Does AC Remove Humidity? How Air Conditioners Dehumidify

How an AC pulls water out of the air

The indoor evaporator coil typically runs at about 40 to 45 °F, with the refrigerant boiling inside it. Room air at 75 °F and 50% RH has a dew point near 55 °F (check with the dew point calculator). As that air passes through the much colder coil, it cools below its dew point and water condenses on the fins, just like a cold glass on a summer day.

The water drips into the drain pan and leaves through the condensate line. A typical central system in a humid climate can remove several gallons a day this way. Air leaving the coil is around 55 °F and close to saturated, but once it mixes with warmer room air its relative humidity drops, and the room ends up drier than before.

Sensible and latent heat

An air conditioner's capacity is split between two jobs:

• Sensible cooling lowers the air temperature that the thermostat reads.
• Latent cooling removes moisture. Condensing water releases heat (about 1,060 BTU per pound of water), and the AC has to absorb it.

The share of capacity going to sensible cooling is the sensible heat ratio (SHR).

SHR = Sensible capacity ÷ Total capacity

Typical residential equipment runs at an SHR of about 0.70 to 0.80, meaning 20 to 30% of its output goes to dehumidifying. A 3-ton unit (36,000 BTU/h) at an SHR of 0.75 delivers 9,000 BTU/h of latent capacity, which is roughly 8.5 pounds (about 1 gallon) of water per hour of steady running.

What changes how much moisture it removes

With the fan set to On, water left on the coil after the compressor stops is blown back into the house. In humid climates that can undo a large part of the moisture removal, so Auto is the better setting.

Why oversized AC leaves a house clammy

When a cooling cycle starts, the coil needs a few minutes to get cold and wet before water begins to drip off steadily. An oversized system reaches the thermostat setting quickly and shuts off, often before it has removed much moisture at all. The house hits 74 °F but stays at 60 to 65% RH, which feels cool and damp at the same time.

That is the most common cause of the complaint covered in AC not removing humidity, and one reason sizing guides warn against simply buying a bigger unit (see the AC tonnage calculator and AC short cycling).

If your system is only slightly oversized, setting the fan to Auto and having a technician lower the blower speed toward 350 CFM per ton can noticeably improve moisture removal. Variable-speed systems often have a humidity setting that does this automatically.

When the AC alone is not enough

An air conditioner only dehumidifies while it is cooling. In shoulder seasons, mild humid days or in a cool basement, the thermostat is satisfied long before moisture is under control. Options include:

• A whole-house dehumidifier ducted into the return, which works independently of cooling.
• A portable unit for a basement or problem room, set using our basement dehumidifier settings.
• Reducing moisture sources: bath fans vented outdoors, kitchen range hoods, sealed crawlspaces.

Aim for 40 to 50% RH in summer; the indoor humidity guide explains why.

Quick checks if your house still feels humid

1. Set the thermostat fan to Auto, not On.
2. Replace a dirty filter; low airflow can freeze the coil and stop drainage.
3. Check the condensate line is draining outside or to a pump; a clog can trip a float switch or overflow the pan.
4. Look for obvious moisture sources such as unvented dryers or open basement windows.
5. Measure RH with a separate hygrometer for a few days.
6. If RH stays above 55 to 60% with the AC running normally, have a technician check airflow, refrigerant charge and whether the unit is oversized.

Frequently asked questions