Ducts & air
Dehumidifier sizing: pints per day, really
A "50-pint" dehumidifier bought in 2018 and one bought today are not the same size. Here is what the rating measures, how to size for a damp space, and why the old numbers still circulate.
What the rating means
A dehumidifier is rated by how many pints of water it removes from the air in 24 hours of continuous running under standard test conditions. The rating is a capacity, like an air conditioner’s BTU per hour. In real use the unit removes less when the air is cooler or drier than the test, and more when it is warmer or damper.
The 2019 test change
Until 2019, the Department of Energy tested portable dehumidifiers at 80 °F and 60% relative humidity, warm, damp air in which a dehumidifier works well. From June 2019 the test moved to 65 °F and 60%, much closer to a basement, where most dehumidifiers run. At the cooler condition the same machine removes less water, so its rating fell by about a quarter to a third.
| Rated under the 2019 test | About the same as under the old test |
|---|---|
| 20 to 22 pints per day | 28 to 31 pints per day |
| 30 to 35 pints per day | 42 to 49 pints per day |
| 50 pints per day | 70 pints per day |
A 50-pint unit today is roughly what was sold as a 70-pint unit before 2019. These are rough correspondences: ENERGY STAR notes that the change differs from model to model, and published conversion tables do not all agree. Many older sizing charts still circulate with the old figures, which makes them look as though they recommend larger units than they do. The dehumidifier size calculator gives sizes on the current test and shows the old equivalent beside them.
Sizing for a space
Sizing guidance from ENERGY STAR and the trade association AHAM grows with area and with how damp the space is. The calculator uses a simple version:
Two basements
A damp 1,000 sq ft basement, musty with damp spots, in an average climate:
- Base: 10 + 1,000 ÷ 100 = 20 pints per day; damp + 5 = 25
- Size to buy: 30 pints per day, about 42 under the old rating
A very wet 1,800 sq ft basement with seepage, in a humid climate:
- Base: 10 + 1,800 ÷ 100 = 28; very wet + 10 = 38; × 1.1 = 41.8 pints per day
- Size to buy: 50 pints per day, about 70 under the old rating
A basement with seepage, standing water or a wet floor after rain has a water problem, not a humidity problem. Gutters, downspouts, grading and drainage come first; the dehumidifier deals with what is left.
What it costs to run
Dehumidifiers are rated for efficiency by integrated energy factor, litres of water removed per kWh. The calculator uses 1.9, typical of ENERGY STAR units on the current test:
The 30-pint unit in the damp basement, running 10 hours a day, uses about 3.1 kWh a day, which at 17 cents per kWh is $0.53 a day and about $16 a month. The 50-pint unit uses about 5.2 kWh a day, about $26 a month.
The target humidity
Dehumidifiers are set by relative humidity, and most should be set between 45 and 50%. Below 40% the unit runs a great deal for little benefit; above 55 to 60%, dust mites and mould thrive. In a basement the relative humidity reading depends on the temperature, so the dew point, which does not, is a better guide. The dew point and humidity calculator converts between them and shows whether a cold surface, such as a basement wall or a cold-water pipe, will condense.
A cold pipe in a summer basement
Air at 72 °F and 50% relative humidity, with a cold-water pipe at 45 °F.
- Dew point: 52.3 °F
- The 45 °F pipe is below it, so it condenses and drips
- It stays dry only if the air is below about 38% relative humidity
Insulating cold pipes is often cheaper than trying to dry the air enough to stop them sweating.
Too big, too small
A dehumidifier that is too small runs all the time and never reaches the setting; one that is too big cycles, which is less of a problem than for an air conditioner because a dehumidifier’s job is only moisture. Where the choice is between two sizes, the larger one usually wins: it reaches the target faster, runs fewer hours, and copes with the wettest weeks. This is the opposite of the advice for air conditioners in the guide to oversized systems.
Whole-house dehumidifiers
In hot, humid climates, a whole-house dehumidifier ducted into the HVAC system handles moisture the air conditioner leaves behind on mild, damp days, when the air conditioner barely runs. They are rated in pints per day on a different test and typically remove 70 to 130 pints. They make most sense in tight, well-insulated houses, where the cooling load is small but the moisture from people, cooking and ventilation air is not, a point the guide to whole-house ventilation returns to.
And in winter
In cold climates the problem reverses: winter air is dry, and the question becomes how much water a humidifier should add. The humidifier size calculator and the guide to humidity and comfort cover that side.