Question about cooling loss due to hot attic

[Brad C]

I have a 2 story home with basement and my 2nd floor is not cooling like it did last summer. This summer isn't really any hotter than last year, but my 2nd floor unit runs a lot during the day, and the thermostats are set to the same values as last summer.

I have a few thermostats/hygrometers I use around the house and taped them to the return/supplies in the master bedroom to check the temp difference. When the attic was around 110 on a hot day, the supply was blowing in air at 68-69 and the return was taking in air at 77. I had a technician come out to inspect and he said the 8-9 degree difference was due to the heat loss in the runs in the attic. He quoted some degree per foot run loss, but I don't fully buy it. He also said they shoot for a 10-12 degree diff for 2nd floors and 15-17 diff for 1st floors.

Today, I measured the temp diff around 8am when the attic temp was 74. Supply was 64, return was 69. I have a 2.5 ton unit for the 2nd floor. I had let the unit run for a half hour to make sure the air was at its coolest. For the downstairs, I had 57 from the supply, with 73 going in the return. The basement temp was 75, so the ducts for the 1st floor, which are in the basement, were roughly experiencing the same amount of heat loss as those in the attic ( attic was at 74 ). The 1st floor unit is a 3 ton.

The house is almost 2 years old. The company that did the install performed a flush/recharge on both units for free since I'm still under warranty, and that didn't help.

So, I experienced a 5 degree diff when the attic temp was at 74. The system was charged to the recommended level. The technician came out 3 days after he flushed/recharged and checked the system again.

The airflow seems to be fine. Any thoughts ?

[Joe Fabeitz]

Are the ducts in the attic insulated? How well? Two years ago I bought a similar house (twenty years old) and immediately replaced both HVAC systems and all of the attic duct work. Although the new systems kept up with Atlanta summers, I chose to also add "radiant" insulation to the attic. The result is the attic temp. is never more than 12 degrees hotter that the outside temp. Something to consider.

it did last summer. This summer isn't really

day, and the thermostats are set to the same

to the return/supplies in the master bedroom

the supply was blowing in air at 68-69 and the

he said the 8-9 degree difference was due to

loss, but I don't fully buy it. He also said

Supply was 64, return was 69. I have a 2.5 ton

sure the air was at its coolest.

The basement temp was 75, so the ducts for

same amount of heat loss as those in the attic

performed a flush/recharge on both units for free

system was charged to the recommended level. The

[Brad C]

I live in Smyrna, so I'm also getting the Atlanta summer.

The house was built in October 2002. It has ridge vents running the full length of it. All ducts are insulated. There is blown insulation in the attic. The return ducts are on the attic floor and get some covering from the blown insulationas well. The supply ducts are suspended overhead in the attic, so they only have the standard insulation around the duct.

The high temp in the attic is definitely causing cooling loss during the day, but the thing that concerns me is during the morning, when I took the last set of measurements, the attic was only 74 degrees. The supply was 64 and the return was 69, so the unit was dropping the temp 5 degrees with an attic temp of 74. When the attic temp was 110, it was dropping the temp 7-8 degrees. This would seem to indicate that the unit can't cool air beyond 64 degrees. That seems like a problem attributed to something other than cooling loss thru the ducts since the unit on the 1st floor drops it

17 degrees under pretty much the same conditions. The difference between the two units is upstairs is 2.5 ton and downstairs is a 3 ton. That shouldn't cause the problem I'm seeing.

I didn't have this problem last summer. I have programmable thermostats set to the same schedule as last year. My power usage this year for May increased 76%, June was 78%, July was 21%, August was 21%. July and August didn't increase as much as May and June, but that is because the AC is running a lot during those last 2 summer months anyway, so there wasn't really much more it could run. We started using the AC in mid April of this year. It's usage was 16% higher. The months where we used heat only had an increase in usage of only 1%~2%. AC is running a lot more this year, and the summer this year isn't really hotter than last year's.

The easiest thing to quantify out of all these numbers is that I know the upstairs runs a lot more. The fact that it only drops the temp from 5-8 degrees upstars between return and supply is proof.

[Zypher]

Brad;

It seems to me, the upstairs unit is not performing well. You should expect approximately 15º - 20º temperature difference between return temperature & supply temperature (similar to what you're finding for the downstairs unit.) Albeit the attic ducts do add some heat, the insulation commonly has a rating of R-4.2 per foot and adds some degree of temperature (or loss) to the air, but shouldn't influence the performance. The contractor should have taken all of that in to account when the system was designed.

Bottom line, you might have either one of two problems, (a best guess from your description) a leaking refrigeration (leak) system loosing refrigerant, or a compressor that has lost the ability to pump (broken suction valves.) I am guessing you're changing your filters regularly. But I'd need more information to really tell. (Also, you could have a breach (opening) in the return duct causing the air handler (furnace) to be drawing in hot attic air, which would hit performance dramatically.)

[Brad C]

There is no refrigerant leak. They did perform a flush/recharge of the refrigerant because the pressure was fluctuating from 55 - 70. After the flush/recharge, they added a little more refrigerant to get it to the optimal pressure. Fortunately, they are doing all this under warranty. They checked the compressor and said there was nothing wrong.

I just checked the returns for leaks and couldn't detect anything. If there were leaks, that wouldn't explain why the unit doesn't cool the air more in the early morning when the attic is between 70 and

73 degrees. As you said, it would have a major effect when the attic is hot during the day.

I change the filters regularly. Downstairs is changed every 2 months, upstairs every

3-4 months. I've never pulled a dirty filter out at those intervals, they are slightly discolored. I check them every month, and they never need to be changed that often. If there was a leak in the return, I would probably need to change them more often, you agree ?

[Mike Fritz]

the thing that concerns me is during

degrees. The supply was 64 and the return

the attic temp was 110, it was

air beyond 64 degrees. That seems

the unit on the 1st floor drops it

units is upstairs is 2.5 ton and

I don't know if this is your problem or not, but you are forgetting a major part of your equation. The condenser. If the condenser cannot remove the heat that's being removed from the house, the system will not work as efficiently. What shape is this particular condenser in?

--Mike

[HeatMan]

refrigerant because the pressure was fluctuating

refrigerant to get it to the optimal pressure.

A flush/recharge? sounds like the installers didn't do the job right to start with. If the pressures were flucuating, that usually means you had non-condensibles in the system. Did the evacuate the system after recovery and installation if a filter/dryer in the refrigerant lines?

compressor and said there was nothing wrong.

there were leaks, that wouldn't explain why the

between 70 and 73 degrees. As you said, it would

upstairs every 3-4 months. I've never pulled a

them every month, and they never need to be

need to change them more often, you agree ?

What kind of filters are you using? Has the company checked blower speeds and how clean the blower wheel/evap coil is?

[Brad C]

I had 2 different techs come out for a total of 3 trips.

1st tech came out the first trip only, he recommended a recovery/recharge on the system because he thought that non condensables were causing the pressure fluctuation ( air ). He also said that air is basically moisture in the system, so a dryer would need to be installed to get rid of all moisture. He gave me an estimate of $480 for the whole process.

I called them back the next day ( after calling other companies to get their take on it ) and said that since the house was only 1.5 years old, and they did the install, they should fix this problem under warranty. The tech had said there were no leaks in the system, so that meant the install was not properly done. They agreed, and sent another tech out to do the recovery/recharge on my upstairs and downstairs units since both were fluctuating in pressure. Downstairs was about 5-8 PSI ( it was working fine ), the upstairs was around 15 PSI. The tech finished and I asked if he put on a dryer, he said he didn't need to b/c the previous tech indicated there was garbage in the refrigerant system, not moisture.

After a day or so, I measured the temps again and both units were blowing out air 2-3 degrees warmer than before. So now, both units are worse off. I called them back and tech #2 came out again. He checked the pressures said he needed to top off the refrigerant since the pressures were just below the optimal value. So he did, now my downstairs unit works fine like before, but the upstairs is still not cooling well.

I am using the standard filters you can get at just about any store. They are checked regularly, and the upstairs is hardly ever dirty, so it isn't changed every month. The tech looked at the filter that had been used for the last 2 months and said it was fine.

I don't know if they checked blower speeds, the air flow seems to be fine from all supplies. If you mean the blower fan in the condenser, a tech checked the amperage it was using and said it was fine.

The condenser fins are slightly damaged on both units from a hailstorm. This happened before last summer, so the units performed fine last summer with the damage. Both condensers are clean as far as I can tell from the outside.

[Brad C]

The condenser fins are slightly damaged on both units from a hailstorm. This happened before last summer, so the units performed fine last summer with the damage. Both condensers are clean as far as I can tell from the outside.