Why Is My House Humid Even When the AC Is Running?

An air conditioner removes moisture as a side effect of cooling. Warm humid air passes over a cold evaporator coil, water condenses on the coil and drains away. That process needs run time. The coil has to get and stay cold, and air has to keep moving across it long enough for meaningful condensation to occur.

An oversized unit satisfies the thermostat in a short burst and shuts down. The coil never reaches its most effective dehumidifying state, and worse, moisture already on the coil re-evaporates into the house when the blower runs on after the compressor stops. You end up at 72 degrees and 65 percent relative humidity, which feels clammy, promotes mold, and is exactly the complaint we hear most in Upstate summers.

• 01Time the cycles on a hot afternoon. A correctly sized system on a 92 degree day should run long cycles, often 15 to 20 minutes or more. Cycles of 5 to 8 minutes point to oversizing.
• 02Buy a $15 hygrometer and check indoor relative humidity. In this climate you want 45 to 55 percent. Above 60 percent with the AC running is the signature of the problem.
• 03Note whether the house feels cold but damp rather than dry and comfortable.
• 04Look at the equipment tonnage against the house size. A rough sanity check in the Southeast is roughly 500 to 700 square feet per ton. A 3,000 square foot house with a 6 ton system is almost certainly oversized.
• 05Ask whether anyone ever performed a load calculation. If the last installer sized the unit by matching what was there before, nobody has actually checked.

Ducts are the other half of the problem and get less attention because they are hidden. Undersized returns starve the blower, which reduces airflow across the coil. Leaking supply ducts in a hot attic lose cooled air into the attic and pull humid attic air into the system. We routinely measure duct systems losing 20 to 30 percent of their airflow before it reaches a room.

Static pressure is the measurement that reveals this, and it takes a technician a few minutes with a manometer. High static pressure means the blower is fighting the duct system, which reduces capacity, shortens equipment life, and makes rooms far from the air handler never quite cool off.

• 01A Manual J load calculation before any equipment is quoted, so the system is sized to the house rather than to the old unit.
• 02Duct repair, sealing, and resizing where the measurements call for it. This is often cheaper than new equipment and delivers more comfort.
• 03Variable speed or two-stage equipment, which can run long low-capacity cycles. This is the single most effective upgrade for humidity control in our climate.
• 04A dedicated whole-house dehumidifier where the load genuinely calls for it, which is sometimes the right answer in a tight, well insulated home.
• 05Air sealing the attic plane, since humid attic air infiltrating the living space adds latent load the AC then has to remove.

We run the load calculation and inspect the duct system before quoting equipment, and every install is commissioned with measured static pressure and refrigerant charge recorded on your paperwork. If your problem is ductwork rather than the unit, we would rather tell you that than sell you a condenser that will not fix it.