Should I buy a compressor or a desiccant dehumidifier?

A compressor dehumidifier is a refrigerator that never makes anything cold except a coil. It chills that coil below the dew point, room air is drawn over it, moisture condenses onto it and drips into a tank. It works brilliantly when the room is warm enough that the coil sits above freezing, and progressively worse as the room cools, because below a certain point the condensate becomes frost and the machine has to stop and melt it off. A desiccant dehumidifier has no refrigeration at all. Air passes over a rotating wheel coated with a moisture-absorbing material, and a small heater then drives the collected water off the wheel into a condenser. That process does not care how cold the room is, which is the whole of the difference.

What you'll need

• The typical temperature of the room you will use it in
• The room's rough floor area or volume
• Whether it will be moved between rooms or left in one place (optional)

Step by step

1. Measure or estimate the room temperature when the machine will be running.Not the temperature when you are in the room — the temperature overnight or while the house is empty, when a dehumidifier usually does most of its work. A bedroom at 16 degrees overnight and a garage at 6 degrees are different decisions.
2. Apply the 15-degree rule.Above about 15 degrees, a compressor model extracts efficiently. Between 10 and 15 it works but noticeably less well. Below 10 it spends much of its time in defrost cycles and extracts very little. A desiccant is close to unaffected across the whole range.
3. Compare running cost the right way round.A compressor model typically draws 200 to 400 watts, a desiccant 500 to 700, so in a warm room the compressor is roughly half the cost per litre removed. In a cold room the compressor removes almost nothing, so its lower wattage is irrelevant and the desiccant is the cheaper machine per litre.
4. Factor in the desiccant's heat output, in both directions.A desiccant returns air one to three degrees warmer than it drew it in, because of its internal heater. In a cold garage or a chilly hallway that is a genuine bonus. In a bedroom in summer it is a nuisance.
5. Compare the noise if it will run where people are.A compressor cycles on and off with an audible click and a low hum, which some people find intrusive at night and others do not notice. A desiccant is a steadier fan noise with no compressor start. Check the decibel figure on the label if it is going in a bedroom.
6. Consider the weight if it moves around the house.Compressor models carry a compressor and refrigerant and are heavier, typically 10 to 15 kilos. Desiccants are commonly 6 to 8. If it goes up and down stairs weekly, that matters more than the specification.
7. Read the litres-per-day rating as a comparison, not a promise.Extraction figures are quoted at 30 degrees and 80 percent relative humidity, conditions no British house ever reaches. Expect roughly a third to a half of the stated figure in a normal room. Compare two machines by their ratings, but do not plan around the number itself.
8. Look for a humidistat, a continuous drain option and a laundry mode.A humidistat is what stops it running once the room is dry and is the single feature that most affects running cost. A continuous drain hose port means it never stops overnight with a full tank. Laundry mode simply runs it hard without cycling, which is what drying washing needs.
9. Check the low-temperature behaviour in the specification if you are still choosing a compressor.Better compressor models have an automatic hot-gas defrost that clears frost quickly rather than just stopping and waiting. If the machine is destined for a room around 12 to 15 degrees, that feature is what makes it usable.

Tips

• If it is for a garage, shed, caravan, boat or an empty holiday property, the answer is desiccant and there is very little to weigh up.
• If it is for a normally heated house and will mostly run in living rooms and bedrooms, the answer is compressor and the running cost difference is real.
• A desiccant's warm exhaust makes it the better choice for drying washing in an unheated room, where a compressor barely functions.
• Whichever you buy, the humidistat setting matters more to the bill than the type does. Set it and let it cycle rather than running it flat out.

Common mistakes

• Buying a compressor model for a cold garage — It will frost its coil, run defrost cycles, and remove far less water than the box implies while still drawing power. This is the most common mismatch and the most disappointing.
• Choosing on litres per day alone — That figure is measured at 30 degrees and 80 percent humidity. Two machines with the same rating can perform completely differently in a 14-degree British bedroom depending on their type.
• Buying either type to fix rising or penetrating damp — A dehumidifier removes moisture from the air. Water arriving through a wall or up from the ground keeps arriving, and the machine simply runs forever.

Alternatives

• Better ventilation first: Extractor fans that actually work, and opening windows wide for short periods, cost nothing to run and address the source. Worth exhausting before buying a machine.
• A small desiccant crystal tub: For a wardrobe, a caravan over winter or a single damp cupboard, a passive moisture trap costs very little and needs no power. It is hopeless for a room.
• A heated airer plus ventilation: If the only reason you want a dehumidifier is drying washing, compare the two properly rather than assuming.

Questions people ask