When should I use one?
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.
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, set it to reduce RH slowly and gradually. Wood dries out slowly, and quick 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.
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.
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.
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.