Buying a Dehumidifier for Damp: the Two Numbers That Decide It
The type you need is decided by how cold the room gets, and the size you need is not the number on the box — a 20-litre machine puts out about eight litres a day in a real British house. Get those two right and the rest is detail.
The short answer
If the room drops below about 15°C — a garage, a conservatory, an unheated bedroom, a caravan — buy a desiccant. Compressor models lose performance as it gets colder and one manufacturer notes they may stop working altogether around 5°C as the coils freeze. Below 10°C, desiccant is the only sensible answer.
If the room is heated and normally sits above 15°C, buy a compressor. It will cost roughly half as much to run — about 4 to 5p an hour against 9p — and that gap compounds over a winter of daily use.
Then ignore the headline litres and size up. The number on the box is measured at 30°C and 80% humidity. A British living room is nearer 20°C and 60%. Meaco’s own published graph shows its 20-litre machine delivering about 8 litres a day in those real conditions — roughly 40% of the box figure.
And one thing before you spend anything: a dehumidifier treats condensation. It does nothing about the cause of penetrating or rising damp. If water is coming through a wall, this is not the purchase you need.
Compressor or desiccant
| Compressor | Desiccant | |
|---|---|---|
| Works down to | Rated 5–35°C on the models we checked. Performance falls off as it cools, and one manufacturer says compressors may stop entirely around 5°C as the coils freeze | Rated 1–40°C. Works in a cold garage or an unheated room |
| Manufacturer’s own guidance | Above 15°C, anything works. 10–15°C, only a large 20–25L model | Below 10°C, desiccant only |
| Power draw | 150–260W at 20°C and 60% humidity | About 330W at the same conditions |
| Running cost | About 4p an hour for a 12L model, on the manufacturer’s own figure | About 9p an hour for an 8L desiccant |
| How it is rated | Headline litres measured at 30°C / 80% — much warmer and wetter than a UK house, so real output is far lower | Often rated honestly at 20°C / 60%. One 8L desiccant quotes 7.5 L/day at real conditions |
| Noise | 35 dB on the manufacturer’s figures | 39 dB — louder, not quieter, on the same manufacturer’s figures |
| Buy it for | A heated house used most days through winter | Garages, conservatories, caravans, unheated rooms, and drying washing in a cold house |
Figures are from manufacturer specifications and UK consumer titles, checked on 1 September 2026. The temperature thresholds and the running-cost comparison are Meaco’s own published guidance; the 5°C cut-out is from Homebuilding & Renovating.
Why the litres figure lies to you
Extraction capacity is quoted in litres per day, and buyers reasonably assume a 20-litre machine removes 20 litres a day. It does — at 30°C and 80% relative humidity, which is the benchmark the figure is measured against. That is a warm, wet test condition, and it is nothing like a British house in February.
At a realistic 20°C and 60%, Meaco’s own published graph shows its 20-litre model delivering about 8 litres a day. That is roughly 40% of the headline. Nobody is lying; the number is just measured somewhere you do not live.
Two consequences. First, size up — the capacity bands consumer titles recommend (7–12L for a flat, 14–16L for a family house, 18L and above for a serious damp problem) already assume this. Second, and less obviously, you cannot compare a compressor and a desiccant on their headline numbers, because desiccants are often rated at the honest condition. An 8-litre desiccant quoting 7.5 L/day at 20°C/60% and a 20-litre compressor quoting its figure at 30°C/80% can do a broadly similar job. Comparing 8 against 20 tells you nothing.
What a dehumidifier will not fix
This is the part most buying guides skip, and it is the part most likely to save you money. Government guidance distinguishes condensation, penetrating damp, rising damp and traumatic damp from a leak or a flood. A dehumidifier addresses condensation. It does not address the cause of the others.
If damp is coming through a wall from outside, or up from the ground, a dehumidifier will run constantly, cost you money and never fix it. At best it speeds up drying after the actual fault has been repaired — which is how one local authority describes its role following a leak.
The reassuring part: condensation is the most common cause of household damp, and the Centre for Sustainable Energy describes rising damp as the least likely one. So for most people this is the right purchase. But diagnose before you buy, and if damp is patchy, tide-marked, or on a wall that backs onto the outside rather than a cold corner or a window, get the cause looked at first.
Damp and mould are a health matter as well as a comfort one — government guidance was updated in April 2026 and covers the health effects and who is most at risk. If a landlord is involved, that guidance is the thing to read.
The features worth paying for, and the ones that are marketing
A humidistat is the one that matters. It lets the machine hold a target humidity and switch itself off, rather than running flat out for eight hours. The Centre for Sustainable Energy puts the sensible range at 40–60% relative humidity, with mould risk rising above 60%. Without a humidistat you are paying to over-dry a room.
Continuous drainage is genuinely useful and slightly oversold. A hose outlet lets you run to a drain instead of emptying a tank — worth having in a garage or a utility room. Note that on both models we checked, the hose is not in the box.
Tank size decides how often you are involved. If you will not be there to empty it, either use continuous drainage or buy a bigger tank; a machine that stops when full has stopped working.
HEPA filtration is a consumable, not a feature. One manufacturer sells replacement H13 filters at £36 for three. Factor that in before it sways the decision.
Smart control is the weakest of the lot. Both UK titles that tested the apps we looked at were unimpressed. If the machine has a humidistat, the app is solving a problem you do not have.
What actually goes wrong
Noise in bedrooms. Manufacturer figures of 35–39 dB sound modest on paper and are not modest at 2am in a quiet room. If it is going in a bedroom, that is the specification to check hardest — and note that the common assumption that desiccants are quieter is not supported by the numbers we could find. On one manufacturer’s own figures the desiccant is the louder of the two.
Cold-room performance. The single most common mismatch: a compressor bought for a garage, doing very little all winter.
Filter cost over time, and the running cost of laundry mode, which is where the electricity actually goes.
One useful thing from the retailer listings: price does not predict satisfaction. On 1 September 2026 a £79.99 12-litre model held 4.5 out of 5 from 185 buyers at one retailer, while a £239 machine held 3.6 from 106 at the same retailer. Spending more is not the same as buying better.
What it actually costs to run
| What | Figure | What it means |
|---|---|---|
| Electricity unit rate | 26.32p per kWh | Ofgem price cap, direct debit, for 1 October to 31 December 2026. Standing charge 54.83p a day on top, which you pay whether the machine runs or not. |
| Compressor, 20L | 207W → 5.4p an hour | Run eight hours a day for ninety days and that is about £39 for the winter. |
| Budget compressor, 12L | 350W → 9.2p an hour | Cheaper to buy, nearly twice the running cost of the better 20L unit. This is where the false economy lives. |
| Desiccant, 8L | About 330W → 9p an hour | The price of working in the cold. Worth it when a compressor would barely work at all. |
| Laundry mode | Up to 650W → 17.1p an hour | One UK title metered laundry mode drawing about five times the energy per hour of normal dehumidifying. Still usually cheaper than a tumble dryer, but not free. |
| Typical price to buy | £99–£189 for most | Range across Currys, B&Q and Screwfix on 1 September 2026 runs from about £32 to over £1,000, but the bulk of the market sits in that band. |
Before you buy, check
- How cold does that room actually get? Not the house — the room. Below 15°C, desiccant. Below 10°C, desiccant without question.
- Does it have a humidistat? If not, put it back.
- What is the wattage, not the litres? That is what you will pay for. Multiply by 26.32p to get pence per hour.
- Where will the water go if you are out all day — a big tank, or a hose to a drain? And is the hose included?
- What do replacement filters cost, and how often?
- Have you diagnosed the damp? Condensation on windows and cold corners, yes. Tide marks, patches on external walls, or damp low down a wall — get the cause looked at before you buy anything.
Where this came from
- Ofgem The energy price cap unit rate used for every running-cost figure here — 26.32p per kWh for 1 October to 31 December 2026 · checked 1 September 2026
- Meaco Manufacturer guidance on the temperature thresholds, the published real-conditions output graph, wattages, noise figures and filter prices · checked 1 September 2026
- GOV.UK damp and mould guidance The definitions of condensation, penetrating and rising damp, and the health effects · updated 1 April 2026
- Centre for Sustainable Energy The 40–60% target humidity range, the mould threshold, and the point that rising damp is the least likely cause · checked 1 September 2026
- Homebuilding & Renovating The 5°C figure at which compressor coils can freeze and the machine stop working · checked 1 September 2026
- Currys, B&Q and Screwfix Prices and buyer ratings with sample sizes, used for the price bands and the price-does-not-predict-satisfaction point · checked 1 September 2026
What we could not check
- We could not find a named BS, EN or ISO standard for the 30°C/80% test condition. It is the benchmark one manufacturer states it uses, and it is widely used, but we are not going to call it a legal standard when we could not verify that.
- The 40% real-world derating is verified for one manufacturer only. It may not transfer to every budget brand, so treat it as the right order of magnitude rather than a universal conversion.
- Which? Best Buy models and scores are behind a paywall, and that organisation publishes noise as a star rating rather than in decibels, so we could not use it for the noise comparison.
- The Good Housekeeping Institute site refused our requests, so none of its testing is reflected here.
- We have no verified-buyer review text. The retailer review pages we tried returned errors, so every complaint theme above comes from consumer-title testing and ratings summaries — none of it should be read as customer quotes.
- The Energy Saving Trust publishes no dehumidifier guidance at all, so nothing here is attributed to it. A tumble-dryer running-cost comparison we found was second-hand and used a stale unit rate, so we have left it out.
- Auto-defrost effectiveness is unverified — one spec sheet we checked does not even list it. Argos is under-represented in the price bands because its category page would not load.
Prices and specifications change constantly. Every figure here is dated — check the one in front of you before spending.