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  #1  
Old 07-22-2008, 05:28 PM
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Home AC - what temp should come out of the vents?

I have two 4-ton units in a house that was built in 1959. It's 98 degrees outside right now, and on one side of the house (west side) the vents are blowing out at 64-65 degrees and other side of the house (east side) is blowing at 68 degrees. Do I have a problem or can this be expected?

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Old 07-22-2008, 05:32 PM
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You need to measure the inlet temp too. Report back with the inlet and outlet temps for each system.

How about air volume? Plenty of volume out the vents?
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Old 07-22-2008, 06:12 PM
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Seems to have plenty of volume - intake 74 on the west side and 73 on the east side.
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Last edited by POS; 07-22-2008 at 06:48 PM.
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Old 07-22-2008, 06:20 PM
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I had an AC guy come out last month. You want to see a 20 degree differential between intake and what is blowing out.
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Old 07-22-2008, 06:50 PM
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And the thermostats are kept at 73-74.

20 degree differential? Meaning that if the intake is 73-74, then it should blow at 53-54 or that there should be 20 degrees between the outside temp and the inside vent?
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Old 07-22-2008, 06:52 PM
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I am not sure about intake air vs supply air.

It used to be that folks designed ac units to drop the temp 20 degrees from the outside temp. I think most folks now probably try for 30 degrees, possibly more down south where it gets over 100 with regularity.

Tom W
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Old 07-22-2008, 06:59 PM
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Quote:
Originally Posted by POS View Post
And the thermostats are kept at 73-74.

20 degree differential? Meaning that if the intake is 73-74, then it should blow at 53-54 or that there should be 20 degrees between the outside temp and the inside vent?
Actually, about 18deg F difference between the inlet temp of the unit and the outlet (the nearest vent to the inside unit) temp is OK. Anything less and you aren't cooling properly. Anything more and you have a different set of problems.

Inside, make sure the ducting is all in good shape and hasn't come loose somewhere in the attic.

Outside, check for obstructions around the condensers (the outside units) that would restrict airflow; make sure that the condenser fans are running.

You could also have an inefficient (worn out) compressor or a low refrigerant charge (unlikely)...hard to diagnose this without looking at it and hooking up a set of gauges.
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Last edited by R Leo; 07-22-2008 at 07:06 PM.
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Old 07-22-2008, 07:00 PM
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Quote:
Originally Posted by thorsen View Post
I had an AC guy come out last month. You want to see a 20 degree differential between intake and what is blowing out.
Agreed, with a healthy unit. Maybe 16 to 20 is typical. You also want to see condensation on the return vapor line near the compressor suction. Sounds like yours isn't operating very well.
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Old 07-22-2008, 07:07 PM
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Quote:
Originally Posted by t walgamuth View Post
I am not sure about intake air vs supply air.

It used to be that folks designed ac units to drop the temp 20 degrees from the outside temp. I think most folks now probably try for 30 degrees, possibly more down south where it gets over 100 with regularity.

Tom W
Hmmm...30? Methinks not.
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Old 07-22-2008, 07:10 PM
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Originally Posted by cjlipps View Post
Agreed, with a healthy unit. Maybe 16 to 20 is typical. You also want to see condensation on the return vapor line near the compressor suction. Sounds like yours isn't operating very well.
A sweat back test for charge isn't 100% reliable down here in the swamp belt where the relative humidity is bumping 70% and the dew point is 75F in the middle of the afternoon. But, I would say that if the big line going to the condenser wasn't sweating after 10 minutes of operation, you definitely have a problem. Any ice on that line is mucho bad news too.
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Old 07-22-2008, 10:56 PM
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Make sure the evaporator is clean - stuff does sneak by filters over time.

Two 4 ton units? Wow - you must have somewhere past 4k sq feet of air conditioned space.

I don't know what my differential is, but if I were to turn the thermostat to 76 or below, I would be looking for a blanket.
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  #12  
Old 07-23-2008, 12:14 AM
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I'm looking at 73.8f in and 55.6f out at the moment.

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