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I am calling a HVAC technician since this is probably too complex a problem for me, but wanted to hear some possible diagnosis.

The symptoms are as follows:

  • The A/C is working well during the day (or so it seems).
  • For the past three nights however I wake up and it feels hot.
  • The thermostat reads 75°F and it is set at 72°F.
  • Most importantly a loud vibrating noise is coming from the basement and it is the HVAC unit. This has never happened during the day.
  • I shut off the AC and turn on the fan only and I still get the loud noise and vibration.
  • After an hour or so, I can hear liquid dripping inside the unit.

Anyone knows what might be the problem? What other information could be useful?

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2 Answers 2

up vote 1 down vote accepted

It sounds like your coils are freezing over. This could simply been the result of not changing your filter, or the coils need cleaning, so there isn't enough warm airflow over the coils. It's also possible that you're running the unit when it's too cold outside. If it gets below 60°F you should be opening your windows instead of running the AC.

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I should mention that when the vibrating noise happens poor to no airflow is felt at the vents. When coils freeze, do they prevent proper airflow? –  sturgman Sep 1 '13 at 19:23
Yes, it will block airflow. Air cannot pass through ice. –  BMitch Sep 1 '13 at 19:24

Because of the time frame you mention, I would suspect that humidity and condensation is exceeding the amount of airflow through your evaporator coils. Check that your filter is not too dirty or restricting airflow, and replace it every 30-90 days depending on the suggested replacement listed on the filter. Airflow could also be restricted by clogged evaporator coils themselves, if dirt, dust or debris managed to get past an air filter or if at any time somebody had run the unit without a filter entirely. If this happens, turn off the AC on the thermostat and wait for the ice to melt and remaining moisture to evaporate before replacing the filter and turning it back on.

The rattling you hear is your blower fan and motor. A blower can wear and become unbalanced over time, which would have similar symptoms and sounds, as well as not cool or heat as effectively, since instability would not allow it to operate at the RPM speed it is intended to, but if that were the case in your situation, it would not likely only be happening when cooler or when there is more condensation or humidity. I suspect that the blocked airflow is causing some vacuum pressure that may cause the fan to rattle, but it may also be possible that the blower fan is in early stages of wearing out, at a point where if airflow is not restricted, the stress on the blower is not as noticeable.

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