You lower the thermostat, the house cools down, but it still feels clammy. An air conditioner removes humidity as a side effect of cooling, and only while the system runs long enough with a cold coil. When the house stays sticky, the usual reasons are short run times (often from an oversized unit), the fan set to ON instead of AUTO, a dirty coil or poor drainage, leaky ducts pulling in damp air, or more moisture coming into the house than the AC can take out.
How an AC takes moisture out of the air
Warm indoor air passes over the cold evaporator coil. The coil is colder than the air’s dew point, so moisture condenses on its fins, drips into the drain pan and runs out through the condensate line. The cooler, drier air goes back to the rooms.
Two things have to happen. The coil must be cold enough, and the system must run long enough. For the first few minutes of a cycle the coil is only getting wet; water starts draining once it has run a while. Short runs cool the air but leave much of the moisture behind.
First, measure it
Feeling sticky is a good hint, but a number is better. An inexpensive hygrometer, or a thermostat that displays humidity, tells you where you stand. The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. Above that for long stretches, mold and dust mites thrive, and a damp, musty smell often follows; see musty smell from vents.
Why humidity stays high
Oversized AC. A common cause. An oversized unit cools the house quickly and shuts off before it has run long enough to dry the air, so you get short cycles that remove little moisture. The lasting fix is correctly sized equipment when the system is replaced.
Short cycling for other reasons. Even a correctly sized system dehumidifies poorly when something else cuts its runs short: a thermostat in a bad spot, a refrigerant problem, a frozen coil. Our guide to short cycling covers the causes.
Fan set to ON. With the fan running nonstop, the water sitting on the coil evaporates back into the air between cooling cycles. Set the fan to AUTO.
Too much airflow. If the blower moves air across the coil faster than the climate calls for, the coil does not get cold enough to pull much water out. A technician can check and adjust the blower setting.
Dirty coil or poor drainage. Dust on the coil insulates it, and a slow or clogged drain leaves water standing in the pan, where some of it evaporates back into the air stream.
Leaky ductwork. Return ducts that leak in an attic or crawlspace pull in unconditioned, often humid air from those spaces, and that moisture goes straight to your rooms.
Air leaks and indoor moisture. If damp outdoor air gets in faster than the AC can remove moisture, humidity stays high. Showers, cooking, laundry dried indoors and exhaust fans that do not vent outside add to it.
Single-stage equipment. A single-stage system runs at full capacity and cycles off quickly. Two-stage and variable-speed systems can run longer at lower output, which gives the coil more time to remove moisture.
What fixes it
Quick fixes you can do:
- Set the thermostat fan to AUTO, not ON.
- Replace a dirty filter.
- On a muggy day while cooling, look for water dripping from the condensate line, if yours ends outside. Standing water in the drain pan under an attic or garage unit means a clog for a technician to clear.
- Run kitchen and bathroom exhaust fans during and after cooking and showers.
System improvements:
- Seal leaky ductwork, especially returns in the attic; see ductwork repair.
- Seal air leaks in the home itself.
- Have the coil cleaned, the drain cleared and the blower airflow and refrigerant charge checked during an AC repair visit.
- Consider a whole-house dehumidifier, which works independently of cooling. An indoor air quality assessment can tell whether that or better ventilation makes sense.
Equipment options:
- Two-stage or variable-speed equipment dehumidifies better than single-stage.
- Some systems and thermostats have a dehumidification mode that slows the blower so the coil gets colder.
- When replacing, size the new system from a load calculation, not from the old unit; see AC installation.
A note for Hayward homes
Humidity in Hayward is moderate most of the year, and many days near the bay are mild enough that the AC hardly runs. That is the catch: on foggy mornings or days when damp air moves in off the bay, the house can feel clammy while the thermostat is already satisfied, so the AC never runs long enough to dry anything. Lowering the setting just makes the house cold. On those days a dehumidifier handles the moisture better than the air conditioner. In older slab homes with ducts in or under the slab, hidden leaks can let moisture into the air stream; if dampness and a musty smell show up together, have the ducts checked. Also make sure bathroom and kitchen exhaust fans vent outside, not into the attic.