After installing a new AC, a homeowner notices mold and learns one fan setting may be trapping humidity

"If it is set to run continuously, it will just blow water off coil right back in."

A homeowner turned to Reddit after a hygrometer showed humidity close to 70% and mold began appearing around the house, even after new coils and a 3-ton condenser had been installed and the thermostat had been lowered to 65 for hours at a time.

People in the discussion pointed to a surprising possible cause. They said the issue might not be the low temperature setting, but a fan mode that could keep moisture circulating through the home instead of allowing it to drain away.

Here's what to know

The homeowner said indoor humidity was averaging about 70% in a Reddit post on r/HVACAdvice. According to the post, the trouble started about four months after the new coils and 3-ton condenser were installed, and the homeowner added, "We started noticing mold around the house."

Even after the HVAC company adjusted the fan motor from 1,800 cfm to 1,200 cfm, the problem did not go away. The homeowner wrote, "It's been 4 days and humidity levels are still averaging in the mid 60%."

Several commenters focused on how the system fan was set to run.

"Do you have the fan switch set to auto? If it is set to run continuously, it will just blow water off coil right back in. It needs off cycles to drain," one commenter wrote.

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Another Reddit user said that setting comes up all the time on service calls. "This is what I find on probably 90% of high humidity house calls lol," the person wrote.

More background

To the homeowner, the timing suggested a new issue tied to the HVAC changes rather than a long-running one. As they put it, "My in-laws have been in this house 40 years and never had an issue."

The homeowner also said the house had replacement windows and was "sealed up well."

Running the AC longer did not fully solve the problem either. After setting the thermostat to 65 "just to let the system cycle for several hours," the homeowner said humidity only dropped to 52%.

A separate response described a similar situation in a renovated home in lower Alabama, where crawl-space moisture was the real source of the problem. That commenter said adding a "sump, dehumidifier, and plastic barrier film" lowered the crawl space to 40% relative humidity, kept indoor moisture below 60%, and meant "total cost of electricity is down a little since a/c does not have to work so hard to cool moist air."

What can be done?

One free step mentioned in the thread is setting the AC fan to AUTO instead of running it continuously. A fan that never stops can re-evaporate water on the coil and send that moisture back into the home.

Another thing to check is whether the airflow has been set properly for the equipment. In this case, the company changed the fan setting from 1,800 cfm to 1,200 cfm.

Rather than only lowering the thermostat, commenters also suggested finding ways to make the system run in longer cycles. One person wrote, "Check if your thermostat allows a wider deadband for longer run time."

If the fan setting and airflow are correct but the house still feels damp, commenters said the moisture source may be outside the unit itself.

One commenter supported that idea by describing improvements after fixing a wet crawl space with a sump, a dehumidifier, and plastic barrier film, then advised, "Do please check to see if infiltration is part of your problem."

The homeowner summed up the concern by asking whether the target humidity range might be too low: "Maybe my target range is off- I've been aiming for under 50%. My understanding is that's the threshold to prevent mold growth/propagation."

Another commenter replied, "No, your target range is good. Aiming for under 50% is correct."

Where can I learn more?

If damp indoor air is part of the issue, other AC habits and common misconceptions can also affect comfort and moisture control. The topics below cover dry mode, airflow mistakes, filter problems, questionable add-ons, and whether shutting doors helps.

• During warm weather, some households use AC dry mode to reduce moisture and bills.

• An HVAC specialist warned that closing AC vents can strain airflow and raise costs.

• A local TV reporter found that closing bedroom doors can backfire while central air runs.