Why a dehumidifier stops collecting water or won't lower the humidity — cold rooms, settings, filters, tanks and open windows — and how to fix each one.
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In UK homes a very common cause is a cold room — compressor dehumidifiers collect much less water below about 15°C and may keep stopping to defrost. Next, check the humidity setting isn't above the room's humidity, the tank is seated properly, the filter is clean and doors and windows are closed. If the compressor never starts or the outlet air isn't slightly warm, it may be a fault.
A dehumidifier that hums away all day while the tank stays empty is frustrating — but it’s rarely broken. Work through these causes in order; the first two explain most cases in UK homes.
A very common cause in UK homes. Compressor dehumidifiers condense water on a cold coil. In a room below about 15°C there’s less moisture to condense, and below about 10°C the coil can frost over, so the unit keeps pausing to defrost.
Fix: warm the room slightly — even a few degrees helps — or use a desiccant dehumidifier, which isn’t affected by cold. See desiccant vs compressor.
A dehumidifier only collects water when the room is above its target humidity. If it’s set to 60% and the room is at 55%, it will run the fan but not dehumidify.
Fix: check the room with a separate hygrometer on Amazon (opens in a new tab) (paid link). If it’s genuinely above 60%, lower the setting to around 50%. If it’s already 45–55%, the dehumidifier is doing its job — there just isn’t much water to collect.
Most units stop when the tank is full or not pushed fully home — the float switch thinks it’s full.
Fix: empty the tank, check the float moves freely, and push the tank firmly back in until it clicks.
A dusty filter cuts airflow across the coil, so less water is collected.
Fix: clean or wash the filter (most are washable) every couple of weeks in regular use.
The unit is trying to dry the whole house — or the outside air.
Fix: close the windows and the door of the room you’re drying. See where to put a dehumidifier for when to leave doors open.
If you use continuous drainage, water may be draining away (so the tank looks empty) or backing up.
Fix: check the hose runs downhill all the way, with no kinks or dips, and isn’t blocked.
A small unit in a large, open-plan space — or with daily laundry — may run constantly without making a dent.
Fix: dry one room at a time with the door closed, or size up. Our size calculator gives a starting point.
If the room is warm (18°C+) and humid (65%+), the setting is low, and after an hour or two the outlet air isn’t slightly warmer than the room and no water appears, the compressor or refrigerant may have failed.
Fix: check the manual’s troubleshooting section, then contact the manufacturer — many dehumidifiers have two-year or longer warranties.
Cold rooms
The usual answer when a compressor model struggles in a cold bedroom or box room.
Compare options on Amazon (opens in a new tab)Paid linkHow we choose: these are types of product matched to the criteria in this guide, not individually tested models. Check current specifications and reviews before you buy.
When a working, correctly sized dehumidifier can’t keep up, there may be more moisture coming in than normal living produces — a leak, rain penetration or rising damp. A dehumidifier can’t fix those. See how to tell condensation from damp.
Most often because the room is too cold for a compressor model, the air is already fairly dry, or the humidity setting is higher than the room's current humidity. A blocked filter, a tank that isn't fully seated, or doors and windows left open are the next things to check.
Usually the space is too big for the unit (doors open to the rest of the house), windows are open, there's a strong moisture source such as laundry drying, or the room is too cold for a compressor model. Check with a separate hygrometer — the built-in sensor can be inaccurate.
Compressor dehumidifiers become much less effective below about 15°C and may ice up and defrost repeatedly below about 10°C. Desiccant dehumidifiers keep working in cold rooms, garages and conservatories.
In a warm, humid room with the humidity set low, a compressor model should hum, blow slightly warm air and collect some water within an hour or two. If the fan runs but the air is cold and no water appears in warm conditions, contact the manufacturer — it may be a compressor or refrigerant fault, often covered by the warranty.
Where to place a dehumidifier, whether doors and windows should be open or closed, what humidity to set, and how long to run it for condensation, damp and laundry.
How desiccant and compressor dehumidifiers work, why room temperature matters so much in UK homes, and which type to buy for each room.
The humidity level to aim for in a UK home, what's too high or too low, how it changes room by room and season by season, and how to measure it properly.
