A useful tool. Almost never the real answer.

Refrigerant dehumidifiers (the heavier, fan-driven ones) work like a fridge: warm moist air passes over a cold coil, water condenses out, drier air comes out the back. They're efficient above ~15°C and lose performance as the room gets colder. The right tool for a damp summer basement or a poorly ventilated bathroom.

Desiccant dehumidifiers use a rotating zeolite wheel that physically adsorbs moisture, with a small heater to regenerate it. They work down to ~1°C with no performance drop, but use more electricity per litre extracted at normal room temperatures. The right tool for cold rooms, garages, holiday homes and winter use.

Most UK households buy refrigerant by default. Most UK damp problems are worst in winter, when refrigerant units underperform. The mismatch is why so many dehumidifiers underwhelm.

Dehumidifier capacity is quoted in litres per day, measured at 30°C and 80% humidity. UK rooms at 18°C and 60% humidity get roughly half that rated capacity. A '20L/day' unit will pull 8–10L/day in real conditions.

Right-size rules of thumb: a 12L/day unit suits a 1-bed flat or single damp bedroom; a 20L/day unit suits a 2-bed flat or 3-bed terrace with one damp area; a 25L/day-plus unit is for whole-house dehumidification or active drying after a leak. Anything bigger is industrial and unjustified for a normal home.

A typical 20L/day refrigerant unit draws 300–500W when running. At 8 hours a day and a UK electricity rate of 28p/kWh, that's £0.70–£1.10 a day, or £20–£35 a month if you're running it that hard year-round. Not catastrophic, but not free.

A desiccant unit at the same extraction rate draws 600–800W; nearly double the electricity. The trade-off is that it works in conditions where the refrigerant unit barely extracts anything. Pick the right type and you'll spend less, not more.

Compare against the alternative: a £700 PIV install pulls 3–5W continuously and prevents the humidity ever climbing high enough to need extraction. Over a five-year window, PIV wins on running cost in almost every leaky-house scenario.

If you dry clothes indoors (winter, no garden, no tumble dryer), a small dehumidifier running in the room is the single cheapest way to do it. Faster than a heated airer, dramatically cheaper to run than a tumble dryer, and the extracted water is the moisture that would otherwise end up condensed on your bedroom windows.

Pair the dehumidifier with a closed door, run it for 4–6 hours, and the clothes come out dry without any of the humidity going into the rest of the house. This is the use case dehumidifier sales reps should lead with.