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How to Diagnose and Repair a Humid Air Conditioner

Ask This Old House plumbing and heating expert Richard Trethewey travels to Orlando to diagnose and repair an HVAC system that is creating way too much humidity

When diagnosing a humid air conditioning system, This Old House Heating and Plumbing Expert, Richard Trethewey, usually follows a series of steps based on the most common issue and works his way to more specific issues.

Steps to Diagnose and Repair a Humid Air Conditioner

  1. First, Richard checks the sizing of the equipment. Square footage alone isn’t enough to determine the appropriate size of a unit, since insulation, building materials, window openings, etc., can all also impact how much work a unit will need to do. He’s often found that units are oversized with the thought that “bigger is better”, but when that happens, the unit won’t stay on long enough to eliminate humidity. If the unit is the wrong size, it will need to be replaced.
  2. The next thing Richard normally checks is thermostat setbacks. If the setback temperatures are too far apart during the day, the air conditioning system will have to work too hard to eliminate heat and humidity in the building, which would explain why the house feels humid. Keep setback temperatures to around five degrees difference to prevent the system from overworking.
  3. If neither of these issues is causing the problem, it’s time to start digging into the specifics of the system. In this case, Richard found that a UV light was requiring a fan to be on all day. That fan was pushing the moisture collecting on the cold coil of the AC unit back into the building, causing the humidity.
  4. To solve this issue, Richard removed the UV light and adjusted the fan so that it only turns on while the unit is in cooling mode.


Richard installed the Healthy Climate Solutions MERV 16 air filter, manufactured by Lennox.

Expert assistance with this project was provided by 4 Seasons Air Conditioning & Heating and by Air Conditioning Contractors of Central Florida.