Can someone help diagnose what's wrong with my air conditioner?

The symptoms are:

  • the air that it is blowing is not very cold
    • It's colder than the ambient air, but not cold enough to cool the house more than 1 degree
  • there is condensation on the supply ducts exiting the furnace
    • The condensation is inside and outside the ducts
  • there is water pooling on the ground below the furnace
    • I suspect this is probably the condensation flowing down the duct into the furnace then down onto the ground
  • On the large copper pipe on the outside unit, some of the insulation had rotted away. I observed frost on the exposed copper.

I've tried:

  • Putting new insulation around the outside copper
    • I have not observed frost since
  • Put insulation around the ducts, but they still have condensation
  • Cleaning debris around and inside the outside unit.
  • Vacuuming the fins condenser coil and the evaporator, and used a alkaline cleaning product
    • neither were very dirty in the first place, and I was very careful not to bend the fins
  • I run a dehumidifier in the basement
  • I've opened all the registers

I have not tried:

  • cleaning the drainage pipe
    • I can't access the pan or outlet from the inside of the furnace
    • the pipe is PVC that's been glued on. I'd have to cut it to investigate
  • talked to a professional
    • I've got an appointment scheduled, but I always like to have an idea of the problem before they come so I can ask intelligent questions.

Other info:

  • this same thing happened 2 years ago. At the time, the professional who came out added refrigerant and that fixed it
    • He said that if there was a leak, it would all just leak out again right away, but it took 2 years for the issue to come back.
    • Does that mean the leak is very slow? or is there no leak?
    • I've read that AC units don't just consume the refrigerant over, it's in a closed loop, so there is no need for regular recharges.
  • the blower belt looks pretty worn, it's at least 5 years old, but it doesn't sound like it's slipping


I had a HVAC company come out and take a look. The refrigerant was low. They gave me various options including finding the leak and refilling the refrigerant, or replacing the whole AC & furnace. The system was installed in 1987, so I'm ok with replacing it. Their quotes seems pretty high though, ranged from $10000 to $16000.

  • Are you getting good airflow through the evaporator? Do you clean/change the filter regularly? Sounds like you have a slow leak, but it's worth it to check the things you can fix first.
    – Tester101
    Commented Jul 10, 2014 at 9:25
  • I just changed the filter ... and try to do it regularly. What constitutes good airflow, is that something I can test? Commented Jul 10, 2014 at 10:17
  • You could measure the airflow, however, I don't think you need to be that accurate. Just make sure there's a "normal" amount of air coming out of the registers.
    – Tester101
    Commented Jul 10, 2014 at 12:31

1 Answer 1


You are correct that the refrigerant is not a consumable. If a system is low on refrigerant, then it has a leak. The technician should have a "sniffer" tool that can detect refrigerant leaks. If they didn't use it last time, then they were irresponsible. Also, refrigerant is expensive so that attitude of "if it gets low again then we know you have a leak" is wasteful of your time and money.

If the compressor in the condenser (the outside unit) is running and the air is not cool, then you are probably low on refrigerant. Frost on the refrigerant line is also a pretty clear sign of low refrigerant, provided you have sufficient airflow through your air handler.

I am additionally concerned about the condensation on your duct work. Is the air handler inside the conditioned envelope of the house? If so, that could explain the condensation you are seeing now but would not be seeing normally.

  • The air handler is in the basement which is not conditioned. But it is naturally cooler than the rest of the house. Would his sniffer tool be able to detect a slow leak like the one apparently in my system? Commented Jul 10, 2014 at 10:09
  • When you say "if the outside compressor is running" is the fan spinning enough to know the compressor is running correctly ... i.e. could the fan spin, but the refrigerant not be circulating? Commented Jul 10, 2014 at 10:15
  • Yes, the sniffer should be able to detect an active leak of any size. If the leak is dependent on something like the dryer being in direct sunlight for 8 hours, then it might be missed. But that is very unlikely.
    – longneck
    Commented Jul 10, 2014 at 12:27
  • 2
    A spinning fan is not enough to know that the compressor is running. The compressor is the tank looking thing in the bottom of the condenser. It sounds like a refrigerator when it starts up. In your case, since you see frost on the refrigerant line we can assume that your compressor is running. There is nothing else in the system that could get the refrigerant cold enough to cause frost.
    – longneck
    Commented Jul 10, 2014 at 12:31
  • A leakdown over two years may be slow enough that a sniffer would not readily detect.
    – mongo
    Commented Jul 21, 2021 at 14:05

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