Where should I put a dehumidifier in a room?

A dehumidifier has a humidity sensor on board, and that sensor reads the air immediately around the machine. Put it in a corner behind a chair and it will dry that small pocket of air quickly, read the target humidity, and switch off while the rest of the room stays damp. It is not broken and it is not undersized; it has simply been given a very small room to work on. Everything about placement follows from that: the machine needs the room's air to move through it, not past it.

What you'll need

• A power socket that does not need an extension lead across a doorway
• A hard, level surface if it runs unattended (optional)

Step by step

1. Give every grille clear space.Most manufacturers specify a minimum of about 30cm at the intake and outlet, and many more than that. Check the manual, because some models draw air from underneath and are crippled by standing on a thick rug or deep carpet.
2. Put it where the room's air will circulate through it.Roughly central, or at one end with the outlet aimed down the longest axis. Dry air thrown the length of the room travels along the far wall and returns, which is the circulation pattern you want. Air blown at a wall 20cm away is not going anywhere.
3. Do not stand it against the damp wall.A common instinct, and the wrong one. The machine dries the air; the cold wall is where the moisture in that air was condensing. Dry the room's air and the wall stops sweating, whether or not the machine is anywhere near it.
4. Shut the door.A dehumidifier set to dry one room with the door open is trying to dry the whole house through a doorway. It will run continuously, cost more and achieve less. Close the door, and if you are drying washing, close it and leave the machine to it.
5. Keep it away from radiators, heaters and direct sun.Local heat raises the temperature and lowers the relative humidity at the sensor, so the machine reads the room as drier than it is and stops early. It is the same failure as the corner, from a different cause.
6. Put it on a hard, level surface if it runs while you are out or asleep.Tanks do overflow, and a machine sitting on carpet with a full tank is a soaked carpet. A level surface also matters for the tank-full float switch to work correctly.
7. Use the continuous drain if you can reach one.Most models have a hose port. Running it to a floor drain, a sink or a condensate pump means the machine never stops overnight with a full tank, which is the most common reason a night's drying is lost.
8. For a whole house, move it rather than buying several.One machine given a full day in each problem room, door shut, does far more than one machine left in a hallway. Damp is usually concentrated in two or three rooms, and those are where it should spend its time.
9. For a cellar, garage or unheated store, place it centrally and off the floor.A cold slab chills the machine's intake and any spillage, and cold air pools at floor level. Standing it on a bench or a couple of blocks improves both circulation and the reading it takes.

Tips

• If drying washing, put the airer and the dehumidifier in the same closed room with the outlet aimed at the washing. It halves the time compared with the machine placed elsewhere in the house.
• In a room with a bad corner of mould, aim the outlet at that corner. It is not that the machine dries the wall, but the air movement stops the still, cold, saturated layer that forms there.
• Check the manual for the intake position before deciding. Bottom-intake models behave very differently on carpet from back-intake ones.
• Wall-hugging is the single most common placement mistake, and simply moving the machine a metre out often changes how much water it collects overnight.

Common mistakes

• Tucking it into a corner behind furniture — It dries the small pocket of air around itself, its sensor reads the target humidity, and it switches off while the rest of the room stays damp.
• Leaving the door open to dry the hallway too — It turns a closed room into an unbounded volume. The machine runs continuously without ever reaching its setpoint, which costs money and dries nothing properly.
• Standing a bottom-intake model on a thick rug — It blocks the intake almost completely. Airflow collapses, extraction falls to nearly nothing, and it can overheat.

If it doesn't work

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