It is one of the most common calls we get from May through October. The AC is running, the thermostat says 72, and the house still feels damp. Sheets feel cool and clammy at night. A closet smells a little musty. The bathroom mirror takes forever to clear.
That is not a temperature problem. It is a humidity problem, and on the Northshore it is usually caused by the house and the system working against each other.
Your AC has two jobs
An air conditioner cools the air, and it also pulls moisture out of it. The moisture comes out on the cold indoor coil and drains away. But that second job takes time. The coil has to run long enough to get cold and stay cold before it removes much water.
When a system cools the house quickly and shuts off, it hits the temperature on the thermostat without ever getting to the moisture. The result is a cool, damp house.
The usual suspects
- An oversized system. It cools so fast that it short-cycles and never runs long enough to dry the air. This is the most common cause we find.
- Too much airflow across the coil. The coil never gets cold enough to condense much water.
- Leaky return ducts in the attic. They pull hot, wet attic air directly into the system.
- The fan set to "On" instead of "Auto." Water sitting on the coil gets blown right back into the house after each cycle.
- Constant exhaust fans. Every cubic foot of air pushed out has to be replaced, and in Louisiana the replacement air is humid.
What healthy looks like
Indoor relative humidity between 40 and 55 percent is the comfortable, healthy range. Above about 60 percent, dust mites thrive and mold has what it needs. Many homes we test sit at 60 to 70 percent all summer and do not know it.
How we fix it
We start by measuring. We log humidity room by room, test airflow and static pressure, check the system's actual cooling output, and run a load calculation on the house. That usually points to one or two root causes.
The fix might be as simple as a blower setting and a thermostat change. It might be sealing the attic return. In some homes it is a whole-home dehumidifier, and when it is time to replace the system, it is sizing the new one correctly so the problem does not come back.