Your AC thermostat setting can create high humidity and grow mold in your house

By · February 15, 2010 · Filed in Consumer Q & A's

I had a customer complaining about respiratory irritations in their house. I brought my testing equipment to their house and started the investigation.

  • I looked into their ducts with our camera and found a lot of debris that shouldn’t be there, an indication that their air ducts should be cleaned, but not necessarily something that should cause extreme respiratory distress.
  • I looked at their main unit, and the evaporator coil was somewhat dirty, indicating it needed cleaning, but again, not enough to cause the unit to malfunction.
  • Mold was growing around the supply registers.
  • I looked at the condensation line, the PVC line that takes the water away from the unit after it removes humidity from the air. The condensation line was almost dry. This was an indicator it was not making condensation, or not dehumidifying the air.

The house felt cool, but clammy, when I walked in.

  • The readings I took in his house registered 78%Rh at 72 degrees, about the same as outside.
  • The indoor RH in any environment needs to be below 60%, ideally it should be below 50%.

I was concerned that the air conditioner was not “making water” and the relative humidity (Rh) inside was above normal.

Upon further questioning, he said he had his thermostat set on “On”, where the unit ran continuously. He said he was instructed by an HVAC contractor to do this. I asked the customer if the HVAC contractor was a commercial or residential contractor. He said he was more experienced in commercial.

Here is the problem. In a commercial unit, the evaporator coils are cooled from a cooling tower, a recirculation of water from the cooling towers on the building to the units in the building. In a residential setting, the evaporator is cooled from Freon gas. The problem is, the Freon gas cannot keep the coils cool with the constant flow of air, which warms the coils. What happens when the unit is set to “on”, instead of “auto”, is, the coils can cool the air a little, but they cannot dehumidify the air, because it does not reach dew point at the coils. The result is, the air is cold and clammy, like a cave. These conditions grow mold and aggravate those with respiratory sensitivities.

The solution is to set the thermostat to “auto”. This allows the evaporator coils to cool enough, while air is not flowing over them, to both cool and dehumidify the air. I talked to him the next day after he set his thermostat to “auto”.

His indoor Rh had dropped to 62% and the measurement of the supply at the vent was 48%Rh. Given a few more days, the Rh in the house would drop to the low 50’s, which is the best reading to maintain for the health the occupants and the home.

Go to your local discount store and buy a cheap hygrometer to monitor your indoor humidity levels to make sure they stay below 50% Rh. You will stop growing mold.

For more information about mold remediation

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