professional home inspections, inc.


Air conditioners should not be run if the outside temperature is not above 65°F. Operating below 65° F can damage the compressor and, since there is little or no heat load on the unit, the test results would not be useful.

To test the air conditioner, let the unit run for ten to fifteen minutes. Make sure it does not vibrate excessively or make unusual sounds. Then measure the air temperature at a cold air return near the furnace. Next, measure the temperature of the air coming out of a supply register.

The supply air temperature should read 14° to 20°degrees F colder than the return duct temperature. This change in temperature is called the "TD."

If the TD is more than 20°, it indicates one or more of the following conditions:

Too little air is moving across the coil. Too little volume means a lower velocity, which means the air spends more time in the coil, therefore gets colder. Causes might be:

Dirty filter, slipping fan belt, direct drive fan motor set at too low a speed, dirt has gotten by the filter and is clogging the bottom side of the cooling coil, or the ducts may be too small.

The first few causes should be inexpensive to correct. A dirty cooling coil can run as much as $ 300-$400 to clean properly. Replacing ducts that are too small can be very expensive.
The cooling coil may be partially blocked by ice. Causes range from low refrigerant charge to a poorly designed system. The cost of correcting this can vary from $50 up to several hundred.

If the TD is less than 14°, it may be caused by:

Too much air is moving across the coil. ( Fan motor set at too high a speed, etc. ) The air moves so fast over the coil that it is not properly cooled.

Undercharged system or restricted refrigerant tubing.

A bad compressor valve, no cure, must replace the compressor at a cost of $600-$800, out of the warranty, usually $300-$400 in the warranty unless the warranty includes labor and parts other than the compressor. Few companies offer such a warranty.

You also should be sure to change the air filters at least once a month with a properly sized filter.