"John Smith" wrote in message
om...
I just bought a house and found the A/C unit not working. I
checked the filters and they were as filthy as you can imagine. I
thought I could make the A/C work by simply replacing the filters.
No. After I replaced the filters, there was still no cool air. I
wonder what has been damaged, if any?

If the filters were VERY dirty, the system could have been pulling in air
from any leak and/or tear in the duct work.since most is not sealed
correctly.
This will lead to the evap coil being covered in the same mess that you see
on the filters.
This will cause:
Compressor failure, or fan motor failure.

The compressor can fail due to the evap coil icing over. This leads to
liquid refrigerant being sent back to the compressor, and the valves in most
units, (piston type) are reed valves. While the compressor may be running,
it may not be pumping, and this will cause a lack of cooling.
It can also lock the compressor up, and a replacement in either case is the
fix. The compressors lock not due to refrigerant lock, but due to the oil
being washed off the bearings. Think automotive style crankshaft, turning
two pistons at about 1750RPM, or more, bathed in oil, and then the oil is
removed.
The blower motor may fail, NOT due to load as there is no load with dirty
filters, but from lack of load. The motors are designed to run at a
particular static pressure, and without the load, the blower speed actually
increases, and with lack of air, the unit overheats.

Get a legitimate AC company out to give it the once over, and make sure that
they check the condition of the return duct, the evap coil, blower etc...
If its all in working order and the compressor is runnning, and the outdoor
fan is as well, and they claim its low on refrigerant, find out WHY...as in,
if its low, WHY since its a sealed system. A dirty evap coil, or lack of
airflow will cause the techs gauges to read low, but the gauges are only ONE
tool used to determine if the systems low. If all the tech does is slap
gauges on the outdoor unit, and proclaim its low, tell him to take a hike,
but be sure to tell him why.....subcool, or superheat, readings HAVE to be
taken to determine if the units low. Without those calculations NO ONE can
tell you HOW low, or how overcharged a unit may be....and if the previous
owners only had someone come out that didnt check the filters, or evap, and
they were that dirty, its possible as can be that the tech simply
overcharged the system not knowing, (not having the training, or caring
enough to check) that the airflow over the evap was reduced.

Have the entire system serviced...and if its over 15 years old, seriously
consider investing in a new higher SEER unit, keeping in mind that Jan 1st,
2006, the minimum standard will be 13SEER...tomorrows cheap unit is a higher
SEER unit of today, so look into a 14 or better SEER rating, and remember,
that if you go that route, if the company does not change out the indoor
evap coil, they simply took your money and ran.