companies out to size the system. They may do the consult for free (ours did, since we told them we were thinking of replacing our AC, which is the case), or they may charge you.
- Second, we had an issue (that my husband just fixed) where there were two many bends in the ductwork, which made the air coming out of those ducts in question not powerful/cold enough. My husband just redid bits of the system to turn various right angles into gentle slopes, which has made a huge difference.
- Third, I wonder whether you have enough return vents. We also have this issue, and it means that the hot air naturally goes up, and then can't get out of the upstairs, so the upstairs is too hot. We're thinking of adding return vents. It's pretty invasive, especially if you have plaster walls (which is almost certain for a 100-year-old house, unless it's been extensively remodeled). So in the interim, we're also thinking of getting an attic fan installed, which apparently can do a great deal to get hot air out of the upstairs. We're hoping this might obviate the need for adding more return vents. One HVAC guy also suggested an upstairs minisplit might help a lot to replace cool air for warm air, if we didn't want to add in several of return vents. This is expensive ($4k+), unless you can DIY.
posted by ClaireBear at 2:06 PM on July 12, 2020 [1 favorite]
- First, I think it's very possible your system is undersized. I'd have two or three other HVAC companies out to size the system. They may do the consult for free (ours did, since we told them we were thinking of replacing our AC, which is the case), or they may charge you.
- Second, we had an issue (that my husband just fixed) where there were two many bends in the ductwork, which made the air coming out of those ducts in question not powerful/cold enough. My husband just redid bits of the system to turn various right angles into gentle slopes, which has made a huge difference.
- Third, I wonder whether you have enough return vents. We also have this issue, and it means that the hot air naturally goes up, and then can't get out of the upstairs, so the upstairs is too hot. We're thinking of adding return vents. It's pretty invasive, especially if you have plaster walls (which is almost certain for a 100-year-old house, unless it's been extensively remodeled). So in the interim, we're also thinking of getting an attic fan installed, which apparently can do a great deal to get hot air out of the upstairs. We're hoping this might obviate the need for adding more return vents. One HVAC guy also suggested an upstairs minisplit might help a lot to replace cool air for warm air, if we didn't want to add in several of return vents. This is expensive ($4k+), unless you can DIY.
posted by ClaireBear at 2:06 PM on July 12, 2020 [1 favorite]
I am assuming you are in the USA based on your temperature units.
HVAC companies should size your AC unit per "Manual J" which includes a large number of parameters of the design of your house (insulation type, number of doors, what side of the house windows are on, etc). Based on Manual J, the unit is sized such that a "1% day" will result in cooling the house to 75° with the unit running at 100%. Even though a "1% day" sounds uncommon, there are, by definition, at least of 3 of them every year (and more on some years). Thus, to a first order, your results make sense and your HVAC company will likely point out they correctly sized the unit.
Without getting into too much details, HVAC contractors will do a lot of work to avoid oversizing a unit. Oversized units will tend to operate for very short amounts of time during "normal" days, which can result in excess wear on the compressor. In addition, oversized units tend to waste energy. As a pragmatic matter, the units cost more and tend to make bids from the HVAC company look uncompetitive compared to companies that "properly" size units.
There are a number of factors that will drive the performance of the unit up or down somewhat. For instance, HVAC units are rated in "tons" of refrigeration, but are usually sold in integer number of tons. If Manual J determined your house needed, say, 3.5 tons of cooling, your HVAC company will likely be forced to choose between a 3 ton unit (and not meet the performance expectations of Model J) or a 4 ton unit (and end up with an oversized unit with associated disadvantages). You should have received some amount of Model J calculations, and you can compare them to what you received to verify. The HVAC company should have the calculations as well - they need them for HVAC unit warranty purposes. For reference, 1 ton of refrigeration is 12,000 BTU/hr of cooling in case they use different units.
If you are finding that Manual J isn't providing the expected cooling to 75°, you should check against the inputs the HVAC technician used for the calculation. Frequently, HVAC technicians will assume house construction, dimensions, or parameters that aren't actually accurate in order to save themselves some time. If an error is obvious (for instance, specifying the wrong number of square feet in the house), the HVAC company should fix it. If the error is not so obvious (for instance, mismeasuring windows by a small amount), the HVAC company is unlikely to do anything about it.
If you are looking to cooling below 75°, you will have to ask your HVAC company to deliberately oversize the unit. This is possibly easy - for instance, in the fractional case I suggested above, it's very easy for them to justify bumping up a notch or fudge a few numbers to bump the required cooling up a bit. It's also possibly difficult - the manufacturer of the unit will only warranty the unit if it's installed to Manual J's requirements, so large deviations from Manual J will result in losing the warranty of the unit.
Assuming Manual J is correctly applied, your unit was correctly installed. If you want something else, you can certainly ask for it, but it won't be the HVAC company's responsibility to change it at their cost.
posted by saeculorum at 2:11 PM on July 12, 2020 [16 favorites]
HVAC companies should size your AC unit per "Manual J" which includes a large number of parameters of the design of your house (insulation type, number of doors, what side of the house windows are on, etc). Based on Manual J, the unit is sized such that a "1% day" will result in cooling the house to 75° with the unit running at 100%. Even though a "1% day" sounds uncommon, there are, by definition, at least of 3 of them every year (and more on some years). Thus, to a first order, your results make sense and your HVAC company will likely point out they correctly sized the unit.
Without getting into too much details, HVAC contractors will do a lot of work to avoid oversizing a unit. Oversized units will tend to operate for very short amounts of time during "normal" days, which can result in excess wear on the compressor. In addition, oversized units tend to waste energy. As a pragmatic matter, the units cost more and tend to make bids from the HVAC company look uncompetitive compared to companies that "properly" size units.
There are a number of factors that will drive the performance of the unit up or down somewhat. For instance, HVAC units are rated in "tons" of refrigeration, but are usually sold in integer number of tons. If Manual J determined your house needed, say, 3.5 tons of cooling, your HVAC company will likely be forced to choose between a 3 ton unit (and not meet the performance expectations of Model J) or a 4 ton unit (and end up with an oversized unit with associated disadvantages). You should have received some amount of Model J calculations, and you can compare them to what you received to verify. The HVAC company should have the calculations as well - they need them for HVAC unit warranty purposes. For reference, 1 ton of refrigeration is 12,000 BTU/hr of cooling in case they use different units.
If you are finding that Manual J isn't providing the expected cooling to 75°, you should check against the inputs the HVAC technician used for the calculation. Frequently, HVAC technicians will assume house construction, dimensions, or parameters that aren't actually accurate in order to save themselves some time. If an error is obvious (for instance, specifying the wrong number of square feet in the house), the HVAC company should fix it. If the error is not so obvious (for instance, mismeasuring windows by a small amount), the HVAC company is unlikely to do anything about it.
If you are looking to cooling below 75°, you will have to ask your HVAC company to deliberately oversize the unit. This is possibly easy - for instance, in the fractional case I suggested above, it's very easy for them to justify bumping up a notch or fudge a few numbers to bump the required cooling up a bit. It's also possibly difficult - the manufacturer of the unit will only warranty the unit if it's installed to Manual J's requirements, so large deviations from Manual J will result in losing the warranty of the unit.
Assuming Manual J is correctly applied, your unit was correctly installed. If you want something else, you can certainly ask for it, but it won't be the HVAC company's responsibility to change it at their cost.
posted by saeculorum at 2:11 PM on July 12, 2020 [16 favorites]
I assume the ductwork was installed as a refit for forced-air heating. If so, it may be undersized for AC. When the AC is running, is there a strong airflow blowing out of the vents? Is the airflow much weaker upstairs? If the ducts are undersized, this may be a very expensive thing to fix.
posted by Kirth Gerson at 2:46 PM on July 12, 2020
posted by Kirth Gerson at 2:46 PM on July 12, 2020
I'm suspicious of your insulation, particularly attic/second floor generally.
Having said that, there are some kludges that might help a bit... For one, you can buy powered vents that will try to pull more conditioned air out of your vents. They're not expensive, they don't always work for every application, but it's probably worth getting one and trying it in a second floor room. Here's one example of the type.
It is also possible to install inline booster fans into the ductwork, assuming you have access to the ducts at any point without cutting holes in your walls etc. Some of these run continuously, others are more sophisticated with speed controllers etc. Your HVAC person(s) should have opinions on inline boosters. I don't like the kind that run continuously, because I don't want them pulling un-conditioned air through the system when the main AC isn't running.
Second-to-last, you probably don't want to do this but if you end up surrendering to Fate, there are high-speed AC systems that use much smaller ducts for retrofitting into old houses. A big name is SpacePak.
Lastly, and something of a tangent, if you have an attic have you considered a whole-house fan? If your climate is right these can be amazingly effective -- I use mine at night when it cools off (pulling outside air throughout the house, venting into the attic and thereby also blasting the daytime hot air out of my attic space) and then again in the morning (to saturate the house with 65F outside air before the day starts). These devices all require certain amounts of attic venting and you may need to install more -- if you don't have enough venting the air pressure they generate will do crazy things like push vent caps off your roof vents, force recessed lights out of their sockets, etc. On a mid-90s day my AC (set for 76F) won't turn on at all until about 3-4pm because my house fan ran in the morning.
posted by aramaic at 3:05 PM on July 12, 2020 [3 favorites]
Having said that, there are some kludges that might help a bit... For one, you can buy powered vents that will try to pull more conditioned air out of your vents. They're not expensive, they don't always work for every application, but it's probably worth getting one and trying it in a second floor room. Here's one example of the type.
It is also possible to install inline booster fans into the ductwork, assuming you have access to the ducts at any point without cutting holes in your walls etc. Some of these run continuously, others are more sophisticated with speed controllers etc. Your HVAC person(s) should have opinions on inline boosters. I don't like the kind that run continuously, because I don't want them pulling un-conditioned air through the system when the main AC isn't running.
Second-to-last, you probably don't want to do this but if you end up surrendering to Fate, there are high-speed AC systems that use much smaller ducts for retrofitting into old houses. A big name is SpacePak.
Lastly, and something of a tangent, if you have an attic have you considered a whole-house fan? If your climate is right these can be amazingly effective -- I use mine at night when it cools off (pulling outside air throughout the house, venting into the attic and thereby also blasting the daytime hot air out of my attic space) and then again in the morning (to saturate the house with 65F outside air before the day starts). These devices all require certain amounts of attic venting and you may need to install more -- if you don't have enough venting the air pressure they generate will do crazy things like push vent caps off your roof vents, force recessed lights out of their sockets, etc. On a mid-90s day my AC (set for 76F) won't turn on at all until about 3-4pm because my house fan ran in the morning.
posted by aramaic at 3:05 PM on July 12, 2020 [3 favorites]
Response by poster: Thank you to all who have weighed in! To answer some of the questions: the house is approx 1450sq ft including the basement. The style of the house is ‘barn roof’ where the shingles go over down over the second floor and the attic space is very small, we have no access to it.
The second floor runs maybe 8 degrees hotter than the main floor. The walls are drywall throughout, which may prove helpful if we add vents or an additional cold air return.
I would describe the airflow as moderate, sometimes weak from the vents when the AC is running, and the air is not cold, it is more so cool.
Thanks again
posted by walkinginsunshine at 4:29 PM on July 12, 2020
The second floor runs maybe 8 degrees hotter than the main floor. The walls are drywall throughout, which may prove helpful if we add vents or an additional cold air return.
I would describe the airflow as moderate, sometimes weak from the vents when the AC is running, and the air is not cold, it is more so cool.
Thanks again
posted by walkinginsunshine at 4:29 PM on July 12, 2020
I’m also not an AC expert in any way. But I can tell you from my own experience growing up in a 2 story house that definitely wasn’t a 100 years old...the downstairs was always noticeabley cooler and the upstairs could get stuffy. From what I hear 2 story houses can be more challenging to cool the upstairs. Some people recommend haven’t different zones. But again I’m not an expert. Might be worth getting a 2nd opinion from a different AC company.
posted by ljs30 at 5:39 PM on July 12, 2020 [1 favorite]
posted by ljs30 at 5:39 PM on July 12, 2020 [1 favorite]
TL;DR - The techs are probably correct.
I had similar issues in my old house. In my case the AC unit was oversized yet ineffective.
The issues were undersized returns and poorly positioned vents.
The "Manual J" was designed to be calculated by grade school educated adults and I had no trouble crunching the numbers myself. I've listed the manuals below. You can search for the name and .pdf to download them.
Manual J - Load calculation
Manual D - duct design
Manual S - Equipment selection
In my case I sealed the attic, windows, doors, and crawlspaces, redesigned the ducts using the above docs, added air returns to the system on the top floor using the doc, and I sealed the existing duct work.
All this was done using free online tools and stuff from Home Depot. It had a side benefit of reducing my AC bill from $130 to $30/month.
Some search terms that will help:
"HVAC air leak"
"Attic air leak"
"HVAC manual J calculation"
posted by pdoege at 6:03 PM on July 12, 2020 [5 favorites]
I had similar issues in my old house. In my case the AC unit was oversized yet ineffective.
The issues were undersized returns and poorly positioned vents.
The "Manual J" was designed to be calculated by grade school educated adults and I had no trouble crunching the numbers myself. I've listed the manuals below. You can search for the name and .pdf to download them.
Manual J - Load calculation
Manual D - duct design
Manual S - Equipment selection
In my case I sealed the attic, windows, doors, and crawlspaces, redesigned the ducts using the above docs, added air returns to the system on the top floor using the doc, and I sealed the existing duct work.
All this was done using free online tools and stuff from Home Depot. It had a side benefit of reducing my AC bill from $130 to $30/month.
Some search terms that will help:
"HVAC air leak"
"Attic air leak"
"HVAC manual J calculation"
posted by pdoege at 6:03 PM on July 12, 2020 [5 favorites]
Insulation is likely the issue, plus heat rising. My 3-story townhouse is always warmer on the 3rd floor because the home isn’t set up properly — it should’ve been set up with distinct units, or more effective zoning, but it wasn’t.
That said, we don’t struggle to get the upstairs cool enough AT ALL and we live in Houston Goddamn Texas, where today it was 98 with a heat index north of 110.
So yes: Insulation is hurting you.
Others have mentioned the “can only cool X degrees” thing, and this is indeed a thing, but remember that “outside minus X” doesn’t therefore represent the best case for your system because your AC system intakes are inside your house. Here in Houston, we hear 20 degrees (F) as the usual delta, which means the system can at best cool 100F air to 80F — but it doesn’t really have to do that unless the system has been off. It takes its best stab at cooling, and cycles the air through, and then gets a little lower, etc.
It is not difficult in our home to get to 72 or even 70 if we want to. We don’t, because that’s just not what folks usually do down here — we usually set for 76 or 78 unless you’re in a super small place where cooling much lower isn’t ridiculous.
Based on OP’s update I might seek a second opinion from an HVAC person your local friends pronounce as trustworthy.
posted by uberchet at 6:06 PM on July 12, 2020 [2 favorites]
That said, we don’t struggle to get the upstairs cool enough AT ALL and we live in Houston Goddamn Texas, where today it was 98 with a heat index north of 110.
So yes: Insulation is hurting you.
Others have mentioned the “can only cool X degrees” thing, and this is indeed a thing, but remember that “outside minus X” doesn’t therefore represent the best case for your system because your AC system intakes are inside your house. Here in Houston, we hear 20 degrees (F) as the usual delta, which means the system can at best cool 100F air to 80F — but it doesn’t really have to do that unless the system has been off. It takes its best stab at cooling, and cycles the air through, and then gets a little lower, etc.
It is not difficult in our home to get to 72 or even 70 if we want to. We don’t, because that’s just not what folks usually do down here — we usually set for 76 or 78 unless you’re in a super small place where cooling much lower isn’t ridiculous.
Based on OP’s update I might seek a second opinion from an HVAC person your local friends pronounce as trustworthy.
posted by uberchet at 6:06 PM on July 12, 2020 [2 favorites]
"the down 20F thing" isn't correct at all. An A/C can cool a house to an insane degree if it's well sealed and the system runs for a while. Think a refrigerator. Think an office building. My office building cools to about 62F, which feels like freezing. You can buy an infrared thermometer at a home improvement store for like $20, shoot that around and check your air temp coming out of the ducts, probably in the 50-60F range - if your house was completely sealed - that's how cold it could get. Then take the temp for the roof, the walls, and windows in various places on a hot day. If it's hot near the ceiling, then you need more insulation. If the walls are hot then more wall insulation etc. On the interior they should be pretty close to the ambient temperature of the inside of the house.
BTW, 1400 sq ft between multiple stories sounds like the roof footprint is pretty small. Even if you can't access the attic, a company can come in and put foam insulation on your ceiling for maybe $3k, and maybe another $1500 or so to fix the ceiling drywall. Extra insulation has a super short payback period - it's one of the best investments in your home you can make.
Double BTW, you don't an oversized system because that will run for too short and your house will be really humid, while also wasting a lot of electricity, and shortening the life of the unit. 1 duct per bedroom is also normal.
posted by The_Vegetables at 9:25 PM on July 12, 2020 [2 favorites]
BTW, 1400 sq ft between multiple stories sounds like the roof footprint is pretty small. Even if you can't access the attic, a company can come in and put foam insulation on your ceiling for maybe $3k, and maybe another $1500 or so to fix the ceiling drywall. Extra insulation has a super short payback period - it's one of the best investments in your home you can make.
Double BTW, you don't an oversized system because that will run for too short and your house will be really humid, while also wasting a lot of electricity, and shortening the life of the unit. 1 duct per bedroom is also normal.
posted by The_Vegetables at 9:25 PM on July 12, 2020 [2 favorites]
One thing you can do right now that isn't very expensive and makes a huge difference is putting blackout curtains on your windows (and keeping blinds closed). I know it can be depressing and feel like you're living in a cave, but that is a little less work that your AC has to do. If you want to level THAT up, you can put a film on the windows, close the blinds, AND put up blackout curtains, which is what I've done on my west-facing sliding glass door.
posted by fiercecupcake at 10:24 AM on July 13, 2020 [1 favorite]
posted by fiercecupcake at 10:24 AM on July 13, 2020 [1 favorite]
This thread is closed to new comments.
Do you know the square footage of what you are trying to cool (including the basement) This website has a calculator you could see if it compares to what you bought. This is the first one I found, i really can't vouch for the accuracy.
posted by AlexiaSky at 1:26 PM on July 12, 2020 [1 favorite]