SPECIAL REPORT:TESTING 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
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
The supply air
temperature should read 14° to 20°degrees F colder than
the return duct temperature. This change in temperature is called the
If the TD is
more than 20°, it indicates one or more of the following
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:
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
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.
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