General Question

robmandu's avatar

It's 105°F outside. What should the temperature of the A/C air at the vent be?

Asked by robmandu (21203 points ) August 24th, 2011

Central air conditioning system. In the morning, when it’s 86°F outside, the cool air coming out of the vent is around 62°F. Later in the day, when it’s over 105°F, it’s around 72°F at the vent.

Are these numbers normal?

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13 Answers

Brian1946's avatar

The cooling air that comes out of my window unit is usually 49º F.

Did you take those readings as soon as the compressor started, or were they taken after it had been running for awhile?

Judi's avatar

What temperature do you have it set at? The A/C at 62 is pretty darned low. I doubt your A/C could maintain that low of a temperature at 104. If you can (and can afford) to keep the air at a chilly 72 I would think your AC is working a well as can be expected. I live where it gets that hot too, but never tried to keep it at such low temperatures. you might want to invest in a meat locker.

thorninmud's avatar

It’s complicated. The critical measure of the operation of the cooling unit itself is the amount of temperature drop over the cooling coil, in other words the difference in the temperature of the air on the upstream and downstream sides of the cooling coil. That should be in the neighborhood of 20 degrees. That’s pretty much independent of outside temperature.

But there’s another factor that does very much depend on outside temperature, and that’s duct loss. Since much of your duct work likely passes through your attic, and since your attic is likely a furnace in the hottest part of the day, you’re probably losing a lot of cooling in that passage through the attic.

robmandu's avatar

@Brian1946, I just hung a thermometer over the front of the vent and left it there for an hour… analogous to when the repair guy points an infrared thermometer at the vent.

@Judi, a typical residential A/C system always just blows the coldest it can. When the temperature at the thermostat has reached the set point, it turns off. It’s all or nothing. It doesn’t matter if the thermostat is set at 60° or 75°... it blows the same cold-as-it-can air. That’s why your a/c actually shuts off… else it’d just run continuously at 72° or whatever your set temp is, like in your car (where you can mix hot/cold air). Fyi: during the day, I have the downstairs set at 78° and upstairs at 81°. At night, I shoot for around 74°.

@thorninmud, was afraid of that… but was also hopeful that the ducting in the attic, once cooled after the A/C had run long enough, wouldn’t be that big of an impact.

30–40° temperature differentials are hell on my electricity bill.

Brian1946's avatar

@robmandu

Is your ducting covered by insulation?

When the compressor starts, the air coming through the vents won’t be that much cooler than the temperature in your attic, but it seems that after the comp has been running for an hour or so, it should get a lot cooler if your ducts are insulated, because hopefully almost all of the standing hot air in your ducts will have been flushed out.

YoBob's avatar

There is no correct answer to this question. It really depends on the conditions between the heat exchanger and the vent. Basically you have a fan that blows air over a cooler and then through a long pipe out the vent. If the pipe is well insulated and relatively short it will not loose much heat on the way to the vent. If, OTOH, the pipe runs all the way across your house through a blisteringly hot attic and is not very well insulated it’s going to pick up quite a bit of heat along the way.

thorninmud's avatar

@robmandu Some of your registers will be served by ducts that have a long trip through the attic, and some will have a shorter trip. If you can figure out which is the longest and which is the shortest, it would be interesting to compare the temps at those two registers this afternoon.

john65pennington's avatar

First, all ac ducts should be vented underneath a building, instead of in an attic. This is for a very obvious reason…..heat. This might be okay in the winter, but it’s not okay in the summer, since heat rises. If your vents are in the attic, they are facing a double-duty situation, in order to give you cool air at the other end of your house.

If your situation is like that described above, you may never receive any cooler air than you are now receiving. Insulating the ductwork may improve your cool air by a few degrees.

Another point…..I will never understand why contractors now place washing machines and dryers on the second level of an apartment. Bad choice.

Judi's avatar

@john65pennIngton, here in central CA almost all the houses are on a concrete slab. I have never seen the ducting under a house here.
I have also never seen insulated ducting in a residential application, but we just did a major remodel and DID insulate the ducting.
The temperature drop is probably happening in the attic which is probably closer to 120–130 degrees.
I thought you were talking about yhe thermostat reading in my previous post.

robmandu's avatar

@Judi and @john65pennington, yup… this is Texas, concrete slab and we have to water that foundation regularly in the summer to keep it from breaking up.

The HVAC ducts in my house’s attic are self-insulated. It’s not a rigid, bare metal box frame, but more like flexible tubing wrapped with fiberglass and shiny foil.

Thanks to all for the replies so far!

Bagardbilla's avatar

On a central HVAC system, a 20–25 degree differential (between outside air and temp of air coming out of duct work) is about as good as you can expect.

robmandu's avatar

Recharged the freon. It’s 103 outside and now the a/c is blowing 66.

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