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New HomeOwner with Central A/C - Is this condition normal?


New to central A/C. Man it’s nice!

I have two zones, up and down.

Here is the situation. I noticed the upstairs was a bit warm air at the registers and the air not as cool. I understand it’s the 2nd floor but it was quite a bit warmer.

Here is what I found after checking around:

Outside temp about 90 degrees

I went out to check the condensers. They were both running.

Downstairs one, the cold line had sweat on it like it was cold, I touched it and it was nice and cold.

Upstairs Unit, was ‘cool-ish’, no sweat

Went inside and got my IR temp gun and measured temps coming in through the wall in the basement. (pulled the insulation on the pipe back a bit)

Downstairs unit, temp of pipe was 49 degrees

Upstairs unit, temp of pipe was 72 degrees

Here are my questions, which I would appreciate some help with.

I assume both temps at the outlets of the condenser unit should be the same temp? eg. 49 degrees?

What would cause this problem? I have the service person coming tomorrow and want to be able to understand what he is talking about.

Any other tips for getting lots of mileage out of the system?

Thanks in advance for your help!



  • Techman
    Techman Member Posts: 2,144
    edited June 2013
    Central AC

    Is this new new? Or is this new to you? Meaning how old are the AC's? Either way it is not normal. Your temp readings of the suction line's should be pretty close to each other. There are a few possibilities for your problem.
  • Cast Iron
    Cast Iron Member Posts: 35
    New HomeOwner with Central A/C - Is this condition normal?

    It's new to me. Both units are about 10 years old. We'll see what they say tomorrow.
  • John Mills_5
    John Mills_5 Member Posts: 935
    Something's not right upstairs

    They won't be identical but they should be a lot closer.

    Every system will be slightly different even same model in same house. Odds are upstairs your temps are warmer, especially if attic heat penetrates the return duct. Also return air temps upstairs will be warmer. So wouldn't be unusual for the superheat to be higher upstairs.

    In a higher efficiency system, the suction line doesn't run "beer can cold" like the old days. 49° could mean an overcharge or could be right if hot out, cool in.

    72° under the right circumstances can be right too. Only way to know is a GOOD tech that knows how to charge units check them out carefully. Upstairs could be low on refrigerant or have a throttled back TXV.
  • Eugene Silberstein_2
    Eugene Silberstein_2 Member Posts: 349
    Number of possibilities

    Although the most obvious possibility is a system undercharge, there are other potential problems with the system. These include liquid line restrictions and deficiencies in the return air stream.

    If there is a liquid line restriction, or underfed evaporator, the cooling coil will not be fed enough refrigerant and the system will not provide adequate cooling. The operating pressures will be low and the evaporator superheat will be high.

    If there is a damaged return duct, which could be the result of improper duct sealing or someone stepping on it, some of the air in the attic will get pulled into the return duct along with the air from the occupied space. This increase the temperature of the air that the cooling coil sees. In this case, the coil will cool the air down, but not to the point that the house will be cool. Here's an example. If the house is 80 degrees and the temperature of the air in the attic is 120 degrees, a damaged return duct can cause this warner air to mix with the air in the space, resulting in, let's say, a 90 degree return air stream at the air handler. If the cooling coil pulls 20 degrees off the air, the supply air will be in the 70 degree range. This 70 degree air will not be sufficient to cool the space.

    So, the bottom line is, have your service company come out, check the system and determine where the problem lies with your system.

    Please keep us posted.
  • Cast Iron
    Cast Iron Member Posts: 35
    The solution


    Thanks all for your input! I appreciate it. I had a good experience with this website on the Steam side, but now I bought a new house and had to give up the radiators... :-(. But I gained A/C.

    The tech found that there was a low charge on the system and added about 18 oz of refrigerant. To me that suggests a leak, but he said at this point it wasn't worth the effort to do leak detection. We'll see if it holds, if it looses it again, we might have a bigger problem. But right now, it's back to being cold again, air vent outlet temp is about 59-60 degrees vs.72.

    thanks everyone.
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