How much water should a dehumidifier collect a day?

Why collection is lower than the rating

Litres a day ratings are measured at warm, very humid conditions, typically around 30 °C and 80 % relative humidity. A UK living room in winter is more like 18 to 21 °C at 50 to 70 % RH. Cooler, drier air holds much less water, so there is less for the unit to remove. Our explainer on extraction ratings covers the test conditions.

Once the room reaches the target humidity on the humidistat, the unit switches off or slows down, so collection drops further.

What is normal

There is no single normal figure, but some patterns are typical. Collection is highest in the first few days in a damp home, and when drying washing. It then falls as the room dries and settles at a lower level. It is usually higher in autumn and spring, when outdoor air is mild and moist, than in a cold, dry spell in mid-winter. A compressor unit in a cold room collects much less than in a warm one.

Temperature and type

Compressor dehumidifiers rely on a cold coil, and their output drops noticeably in cooler rooms. In an unheated room in winter, a large compressor unit may collect very little. Desiccant units hold their output much better in the cold. If you are drying a cold garage, cellar or conservatory and collection is low, the type may be the issue; see desiccant vs compressor.

How much can it lower humidity?

A dehumidifier lowers relative humidity in the space it serves to the target you set, as long as it is big enough for the space and moisture load. In a closed room that is often achievable down to around 40 to 50 %. In an open-plan home with lots of moisture sources, a small unit may struggle to bring readings down. Use a hygrometer to check, and see what humidity to set a dehumidifier to for sensible targets.

If readings will not fall, try closing doors, moving the unit to a more central spot or sizing up; see what size dehumidifier you need.

When low collection means a fault

If the room is clearly damp, warm enough and the humidity reading is well above the target, but the tank stays almost empty, something may be wrong. Common causes are a dirty filter, the humidistat set higher than the room reading, ice on the coil, or a refrigerant fault. Our guide on a dehumidifier not collecting water walks through the checks.

1. 1

   Check the room's humidity on a hygrometer

2. 2

   Make sure the target is below that reading

3. 3

   Clean the filter

4. 4

   Look for ice on the coil

5. 5

   Feel for warm air from the outlet

6. 6

   Contact the maker if it still collects nothing

Compare the best dehumidifiers for UK homes →

Tracking collection usefully

Note how much is in the tank each day for a couple of weeks, along with the room humidity reading. Falling collection with falling humidity shows the unit is doing its job. Collection that jumps up when you dry washing or after a wet spell is normal. This simple log also helps you choose the right humidity target and running hours, which affects your running costs.

Frequently asked questions

Is it normal for my dehumidifier to collect only a litre a day? Often yes, especially once the room has dried or in cold, dry weather. Check that humidity is at or near your target.

Why did collection drop after the first week? Because the first days removed the built-up moisture from the air, furniture and walls. Lower collection means the room is drier.

How much water does a 20L dehumidifier collect? In a heated UK home, usually a fraction of 20 litres. The rating is measured at warm, very humid test conditions.