Home>Discussions>INSULATION & HVAC>My second floor central AC unit in the attic does not cool sufficiently in the daytim
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Daveh839
My second floor central AC unit in the attic does not cool sufficiently in the daytim

At night the outlet registers blow air between 60 - 64 degrees, the return register air leaves at about 76. During the day the outlets blow only 72 - 75 degrees, with the air exiting into the return register at about 78. The overall upstairs of the house is approximately 50 x 25 and I have a 3 ton unit. The evaporator coil and condenser were replaced last fall. By my calculations the unit requires 1200cfm on the returns, and I have 4 return registers in different rooms running 6" return flex duct. From some web sites it sounds like that is not sufficient to achieve the 1200cfm requirements of a 3 ton system, but the flow into the rooms doesn't seem bad. Would inadequate return flow cause the symptom of increased temperature of the air coming from the outlets during the day? Everything seems insulated in the attic, what could be causing the attic temperature to have such a profound effect on the temperature of the outlet airflow into the rooms?

Many thanks for any advice,

Dave

dcalabro
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

Obviously attic temperature will be a lot hotter during the day and decrease the efficiency of the unit.

It sounds like your ducts might not be insultated and is more easily being effected by the hotter temperature during the day.

If you already have insulation around the ducts maybe you should add some more. If you have ducts running inside exterior walls, the extra heat can also be rising the temperature but i'm not sure of any workarounds for that since theyre already in the wall.

Daveh839
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

Thank you - I do have ducts in exterior walls, and the runs are also pretty long in the attic (in excess of 25' in several cases). I have a contractor who said that because I changed the AC condensor and coil from 10 seer to 13 seer last fall that I should increase my return capacity to blow more air across the 13 seer coil. Does that make sense in that it would lower the air temperature of the air coming into the rooms?

Thanks again,

Dave

Sten
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

It is very important that you have at least as much return air as supply, you can never have to much return air. I don't know where your returns are, but you need more !! As for the attic temp. , it should have no bearing on your supply air if your main trunk is properly taped, sealed and, insulated. Your 25 foot flex runs are way to long, 10' max. and they must have no dips and be as straight as poss. I don't know the layout of your house but it sounds like you have to have your main trunk worked on to decrease your supply length.

ZZZ
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

If your a/c is working properly you will have about a 17 degree temperature drop across the cooling coil. You have this number at night, as is seen by subtracting the supply temp. from the return temp. In the daytime you will still have the same temp. drop but it is picking up additional heat through the ductwork and your net difference is only 3-5 degrees. This will not cool. Four six" round ducts have a total area of 113 square inches. If that is your total return area. you are about 66% short. Install an attic fan and larger returns.

Sten
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

As for the attic temp. , it should have no bearing on your supply air if your main trunk is properly taped, sealed and, insulated.:)

Sten
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

Without doing a Load Calc. because I know nothing of your house your 4 6" returns are pulling back between 300-400 CFMs. Your 3 ton should be 1200 CFMs as you say, you need at least 900 more CFMs. Have a Qualified Co. put in 4 9" returns or 5 8" returns, choose the warmest area in your house to pull the most heat back. If you do not have your 25' flex runs fixed at least pull them tight with no sags. It may not seem like it matters but the spring inside will cause turbulence and you want the air to be smooth. There have been studies done on the correct way to install Flex, length, etc. Airflow is key, if your not going to fix your 25' supplies, at least add the returns, it will make all the diff. in the world. I have no idea how ZZZZ came up with the duct work picking up 3-5 degrees when he doesn't know your Attic Temp. Once again if all is properly taped, sealed and, insulated and you add the returns you should be fine. I doubt you need an Attic Fan.

Bob Gabrilson
Re: My second floor central AC unit in the attic does not cool sufficiently in the da

It's another poorly designed system.
Everything is wrong.
It's so sad.
Does it take THAT much longer to do the job right?

Sten
Re: My second floor central AC unit in the attic does not cool sufficiently in the da
Bob Gabrilson wrote:

It's another poorly designed system.
Everything is wrong.
It's so sad.
Does it take THAT much longer to do the job right?

No it doesn't and your right it is sad.

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