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 higher total costs 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 a damp cellar 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.

Types

Compressor

Most dehumidifiers are compressor based. A refrigeration circuit causes condensation in the machine, drying the air. By the time ambient temp drops to around 12°C 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.

Peltier

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 survive regular switching 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. They are effective for one-off drying out a plumbing leak & similar, but for most tasks they're not a good choice.

Lime chips

Fine for a closed cabinet, useless for a room. The chips need renewing periodically. Silica cat litter is normally cheaper than the chips sold as dehumidifiers, and can be dried for reuse in an oven at 120°C for 2 hours.

Features

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. Generally set unnecessarily low to work with every situation
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.

The worst 'humidistats' use a strip of plastic film that expands and contracts, operating a switch. They have huge hysteresis and rely on the right speed of dried airflow to counter this. But they can still behave poorly, switching off when they should be on, and on when they should be off. The result is compromised performance & electricity wasted. They can also stick - a whack sorts that out temporarily. Prone to a large amount of drift over time. Electronic humidistats provide much better control.

Tank

Small condensate tanks need more frequent emptying. Large containers are more of a spill risk.

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 tank. Domestic machines usually don't have one.

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.

Mouldproofing

Some machines claim to have mouldproof tanks. If you get mould in the tank, a thorough clean denies mould anything to eat. It's easier to clean it before mould takes hold.

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.

Cabinet

Nearly all domestic units are plastic cased. Metal are better for abuse resistance if required, but rust through eventually.

A flat top with no vents or controls enables other things to be stacked on the unit.

Castors are sometimes useful

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. (But doing this often defeats the de-icing cycle, which in most cases isn't required, but if it is the machine will fail to extract much.) A snubber on the switch contacts is a good idea (a 0.1uF class X2 capacitor & 100ohms 400v resistor in series, across the switch contacts).

2 speed fans give a slower quieter option, with a lower extraction rate sufficient for most household jobs. (Of course some 2 speed machines are noisier than some single speed.)

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.

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 for a few pounds 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.

Modes

Maximum extraction rate is the default mode on most machines. Some machines have extra modes.

Useful modes include:

low speed fan only for bedrooms - basic 2 speed machines have this, and only some computerised models
maximum energy efficiency, which does reduce extraction rate to some extent

Not so useful modes:

Clothes drying mode. Typically this bypasses the humidistat, so it runs non-stop. 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 modes. I have seen mode names & instructions that make no sense, with an algorithm I can see no use for. More isn't always better.

Reliability

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

Ions, ozone etc

Ozone has some mould inhibition ability, so can enhance the machine's mould prevention action to an unknown degree. Whether they're worth paying for is a matter for debate - probably if you have a big mould problem. The main issue is that machines often generate very little. Ozone generators are cheap to buy separately.

Filter

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

Brand

In most cases I've not found brand to be much of an indicator of anything. Most machines are rebranded generic Chinese designs, with any given design getting many brand names.

Efficiency

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

The refrigeration heat exchanger can be fine or coarse. Fine ones are normally more effective & more energy efficient. An air filter is a must with fine ones, don't run without one or it will clog.

Repairability

Most machines' innards are accessible without difficulty, but not always. Simple controls are more often fixable than computer systems. Some machines have replacement boards or controllers & other parts available.

Commercial machines sometimes have a port rather than a crimped refrigeration pipe, making regassing easier. But if gas has been lost it has a leak, and most are not worthwhile regassing.

Safety

Not normally an issue, but I found one model where one of the knobs can be pulled off with fingernails, exposing a live fixing nut. The machine had other safety issues too. It had what most people think of as a high quality brand name.

Looks

A lot are unattractive, some look like a large carbuncle, some actually look pleasant.

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 invariably ineffective below about 12°C (and some barely work at 16°C due to persistent icing). Boxes of lime chips have so little effect as to be pointless for a damp room. They are only suited to a small enclosed space.

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, real world extraction rate tends to be much less than specs indicate.

A humidistat is important, otherwise the RH is uncontrolled. This can result in woodwork warping & cracking, wasted electricity, and no ability to respond to the changing vapour load of cooking, shower use & occupancy.

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 litre either require unnecessarily frequent emptying or don't collect much. 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 (i.e. to under 80% RH and no visible condensation) 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, it's best to put it either at the main source of damp, or 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, large fans can increase air circulation and evaporation, much improving extraction rates. High speed air is not required, just get it moving. Severely damp buildings benefit from use of a larger 400w dehumidifier.

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 £130. Second hand units are much cheaper, but not all are humidistatic. Desiccant wheel units are much more expensive, despite being simpler. Mini-dehumidifiers tend to be peltiers, which can not be recommended.

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 6p.
A 200w dehumidifier running 2/3 of the time day and night in a severely damp house would use around 81p a day.
A 200w machine running 15% of the time for 8 months of the year at 36p/unit costs £63 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.

Repair

The main parts 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. I've seen machines with motor mounts that push on the bearings, causing stiffness. Loosening the mounting a little solves the problem.

The fans in many different machines are similar, so fitting a fan from another dead machine is often possible. Mountings, 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 & 10°C.

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.

I've also seen a bypassed controller so the machine always runs when plugged in, with no humidistat, no speed select and no de-icing 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. Using an external humidistat is possible, if losing the de-ice function isn't a problem, ie if the machine is only used in a warm room.

Compressor system: beyond the scope of this article, beyond most diyers.

Cabinet: Plastic cracks/breaks can have staples melted in across the break from the inside with a soldering iron, or sheet plastic or metal screwed & glued 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.

Freezing: refrigeration machines can freeze if the air is cold, or airflow is compromised. Once frozen they have very poor extraction & energy efficiency. They are designed to defrost themselves, but it doesn't always happen. If a cold room is not the cause, check the airpath is clean & clear. Clogged filter & muck on the exchanger are usually the cause. Debris buildup on fanblades can reduce airflow too.

Filter: Small filter & screen holes / breaks can be taped over if the tape only covers a small percentage of the area. Large broken away areas can have something glued on around the edges such as sieve material, plain net curtain or plastic grille from some other donor machine. 3d printing a new grille or filter is a neater option, with artistic possibilities.

Maintenance

Air filters need regular cleaning, they don't need replacing unless damaged.

Clean water tank to prevent mould, check the float moves freely.

Clean the drainage path from cold heat exchanger to tank to avoid blockage. Ignoring can eventually cause puddles or heavy rusting.

Clean cold heat exchanger, fan blades, filter & grilles. Ignoring these eventually ruins effectiveness & energy efficiency, and can cause icing.

Testing

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

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?

Fan

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. @30p/unit electricity this is £108 per day, or at 6p/unit gas with 80% efficiency this is £27 a day. Medium and large heating systems would cost more. When comparing for temporary use, a dehumidifier has resale value and you can use it as a good Clothes dryer.

Gotchas

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

Dehumidifier