Upstairs AC unit can't keep up on these hot days

jcramsey

Well-Known Member
Joined
Jul 22, 2009
Location
Marion, NC
Upstairs AC in our new house cant keep up on hot days. I assume I'm fighting physics and all the cool air is just going down the stairs, because downstairs is always comfy even when the thermostats say the temps are the same or at least pretty close. Anybody else have/had the same struggle? Would a door at the top of the stairs help? Only have thin curtains so nothing that really blocks any heat gain from those so I've been thinking of getting some blackout shades to see if they help. I think its 680 sq. ft with a 1.5 ton unit.
 
Have you used a temperature gun and an anemometer (air flow meter) to measure what's happening at the vents?. Cheap versions of both would be fine.
Its not uncommon for there to be leaks in the plenums or they aren't properly sized so that the more distant areas just aren't getting the right volume of cooled air.
I was shocked at the difference it made when I went around and sealed everything up.
 
Ours struggles when we get above 85-90 outside. It is about 12 years old. Yesterday upstairs was barely maintaining 75 and downstairs was at 72 easy all day long. Our windows are all 26 years old original builder grade and I'm sure all the gas is probably gone between the double panes. Top of the stairs is also the hottest place in the house but we have a whole house attic fan that is uninsulated at the top of the stairs also. I also think a contributing factor is the blown in insulation has settled too. I don't remember the problem when we moved in and we had R22 refrigerant IIRC which was a less efficient system back then. I never considered seeing how well the ducts were sealed or not I also had a gable vent fan that kicked on at 120° but has burnt out and I never replaced it. It all contributes and the family complains about it, but when it cools off in the evening it'll drop to 66 for sleeping. I don't spend much time up there until we go to bed so I'm not as concerned but I probably should be. because it is running our power bill up.
 
Is air handler and ducting in the attic?

If so a lot of your loss can be coming from attic heat warming the air in the duct work prior to it reaching the vents.

Venting or conditioning the attic space can help here as can better insulating the duct. Each has their own related consequences that have to be considered relating to attic moisture and such.
 
Ron said:
Is air handler and ducting in the attic?

If so a lot of your loss can be coming from attic heat warming the air in the duct work prior to it reaching the vents.

Venting or conditioning the attic space can help here as can better insulating the duct. Each has their own related consequences that have to be considered relating to attic moisture and such.
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Yea the handler and ducting are in the attic. There's a lot of roof and it definitely gets toasty in there. Just have ridge vents but I have wondered if some sort of fan would help pull the hot air out.
 
Go hose down the coils in the unit outside so they're clean. And if you get real desperate rig up a means to drip water over the coils continuously when it's hot. Water out the hose is pretty cheap and your lawn probably needs watering anyway
 
serviced the unit? Wash the outdoor unit with water and coil cleaner.
 
House was finished last summer and unit was brand new and has always struggled to keep up when temps are high. @Ron 's theory that the air is getting heated up makes sense.
 
Attic fans or increased ventilation otherwise will help keep the attic cooler and help keep from the duct picking up as much heat.

I would take a look at the duct work sooner than later to make sure its sealed, especially the return side. If your unit is drawing in attic air (could be 120 plus) that's not good.
 
jcramsey said:
House was finished last summer and unit was brand new and has always struggled to keep up when temps are high. @Ron 's theory that the air is getting heated up makes sense.
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If everything is new, then its a design problem. Very likely those attic temps are getting you.

If only there was some way to diagnose whether the temp at the vents was proper or not....
 
RatLabGuy said:
If everything is new, then its a design problem. Very likely those attic temps are getting you.

If only there was some way to diagnose whether the temp at the vents was proper or not....
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RatLabGuy said:
If everything is new, then its a design problem. Very likely those attic temps are getting you.
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That assumes it was installed properly.

A leak in either the supply or return ducting could cause the same issues.

Ive seen homes with the blown in insulation all the way out to and covering soffit vents. Ive seen ridge vent caps covered with plywood all the way with no gap.

Or..it could be the upstairs unit was never purged properly and isnt cooling completely.

You are right though, Id start with a temp gun reading of the ducts. Then if its low diagnose why its low. Next you would want to measure the air velocity and volume. A supply side leak will likely show up as low velocity and high temp.. A return side leak or coolant issue will have adequate velocity but high temp.

With an open stairwell some air will migrate to lower levels, but it is often met/combatted by rising higher temp air from the first floor. I would put air migration as suggested in original question lowest on the list.

It's unlikely that a new home was built and the entire system was designed inadequately. Most areas require a manual J calcs at least to be submitted along with any new building permit.
Thee actual design should have J,S,T and D calcs done. But I'm out of my element there other than knowing they are important. At that point I just call in te experts like @CasterTroy ...who I think OP met once or twice ;)
 
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Hey look for 15 bucks OP could have had this diagnosed before this thread even finishes rambling

Although I find laser thermometers really useful, so he'd have to skip another half a pizza. I've used one of these for years
 
Ron said:
It's unlikely that a new home was built and the entire system was designed inadequately. Most areas require a manual J calcs at least to be submitted along with any new building permit.
Thee actual design should have J,S,T and D calcs done. But I'm out of my element there other than knowing they are important. At that point I just call in te experts like @CasterTroy ...who I think OP met once or twice ;)
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When I said bad design I meant dumb shit like running the lines through the attic but not properly blocking them from the heat or having proper attic ventilation to not cause problems.
I'm just a dumb neuroscientist and @CasterTroy will correct me when I'm wrong but those calculations don't prevent that kind of thing.

But even the best designs are easily counteracted by sloppy installers anyway.
 
Attic insulation or venting as said. Radiant heat is killing your coolness.
 
RatLabGuy said:
Hey look for 15 bucks OP could have had this diagnosed before this thread even finishes rambling

Although I find laser thermometers really useful, so he'd have to skip another half a pizza. I've used one of these for years
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More data would be helpful, but it doesn't tell you why the temp at the duct is what it is.

Ron said:
Most areas require a manual J calcs at least to be submitted along with any new building permit.
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Strong argument it's GIGO.
 
@Curtis_H could offer a great deal more expertise here than I ever could in a residential situation.

To me, What it SOUNDS like is a few issues; design, installation and/or envelope issue.

Today is one of those "design days" where we're at optimum humidity and optimum heat design for our area. When sizing a unit, you don't necessarily throw ALL the cooling BTU's you can at it for those few days, because 95% of the other days of the year, you don't want the unit cycling constantly (kills a unit)
Unfortunately that means for 12 days a year you're not going to achieve 69° at 4pm.

However, you can't run a unit in 130° attic with sub par package insulation and running 50ft of flex and expect 45°-55° discharge air.
 
All good suggestions and info. I’ll hurry up and get a temp gun before @RatLabGuy has a stroke.

I actually brought this issue up with the HVAC company after they installed and I noticed upstairs wasn’t cooling too well and was told “That’s just the nature of an upstairs system, I’m dealing with the same thing at my house”. Looks like I should have pushed back harder on that. :rolleyes:

@Curtis_H as soon as them crab legs go on sale at Food Lion I’m gonna invite you up for dinner and let you take a look lol
 
jcramsey said:
All good suggestions and info. I’ll hurry up and get a temp gun before @RatLabGuy has a stroke.

I actually brought this issue up with the HVAC company after they installed and I noticed upstairs wasn’t cooling too well and was told “That’s just the nature of an upstairs system, I’m dealing with the same thing at my house”. Looks like I should have pushed back harder on that. :rolleyes:

@Curtis_H as soon as them crab legs go on sale at Food Lion I’m gonna invite you up for dinner and let you take a look lol
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That's a lazy answer. I got the same response when a new unit was installed upstairs at our church. It never could make the upstairs comfortable. They came out a few times and gave it a hasty once over and told me it was what it was. I got lucky and happened to be beside it one afternoon when it cycled off for no apparent reason, the thermostat was definitely not satisfied. Got them back out and they finally realized that there was a leak in the brand new evaporator.

Edit: my upstairs is almost the exact same sqft as yours, maybe a little more, and I have a 15k btu mini split up there that can have it comfortable up there from a dead stop in 30mins. We only turn it on when we go up there and my kids and wife watched a movie up there yesterday afternoon and my wife is only comfortable if it's below 75.
 
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We have ~1000sqft upstairs and 1.5 ton heat pump and it has no problem keeping it cool. Our stairwell is completely open with hallway running parallel so there’s a lot of mixing and down flow.
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The vents are in the ceiling, and the unit is in the attic, but the attic is spray foamed and stays around 80-85 degrees so not much different than the conditioned space.

If your upstairs vents are in the floor, that is probably a significant contributor to the problem. On our garage apartment, the vents are in the ceiling, and it is terrible at heating, but great at cooling.
 
jcramsey said:
I actually brought this issue up with the HVAC company after they installed and I noticed upstairs wasn’t cooling too well and was told “That’s just the nature of an upstairs system, I’m dealing with the same thing at my house”.
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That is bullshit. I would ask around in your area and hire a competent HVAC company to come figure out what is wrong.
 
^Is right. If the guy is telling you he has the same problem at home, then maybe HE is the problem 🤷‍♂️ The main purpose of a second unit is to properly match it to the secondary space so it can effectively heat and cool it. Otherwise it would be simpler (and cheaper) to just use a 4 ton unit instead of a 2.5 ton and a 1.5 ton (or whatever sizes yours are). I'm sure in the summer the 1.5 ton upstairs works harder than the 2.5 ton downstairs, and in the winter the 2.5 downstairs works harder than the 1.5 ton upstairs, because of natural convection. But overall the system is able to balance that instead of running one unit hard and overcooling or overheating one area.
 
When you want to get serious about your HVAC systems, call on Harry Boody. A name you can't forget! Energy Innovations | HVAC Design

I've never dealt with him, nor seen his work, nor know anything about him other than referrals. I can't afford his work, I just always have that name pop up in my head when someone mentions HVAC efficiencies. He caught some heat (no pun intended) a while back for not having an engineering degree to do what he does. But his designs are proven. Granted it may be a bit late since your house is already built. He designs the system along with the construction. One of the biggest factors is how air tight your house is built.

"That's just the nature of an upstairs system" is a massive cop out.
 
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