Dehumidifiers: the other half of feeling cool

This site spends most of its time on machines that move heat. A dehumidifier is here because, for a large part of the year, the thing making a room unbearable is not its temperature.

Your body sheds heat mainly by evaporating sweat, and evaporation only continues while the surrounding air has spare capacity to absorb water. Humid air has less of that capacity, so the same air temperature feels markedly worse and a fan does markedly less. The mechanism in full is the single idea that connects every category on this site.

Which means there are two ways to make a damp room comfortable. You can cool the air, which also condenses moisture out of it as a side effect — that is what an air conditioner does, and dehumidifying is the most energy-intensive part of its job. Or you can remove the moisture directly and leave the temperature alone.

In a basement, a laundry room or a shut-up house in a humid climate, the second is frequently the cheaper answer, because you are not paying to cool air you did not need cooler.

Dehumidifiers are sold on pints per day — how much water the unit pulls out in 24 hours under test conditions. That is a capacity figure, and capacity tells you whether a unit is big enough, not whether it is good.

The efficiency figure is IEF, the integrated energy factor, measured in liters of water removed per kilowatt-hour consumed. It is tested under a federal procedure and published for every certified model, which makes it the one number a manufacturer cannot write itself.

Across the 502 certified portable units we hold data for, IEF runs from 1.70 to 2.24 L/kWh with a median of 2.01. That is a narrow band, and the narrowness is the story: most of this market is built on a handful of shared platforms, and a large majority of household-name brands land on exactly 2.01.

Annual energy use is the figure to compare in dollars. Certified portables run from about 283 kWh a year for a 27-pint unit to 526 kWh for a 50-pint one — $52 to $96 at 18.34¢/kWh. That is real money for an appliance many households run continuously for months.

We say this on every category page on this site where it applies, and it applies twice here.

No dehumidifier publishes a noise rating. The certification program records capacity, efficiency, annual energy, refrigerant and connectivity, and has no acoustic field at all. Bathroom fans are rated in sones and room air conditioners are not; dehumidifiers are in the second group. Any decibel figure you see on a listing is the manufacturer’s own, tested however they chose.

The Most Efficient list skews small. Of the 111 certified portables carrying the ENERGY STAR Most Efficient designation, 89 are under 25 pints per day. In the 35 to 45 pint band, 2 of 68 units qualify. If you need real capacity, the label will barely narrow the field for you and IEF has to do the work instead.

Portable units are the 500-odd models most people shop: a box with a tank, a filter and a drain port, sized in pints per day and moved between rooms. They top out at 2.24 L/kWh.

Whole-home units are a much smaller field — 24 certified models — and they are installation projects rather than purchases, ducted into a system or plumbed into a drain in a basement. They are also substantially more efficient: a median of 2.30 L/kWh against 2.01, and a maximum of 3.26 against 2.24. If the whole house is damp, the larger machine is not just bigger, it is better per unit of water removed.

A dehumidifier treats a symptom. It is very good at that and it is not a substitute for finding out where the moisture is arriving from, because a source that keeps replenishing will keep the machine running indefinitely.

• A dryer venting indoors is the single largest fixable moisture source in most homes — how to check yours.
• Bathroom and kitchen extraction that is missing, blocked or ducted into a loft. A certified fan removes moisture at 16.7 CFM per watt, which is a fraction of a cent an hour.
• Groundwater and drainage in a basement. That is a building problem, and a dehumidifier will run forever against it.
• Drying laundry indoors in an unventilated room.

Fix what is fixable, then size the machine for what is left. How to size it.