Dehumidifiers: which type, what size, and when they are the wrong answer
Buy one if your humidity stays above 60% after two weeks of proper ventilation, or if the damp space is a basement or crawl space you cannot air out.
The short version. A dehumidifier treats the air, not the cause. If water is coming through a wall, from a leak, or from drying laundry indoors, the machine will run forever and your power bill will show it.
A dehumidifier is the only device on this site that removes water from your air at a rate you can measure in gallons. Everything else — vents, fans, absorbers, habits — works in pints or fractions of pints. That makes it the obvious purchase, and it is why so many people buy the wrong one.
Three questions decide it: how cold the space gets, how big it is, and whether the moisture has a source you could remove instead.
- Typical home unit
- 20–50 pt/day
- Works down to
- 65 °F
- Running cost
- $15–40/mo
- Target setting
- 45–50%
Measured at 65 °F and 60% RH under the DOE 2019 method
Compressor. Desiccant keeps going to about 33 °F
30-pint unit, roughly 300–500 W, 8–12 hours a day
Lower settings cost more and buy you nothing
How they actually work
A compressor (refrigerant) dehumidifier is a small air conditioner folded in on itself. It chills a coil below the dew point of your room air, water condenses onto the coil and drips into the tank, and the now-dry air is reheated slightly and blown back out. This is why a compressor unit warms the room a little, and why it stalls in the cold: below about 65 °F the coil starts icing instead of draining.
A desiccant dehumidifier has no compressor. Air passes over a rotating wheel coated in a moisture-absorbing material, a heater drives the collected water off the wheel, and it condenses into the tank. Because nothing has to be chilled below freezing, it keeps working down to about 33 °F. It also puts more heat into the room, which in an unheated garage is a feature.
A thermoelectric (Peltier) mini dehumidifier uses a solid-state chip to make one side cold. It is silent and cheap and it removes 8–20 ounces a day — roughly a pint. That is a closet device. Sold as a room dehumidifier, it is a waste of money.
| Type | Output | Temperature range | Best for |
|---|---|---|---|
| Compressor | 20–50 pt | Above 65 °F | Living space, finished basement |
| Desiccant | 7–20 pt | 33–80 °F | Garage, crawl space, unheated room |
| Whole-house, ducted | 70–100 pt | Above 60 °F | Whole home in a humid climate |
| AC in "Dry" mode | Varies | Above 68 °F | Free if you already run AC |
| Thermoelectric mini | 0.5–1 pt | Above 70 °F | Closet or safe only |
When it does not work
The limits matter more than the benefits — this is where most money gets wasted.
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It will not stop water coming through a wall
Rising damp, penetrating damp and plumbing leaks all deliver liquid water faster than any dehumidifier removes vapor. If a damp patch has a defined edge, sits low on the wall, or gets worse after rain, stop and read structural damp first.
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It cannot beat a moisture source you keep feeding
Drying one load of laundry indoors releases 2–4 pints. If you run a 30-pint machine and dry two loads a day inside, most of its capacity is spent undoing that one habit.
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A compressor unit in a cold garage is dead weight
Below 65 °F it ices up and cycles into defrost more than it dehumidifies. In an unheated space you need a desiccant model, whatever the price difference suggests.
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Drying the air does not warm the wall
Mold grows where the surface is cold, not where the room average is damp. If one corner keeps growing mold while the room reads 50%, the fix is insulation and heat, not more dehumidification.
Doing it properly
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Rule out a water source first
Check under sinks, around the water heater, at the base of exterior walls, and outside at gutters and grading. A dehumidifier bought to mask a leak is money spent twice.
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Measure the space and the temperature
Square footage sets the capacity; the coldest temperature the space reaches sets the type. Write both down before you shop.
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Size it with room to spare
Use the sizing calculator. An oversized unit reaching its target and idling costs less to run than an undersized one running flat out all day.
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Set it to 45–50% and leave it
Every 5% lower costs meaningfully more electricity. Below 40% you are drying wood and skin for no health benefit.
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Plumb the drain if you can
Most units take a garden hose on a gravity drain. Emptying a tank twice a day is the single most common reason a dehumidifier ends up in the garage unused.
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Give it air on all sides
Keep it clear of walls and furniture and put it near the damp, not in a doorway. Close the windows in that room while it runs.
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Re-measure after two weeks
If the reading has not moved, the machine is losing to a source you have not found yet. Go back to step one.
Common mistakes
Buying on tank size
Tank capacity tells you how often you empty it, not how much water the unit removes. Pints per day is the only number that matters.
Setting it to 30%
It will run almost continuously, cost far more, and dry out wood floors and furniture. 45–50% is the target.
Running it with a window open
You are then dehumidifying the outdoors. Close the room while it works.
Using it instead of a bathroom fan
A shower dumps moisture in fifteen minutes. An extractor fan removes it at the source in real time and costs a fraction to run.
Common questions
What size dehumidifier do I need for a basement?
For a typical 1,000 sq ft basement that smells damp, a 30–35 pint unit under the current DOE rating is the usual answer, moving to 50 pints if there is standing dampness or the space is above 1,500 sq ft. Use the sizing calculator for your numbers.
How much does a dehumidifier cost to run?
A 30-pint unit draws roughly 300–500 watts. At 16 cents per kWh and 10 hours a day, that is about $15–25 a month. Larger units and lower humidity targets push it toward $40.
Should I run a dehumidifier all the time?
Run it on a humidistat rather than continuously. Set 45–50% and let it cycle. Continuous operation at a low target is the most expensive way to get an outcome you did not need.
Can a dehumidifier make mold worse?
No, but it can hide the cause. Drying the room air lowers the surface humidity on cold walls a little, so growth slows without stopping. If mold keeps returning in the same spot, the wall is too cold — that is an insulation problem.