A Guide to Domestic Dehumidifiers

The most frequent question customers ask when enquiring about domestic dehumidifiers is whether to buy a desiccant unit or a compressor unit.
Desiccant units tend to be smaller, lighter, quieter and more effective and efficient at extracting moisture in cooler environments so are ideal for lots of scenarios in the UK.
Compressor dehumidifiers only work well in warmer temperatures (over 18 degrees celsius). In contrast, desiccant units extract the same amount of moisture in any temperature between one degree celsius and 20 degrees celsius.

Whilst a desiccant unit may consume more power on an hourly basis than a compressor unit, they are much more effective at extracting moisture and achieve a better result in a much shorter time. Therefore in real terms desiccant units can actually be cheaper to run than compressor units.

For example, at a normal room temperature of 21 degrees, a 7 litre per day extraction desiccant (power consumption 660 watts per hour) will normally extract more moisture in one hour than a 20 litre per day extraction compressor driven unit (power consumption 370 watts per hour) will extract in one and a half hours. However, at 15 degrees it takes the compressor unit much longer to extract the same amount of moisture so all of a sudden the desiccant unit becomes much cheaper to run to achieve the same result. 
Desiccant dehumidifiers use a desiccant material, typically Zeolite, which absorbs water vapour from the air in a similar way to silica gel. A fan draws air into the dehumidifier and passes it through a section of a slowly rotating wheel which holds desiccant which dries the air. The moisture is extracted from the desiccant by heating a portion of the wheel not being used to dry the air.

All desiccant units are rated in test conditions of 60% humidity (as opposed to 80% for compressor units) and 30 degrees centigrade temperature. They will extract the same amount of moisture irrespective of the temperature in which they are working.
There are no consumables on a desiccant unit: the desiccant material does not expire and will not need to be replaced / topped up. The lifespan of a desiccant unit will be much longer than a compressor unit as it is a much more simple process with far less moving parts, no compressor and no refrigerant gas being held under high pressure to potentially fail.
Compressor dehumidifiers work on the principle of moisture condensing on a cold refrigerant pipe inside the unit. As a result, compressor dehumidifiers work well in very warm conditions, but as the temperature falls so their effectiveness drops off dramatically. All compressor driven units are rated in test conditions of 80% humidity and, critically, 30 degrees centigrade temperature.
A three degree drop in temperature to 27 degrees results in up to a 50% loss of performance with Compressor units. At 21 degrees centigrade (which is room temperature) a compressor unit will tend to extract around 30% of its theoretical maximum. Below 15 degrees centigrade most compressor units extract about a tenth of their theoretical maximum and in cooler temperatures below eight degrees basic compressor units stop working as the evaporator plates ice up.
Therefore a compressor unit rated at 20 litres per day extraction will extract up to 5 litres in a normal domestic environment; a unit rated at 18 litres will extract up to 4.5 litres and a 10 litre will extract up to 2.5 litres.

Here are the test results from an independent test undertaken by Southampton University for an article on dehumidifiers published in November 2009 that clearly demonstrates that the Meaco DD122FW mk3 7 litre desiccant (the predecessor to the Eco Air DD122FW mk5 7 litre desiccant) extracts more than twice as much mositure as the next beast compressor unit which is the Ebac 2650e 18 litre per day compressor unit. It is worth noting that this test was conducted in cool conditions or around 15 degrees.


Dehumidifiersuk PBO Dehumidifier test Results



It is also worth noting that a desiccant unit can easily reduce the humidity level to 40% in a well sealed environment whereas a compressor unit will find it harder. 40% humidity is below the rusting point of metal which is another reason why desiccants are used for protecting vehicles, tools and equipment (as well as the fact that they will work in cold temperatures).
Some other useful features that the desiccant units have:

  • Continuous drain off facility which means that rather than just collecting the extracted moisture in a holding tank, there is option to drain off the moisture through a small hose which can be fed into a sink, drain point or separate holding tank.
  • Auto restart which means that after a power failure or power cut the unit will start back up automatically and continue to run at the same setting it was running at prior to the cut in power rather than going into standby and having to be manually reset. It also means that the unit can be left running on a plug timer if required which can be useful as it provides even greater flexibility of use than just the inbuilt timer and variable humidity level that can be adjusted by the built in humidistat.

Our best selling domestic desiccant dehumidifiers are

  • Eco Air DD122FW 7 litre per day extraction at £165 including VAT and delivery
  • Toyotomi TDZ80 8 litre per day extraction at £169 including VAT and delivery
  • MEACO DD8L 8.5 litre per day extraction at £179 including VAT and delivery