How a dehumidifier works

What it’s for

A dehumidifier lowers the humidity of a room or a home. The EPA’s target indoors is below 60%, and ideally 30 to 50%. The ENERGY STAR® sizing table calls a space at 50 to 75% damp, and one at 90% or more wet.

It doesn’t clean the air or cool it. Its one job is moisture.

The four parts

Federal rules define a dehumidifier by what it contains. 10 CFR 430.2 lists four parts in one electrically powered case:

1. A refrigerated surfaceThe evaporator, which condenses moisture from the air.
2. A refrigerating systemWith an electric motor, which keeps that surface chilled.
3. A fanIt pulls room air across the cold surface.
4. A way to collect or drainThe condensate goes to a bucket, a hose, or a pump.

Damp air meets the chilled coil, and the vapor in it turns to drops, the way a glass of iced tea sweats in summer. The drops run off into the case. The air then crosses the warm side of the system and goes back to the room drier, and slightly warmer.

Why cold rooms are hard

The coil has to stay below the air temperature. In a cool basement or garage, that gets difficult. ENERGY STAR notes:

Frost blocks the coil. The compressor may then cycle on and off repeatedly without removing moisture, ENERGY STAR says. Some models have a frost sensor that stops the unit until the ice melts.

How it knows when to stop

Every certified model has a built-in humidistat. You set a humidity, and the machine runs until the room reaches it, then cycles on and off to hold it.

Where the water goes

Into a bucket first. ENERGY STAR describes the bucket shutting the unit off when it’s full. You can also connect a hose and run it to a floor drain or sump. With no drain lower than the unit, a built-in pump pushes the water up and out.

Portable or whole-house

The federal rules split dehumidifiers by ducts. A portable works within the room, with no ducting needed. A whole-house unit works with ducts that bring air to it and carry the dried air to one or more rooms.

On the EPA list, 21 of the 482 certified models sold in the United States are whole-house units.

The refrigerant

The cold comes from a refrigerant, as in a fridge. Most certified models use R-32: 421 of them. Another 44 use R-290, a propane refrigerant, and 4 use R-454B. The EPA record of each model names its refrigerant.

A dehumidifier that dries air with a desiccant alone, with no refrigerating system, doesn’t fit this definition. The rules do cover a whole-house unit that adds a desiccant to a refrigeration system.

Sources

Every figure and quote on this page comes from these, each with the date we read it.