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When should I use one?

When you need to reduce water vapour produced by showers, cooking or in bedrooms.

When damp is bad enough that it will take a while for the house to dry, and mould would present a health risk in the meantime.

When other practical measures have failed, or it is known they will not work.

When damp is severe.

In preference to external venting fans when the fresh air is not required. Fans have a higher total cost of ownership than a dehumidifier, due to throwing heat out of the building.

Deuhmidifiers are the standard method to dry a house after a flood

Dehumidifiers combat condensation, mould & rot. In most cases these are better tackled by sorting out the cause of the problem, but a minority of causes are intractable or impractical to resolve, and a dehumidifier can dramatically improve the situation.

A dehumidifier is a simple way to make damp cellars habitable.

Dehumidifiers are sometimes used for drying timber. This can be of use for species such as holly, which require quick drying. It can also be of use for drying large quantities of timber under cover.



Most dehumidifiers are compressor based. A refrigeration circuit causes condensation in the machine, drying the air. By the time ambient temp drops to around 12C they've pretty much stopped working.

200w domestic and 400w commercial machines are common.

Dessicant wheel

Higher price and about double the energy use of a compressor type, but these work well at all temps.


Small very cheap dehumidifiers use peltier junction coolers. Much less energy efficient means far less extraction. Designed to dry one room only. Peltiers don't like switcing on & off, so they run continuously with no RH control, which can damage woodwork if left running too long once the room has dried out. Although being very compromised machines they are effective for one-off drying out a plumbing leak & similar apps.

Lime chips

Fine for a closed cabinet, useless for a room. The chips need renewing periodically.


===Humidistat===RH control options: No humidistat control - only seen on very low end machines eg peltiers & old machines. Wastes energy & money, risk of woodwork damage.

Fixed setpoint - likely to be the wrong setpoint & waste money

Adjustable but no numbers on dial - who knows what it's set to. Often wastes money

RH numbered dial - sensible

Computerised choice - tend to be more accurate than numbered dials. But they tend to reset to default value every time power is lost.

Mechanical humidistats use a strip of plastic film that expands and contracts, operating a switch. They have large hysteresis and rely on the right speed of airflow to counter this, unfortunately the airflow speed isnt always suited to the sensor. They're prone to misbehave - a whack sorts that out temporarily. Prone to a large amount of drift over time. Electronic humidistats provide much better control.

Tank size

Small condensate tanks need more frequent emptying.

Drain tube

Enables machine to drain continuously, eliminating manual emptying. Typically slides on in place of the tank.

Condensate pump

Enables water output to be pumped to a point higher than the dehumidifier's tank.

Fan noise

Varies by model from intrusive to completely silent. An often effective workaround for a noisy machine is to run it when you're not using the room, on a timer.


Some machines have mouldproof tanks.

Air filters

These are quite common. By preventing muck landing on the condenser they prevent mould growth there. The filter must be cleaned out of course.

Metal v plastic cabinet

Metal's more robust and shouldn't yellow. It can rust though.

Fan control

Fans on some machines run all the time, some switch off when not needed. 1 & 2 speed fans are both common.

Fans always on waste energy & money. This tends to be the only option on machines with a filter. This can be fixed by adding a relay to turn the fan off whenever the compressor isn't running. A snubber on the switch contacts is a good idea (0.1uF class X2 & 100ohms 400v in series).

Automatic computerised fan speed selection works well in many cases, but often the settings are not suitable & not user overridable. Such machines can be unusable in bedrooms. Manual speed select works better in most situations.

2 speed fans give a slower quieter option, with a lower extraction rate sufficient for most household jobs.

Computer control

Can employ more complex algorithms than just switching on & off. These have some advantages but realistically just switching off when RH falls or the evaporator ices is usually fine, making computerisation of small benefit. More complexity can make the UI experience frustrating.

RH meter

I very much recommend a display of the current RH, though most machines omit this. Very cheap RH meters are available from China. If you know the RH you know what the humidistat is set to & whether to change the setting, you get to find out what RH will keep the space symptom free, and thus end up using less electricity than working blind. The saving far outstrips the cost of a meter.


Clothes drying mode. Typically this bypasses the humidistat, so it runs nonstop. A surprising amount of money & energy can be wasted this way, with almost no benefit. Clothes dry fine on standard setting. Setting the fan to high speed is a minor benefit if the clothes are close.

Senseless mode. I have seen mode names & instructions that make no sense, with an algorithm I can see no use for. More isn't always better.


Machines with basic controls tend to be more reliable than computerised.

Ions, ozone etc

These do have some mould inhibition ability, so can enhance the machine's action slightly. Whether they're worth paying for is a matter for debate - probably if you have a big mould problem. They're cheap to buy separately.


Removes dust, saving a little on cleaning time. But filtered machines tend to run the fan nonstop. A 10w fan costs in the region of £30 a year to run in 2022.


In most cases I've not found brand to be much of an indicator of anything. There are a few exceptions.


Peltier based mini-dehumidifiers have dire energy efficiency & should be avoided.

There are 2 styles of refrigeration type heat exchanger, fine & coarse. I've noticed the fine ones seem more effective & more energy efficient. An air filter is a must with fine ones, don't run it without or it will clog.


Most machines' innards are accessible without difficulty, but some are problematic. Simple controls are more often fixable than computer systems. Some have replacement boards or controllers & other parts available.


Not normally an issue, but I've found one model where one of the knobs can be pulled off with fingernails, exposing a live fixing nut.

Flat top

Enables other things to be stacked on the unit.

Which model should I choose?

A standard compressor based unit is good for most tasks. For use in cold locations a desiccant wheel unit is needed, as compressor dehumidifiers are ineffective below about 12 deg C. Do not use the £5 units that are simply boxes of lime chips, these have so little effect as to be pointless.

For small and medium houses where damp is not severe, the smaller 200w models normally extract enough water. Note that a dehumidifier's water extraction rate is normally specified under different conditions than it sees in its end use, and real world extraction rate can be much less than specs indicate.

A humidistat is important, otherwise the RH is uncontrolled. This can result in woodwork warping, wasted electricity, and no ability to respond to the changing vapour load of cooking, shower use & occupancy. Humidistatic units cost more than the cheapest dehumidifiers, but this feature is a safeguard to avoid wood damage.

A mouldproof water tank is a good idea. Older units may lack this feature, and some types of mould can pose a health problem.

The ability to connect a continuous drain hose occasionally gets used, so this facility is a minor plus point even if you don't plan to use it initially.

Units with water tanks under a gallon are best avoided, as they require unnecessarily frequent emptying. A gallon tank should fill in anything from 1 day in severe damp to several days when used to dry a shower room.

How should I use it?

If using it to reduce overall house RH, get it out of the danger zone then reduce RH slowly and gradually. Wood dries out slowly, and quick large air RH reduction would cause a humidity gradient between the inside and outside of all woodwork. This can cause woodwork warping & cracking.

If using it to dry out shower rooms, set it so it just fails to come on during dry conditions in the room. An increase in RH should trigger it, and it will keep going until RH is back down to a normal dry level. 65% should be good.

If using it to dry the whole house, its best to put it in the middle of the house to minimise the distance of any location from the unit, and thus maximise effectiveness and reduce RH variation.

If drying a very damp house, the use of large fans can increase air circulation and evaporation, and thus improve extraction rates. Severely damp buildings benefit from use of a larger 400w model.

Humidistat settings: 65% is usually low enough to prevent any problems, but in some cases lower may be needed, especially if there is a house-wide damp problem. 50% works for all situations, but of course uses more electricity than 65%.

Should you close internal doors when drying a room? If RH in the rest of the house is lower than the damp room, leaving the door open it'll dry faster. If RH in the rest of the house is equal or more than the damp room, it needs dehumidifying too, best leave the door open.

Should clothes be hung above the machine? Dehumidifiers don't output significant heat, there isn't a danger from clothes falling on the machine. However it's not best practice as electrical safety features can occasionally be defective or missing.

What will it cost?

At time of writing, basic non humidistatic units go for around £100, and humidistatic units for around £150. Second hand units are much cheaper when available, but many are not humidistatic. Desiccant wheel units are much more expensive, despite being simpler.

There's no fitting cost, just plug it in.

Run cost depends on machine rating, amount of use and type of heating. During summer the unit can be switched off and external ventilation used instead, and during winter any electricity used becomes heat for the house. A typical unit uses around 200w

  • when used to dry a shower room it might run for an hour a day. This is 0.2kWh a day, or approx 2p.
  • A 200w dehumidifier running 2/3 of the time day and night in a severely damp house would use around 27p a day.
  • A 200w machine running 15% of the time for 8 months of the year at 12p/unit costs £21 a year.

The energy used is all released as heat into the house. Heat is also given up by the water vapour as it condenses, so the heat output is actually slightly greater than the energy consumed. Total heat production is small, but the 200w is not wasted.

  • If gas CH is used, and gas costs 1/3 the price of electricity, then about 1/3 of the run cost is deducted from the gas heating cost, so the cost to the householder is 2/3 of the figures above.
  • When using day rate electric heating, the dh adds no run cost at all, as all its electricity becomes heat.
  • With economy 7 type tariffs, the dehumidifier can be set to run only at low tariff times using a timer, and the added run cost is zero.

A dehumidifier is cheaper than an extractor fan because a fan throws a fair amount of heated air outside, costing higher heating bills.

Dessicant wheel dehumidifiers have in the region of twice the run cost of comparable compressor units.


The main 3 failure items are fan, controller, compressor.

Fan: a machine with a struggling fan must not be left to run in poor condition, fans can catch fire when they stall. Replace it. Motor bearing lubrication & keeping it on high speed only can help temporarily, but replace it asap.

The fans in many different machines are similar, so fitting a fan from another dead machine is often possible. But noise levels & airflow rates vary, so a different fan speed can sometimes mismatch the machine's requirements & not work well. Check the evaporator temp after replacement to ensure you have a good match. It should be between 0 & 10C.

Controller: If not repairable, replacement boards are available for some machines. Occasionally repairers have fitted an old simpler control system from another machine, or even made a new PCB.

Occasionally I've seen a bypassed completely so the machine always runs when plugged in, with no humidistat, no speed select and no deicing cycle. Not normally recommended, but running a few times a day for 30 minutes on a timer can sort out a minor flood. Don't use machines like this long term, the extra run cost does not make sense.

Compressor system: beyond the scope of this article.

Cabinet: Plastic cracks/breaks can have staples melted in across the break from the inside with a soldering iron, or sheet plastic /metal screwed across. Rusted metal can be repaired the same way as car bodies. Rust spots can be stripped with HCl or phosphoric acid & painted with radiator paint or car paint.


Clean hot & cold heat exchangers, fan blades, filter & grilles. Ignoring these eventually ruins effectiveness & energy efficiency. Filters need regular cleaning, they don't need replacing unless damaged. Small filter & screen holes / breaks can be taped over if the tape only covers a small percentage of the area.

Clean drainage path & water tank, check the float moves freely. Ignoring can eventually cause puddles or heavy rusting.


When buying used, check the fan & compressor come on, and go off when the RH dial is turned up.

Refrigerant leakage can cause no cooling thus no dehumidifying, the only 2 ways to check this are

  • feel the evaporator coil after a couple of minutes, it should be cold.
  • or run it a while until it produces water

Are there any cheaper options?


To dry a localised spot of dampness caused by a leaking pipe etc, a desk fan may be used instead. The air movement causes much increased evaporation rate, and normally a house will handle the extra airborne water load without issue. Air movement also discourages mould growth while the material is still damp.

For a desktop fan on low speed, run cost is similar to a dehumidifier running intermittently.

Home made dehumidifier

Desiccant wheel dehumidifiers are not particularly difficult to make, and the parts are simple and cheap. The high sale prices seem to be down to low sales volume more than anything else. However these are not a great choice for long term use in situations where a compressor unit would work, since they use a good deal more electricity.

Heating & ventilation

Turning the heating up and opening the windows is not normally a cheaper option. Even a small 15kW heating system can use 15x24= 360kWh per day. @10p/unit electricity this is £36 per day, or at 3p/unit gas with 80% efficiency this is £13.50 a day. Medium and large heating systems would cost more. When comparing for temporary use, don't forget the resale value of the dehumidifier, and the fact that you can use it as a good Clothes dryer.


Setting RH much too low or using a machine with no humidistat in very damp properties runs the risk of causing some wood damage.

If a dehumidifier has been tipped over, let it sit upright for a day before plugging in. Omitting this risks causing compressor failure.

See Also