Why is my dehumidifier not collecting any water?

Six causes in order of likelihood, most of which are not faults at all — the room being colder or drier than the machine can work in accounts for the great majority of empty tanks.

Difficulty: beginner
Time: 25 min
Read: 6 min

Short answer

Check the room temperature first. Below about 15 degrees a compressor model extracts very little and below 10 it barely works at all, which is not a fault. Then check the room is not already at or below the humidistat setting, that the filter is not clogged, that the tank and float are seated correctly, and that the coil has not iced up.

An empty tank is the most common dehumidifier complaint and it is usually the machine behaving exactly as designed. A dehumidifier can only collect water that is in the air, and only at a rate the physics of its own mechanism allows. If the room is cold, if it is already dry, or if the airflow through the machine has been restricted, then no water is a correct outcome rather than a fault. The genuine faults are further down the list and are recognisable by different symptoms.

What you'll need

A room thermometer, or a cheap humidity and temperature monitor
The machine's manual (optional)

Step by step

Check the room temperature, and note the machine type.If it is a compressor model and the room is below about 15 degrees, extraction will be poor; below 10, close to nothing. That is inherent to the design and no amount of adjustment fixes it. A desiccant model in the same room should still work, so if that is what you have, move on down the list.
Check the actual humidity against the setting.If the room is already at or below the humidistat target, the machine is correctly doing nothing. A cheap hygrometer settles this in seconds. In a normally dry heated room in summer, a dehumidifier will collect very little because there is very little to collect.
Take the filter out and look at it.A clogged filter throttles the airflow, and a dehumidifier that cannot move air through itself cannot extract from it. Most have a washable mesh at the intake that wants rinsing monthly. This is the most common genuine cause.
Check whether the coil has iced over.Look through the grille on a compressor model. A white frost layer on the fins means it is icing rather than condensing, and the machine will be cycling through defrost. Switch it off, let it thaw completely, and move it to a warmer room or accept it is the wrong machine for that space.
Confirm the tank is seated and the float is free.Most machines refuse to run without the tank correctly home, and many display a tank-full warning if a float sticks in the up position. Remove the tank, check the float moves freely, wipe the sensor contacts and reseat it firmly.
Check the continuous drain hose if one is fitted.If a drain hose is connected, the tank will stay empty because the water goes down the hose. Follow it and check it runs downhill throughout with no kinks or trapped loop, and that the end is not submerged or blocked. An unnoticed drain hose accounts for a surprising number of empty-tank reports.
Confirm the compressor is actually running.On a compressor model, the fan runs constantly but the compressor cycles. Put a hand near the exhaust: air noticeably warmer than the room means the compressor is working. Air at room temperature with the fan running and the humidistat calling for extraction points at a compressor or refrigerant fault, and that is a repair, not a DIY job.
Give it clear space and shut the door, then judge it again.A machine in a corner behind furniture dries its own pocket of air and switches off. Move it into clear floor, aim the outlet down the room, close the door and give it a few hours before concluding anything.
Reset by unplugging it for a few minutes.Many models will not restart the compressor for several minutes after switching off, to protect it. If you have been switching it on and off testing things, leave it plugged in and untouched for ten minutes before deciding it does not work.
Judge a genuine fault by the pattern.No water in a cold or dry room is normal. No water in a warm, visibly humid room with a clean filter, clear space, a running compressor and no drain hose connected is a fault worth a repair call — most likely a refrigerant leak or a failed humidity sensor.

Tips

A cheap hygrometer is the most useful thing you can own alongside a dehumidifier. It turns every argument about whether it is working into a reading.
In summer in a normally heated house, very little water in the tank usually means the house is fine, not that the machine is broken.
If it collects well for the first two days and then very little, that is the correct pattern — the room's fabric had moisture in it, and now it does not.
Rinse the intake filter monthly. It costs nothing and it is the most common fixable cause of poor extraction.

Common mistakes

Assuming an empty tank means a fault — A dehumidifier can only remove water that is present in air warm enough for its mechanism. In a cold or already-dry room, an empty tank is the correct result.
Running a compressor model in an unheated garage and expecting results — The coil frosts instead of condensing, and the machine spends its running hours defrosting. That room needs a desiccant model.
Forgetting a drain hose is connected — The tank stays empty by design. Several hours of concern usually end with the discovery of a hose running into a nearby sink.

If it doesn't work

Ice on the coil

Cause: Room too cold for a compressor model, or airflow restricted by a clogged filter — Fix: Switch off and let it thaw fully, clean the filter, and move it somewhere warmer. If the room cannot be warmer, it needs a desiccant machine.

Runs constantly, tank never fills, room still damp

Cause: An active moisture source, or an open door — Fix: Close the door and look for the source: a leak, unvented showering, drying washing, or damp arriving through a wall rather than in the air.

Fan runs but the exhaust air is not warm

Cause: Compressor not running — thermal protection, a start fault or lost refrigerant — Fix: Unplug for ten minutes and retry once. If the exhaust never warms with the humidistat calling, it needs a repair rather than further testing.

Questions people ask

Why is my dehumidifier not collecting water in winter?

Almost certainly because the room is too cold for a compressor model. Below about 15 degrees extraction falls away sharply, and below 10 it is minimal because the coil frosts instead of condensing. A desiccant model works at those temperatures; a compressor one does not.

How much water should a dehumidifier collect in a day?

Far less than the box says, because that figure is measured at 30 degrees and 80 percent relative humidity. In a normal heated British room expect roughly a third to a half of the rated litres, less as the room dries, and very little once it has reached the target humidity.

Is it worth repairing a dehumidifier that has stopped extracting?

If it is out of warranty and the fault is refrigerant or the compressor, usually not — those repairs approach the price of a new machine. If it is a filter, a sensor, a fan or a float switch, it is usually cheap. Get the fault identified before deciding.