By unit size

30 pint dehumidifier: what it actually covers

A 30 pint unit removes 30 pints of water in 24 hours under test conditions. The area it covers depends entirely on how wet the space is, so the same unit is right for a large dry basement and too small for a wet crawl space half that size.

Coverage by condition

Rounded down to the nearest 50 sq ft. Fixed additions for a vented crawl space, a sump pit, or a bare dirt floor come off the top of this and can take 100 to 400 sq ft out of the figures above.

See AlorAir crawl space and basement units

The rating is a test number, not a promise

Capacity is measured at a fixed temperature and humidity. Run the same unit in a 50 F crawl space and it will pull far less than the label says, because cold air holds less water for the coil to condense out. This is the single most common reason a correctly sized unit appears to fail: it was sized on the box number and installed somewhere the box number does not apply.

When to go one size up

• The space runs below 60 F for months at a time.
• There is standing water, a sump pit, or bare earth with no vapour barrier.
• You want the unit to hold humidity through a wet week, not just an average one.

When to go one size down

• The space is sealed, insulated, and already reads under 60% most of the year.
• You are running it in a conditioned basement that the house heating already dries out.