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Sheet 07 · Heating and damp

What size dehumidifier do I need?

Condensation on the inside of the window every morning. A wee bloom of black in the corner above the wardrobe. Washing on the airer for three days and still damp at the seams. That is not bad luck, that is litres of water going into the air of a home that was never built to let it out, and the fix is a bucket with a fan.

The size of the bucket is the only decision that matters, and the shops will not help you make it, because the big one costs more. Put the rooms in. The sheet does the rest.

1Work it out

The home
How wet, and how cold
What you are looking at
Washing dried indoors?
Where it will run
20litres a day

A compressor model rated 20 L a day. In a heated home that is the cheaper kind to run, and the one the makers point you at for a 3-bedroom house.

The litres figure is what the machine pulls out at the makers’ test conditions, not what it will pull out of your bedroom in November. That is why the sheet sizes up rather than down.

Get a 20 litre compressor dehumidifier with a laundry mode (MeacoDry Arete Two) on Amazon.co.ukor search Amazon.co.uk for “20L dehumidifier laundry mode”

2What the litres mean

A dehumidifier is rated by how much water it can pull out of the air in a day under the makers’ test conditions, which are warmer and wetter than any British bedroom. A 20 litre machine in a cool flat in November might take out five. That is not a con, exactly: every maker tests the same way, so the numbers compare. It does mean the rating is for choosing between machines, not for predicting the bucket, and it is why this sheet sizes up for a damp house rather than down.

3Compressor or desiccant

A compressor machine is a small fridge: air goes over a cold coil, water drops out. Cheap to run, and it works well in a heated room. In a cold room the coil ices up and the machine spends its day defrosting, so in a garage, a conservatory or an unheated spare room it is nearly useless. A desiccant machine passes the air through a drying wheel and heats it to drive the water off. It costs more per hour, it warms the room a degree or two (in a cold room, that is the point), and it does not care how cold it is.

Meaco’s own line, and EcoAir agrees: below 10 degrees, desiccant only; between 10 and 15, a desiccant or a big compressor; above 15, anything. Heated home: compressor. Cold space: desiccant. That is the whole decision.

4Washing on the airer

A load of washing holds a couple of litres of water and every drop of it goes into the room. The makers list washing drying indoors as a sign of a very damp home and size for it accordingly, which is what the sheet does when you tick the airer: one size up. Buy the machine with a laundry setting, put the airer next to it and shut the door. The washing dries in hours, not days, and the window stops streaming.

5What the number is not

It is not a cure for a leaking gutter, a bridged damp course or a bathroom with no extractor. A dehumidifier takes water out of the air; it does not stop water getting into the wall. If the patch is wet to the touch and getting bigger, find where the water comes from first. Then buy the bucket with a fan.

6Where to get it

Those links go to Amazon.co.uk and earn this site a commission if you buy. The price is the same either way, and nothing on this sheet changes because of it.

7Where the numbers come from

  1. How to choose the best dehumidifier: capacity by home size and how damp it is, Meaco. Slightly damp: 10 L up to two bedrooms, 12 L for three, 20 L for four or five. Moderately damp: 12, 12, 20, 25. Very damp (washing drying indoors is one of its signs): 12, 20, 25, two 25s. Wet: 20, 20, two 25s (read 17 October 2026)
  2. Which dehumidifier for a flat, Meaco. A 10 L for a one or two bedroom flat, a 12 L if the problem is serious or there are more than two people (read 17 October 2026)
  3. Which dehumidifier for a two bedroom house, Meaco (read 17 October 2026)
  4. Which dehumidifier for a 3, 4 or 5 bedroom house, Meaco. 12 L for a three-bedroom house, 20 L for four, at least 25 L for five or more (read 17 October 2026)
  5. When to buy a desiccant dehumidifier and when to buy a compressor dehumidifier, Meaco. Below 10 °C desiccant only; 10 to 15 °C desiccant or a large compressor; above 15 °C either (read 17 October 2026)
  6. Desiccant vs compressor dehumidifiers, EcoAir. A desiccant machine is recommended wherever the room is likely to fall below 15 °C (read 17 October 2026)

Data on this sheet last changed 17 October 2026.