London Southern England

Dehumidifier Performance

The amount of moisture which a dehumidifier can remove is a function of four things

  1. The temperature of the surrounding air
  2. The relative humidity of the air
  3. The type of dehumidifier
  4. The performance rating of the machine

It should not be assumed that a machine rated at 50 litre/day of moisture removal will automatically remove a spillage of, say, 500 litres absorbed into the fabric of the building, in 10 days. Refer to drying time for a better understanding of this process. The rate of moisture removal of a dehumidifier is stated at a certain temperature of relative humidity.

For example a refrigerant based dehumidifier might be rated for 60 litres/day moisture removal at 35°C and 100% relative humidity (RH) but will have a rating of around 20 litres/day at 20°C and 70% relative humidity.

This is because the amount of moisture held in the air and available to be extracted depends on the relative humidity and temperature of the air. See the table below for a more detailed understanding.

Moisture in Air Graph

The rate at which moisture can be removed by a dehumidifier is a function of the amount of moisture in the air and this in turn is determined by the temperature and relative humidity. A refrigerant based dehumidifier relies on moisture condensing on a cold coil before collection which makes it effective in warm humid conditions but less so in lower temperatures and where the relative humidity is lower (unless heating is also provided.) In typical UK winter temperatures of around 10°C the effectiveness of refrigerant based dehumidifiers is very limited. In these conditions a desiccant dehumidifier will be needed to further reduce the relative humidity and so accelerate the drying process.

The table which follows is a good guide as to the conditions in which each type of unit is most useful.

Dehumidifier Choice


We can provide dehumidifiers,drying equipment & services for water leak and flood drying projects including:-

  • Dehumidifiers
  • Heaters
  • Fans
  • Blowers
  • Damp measurement
  • Surveys and advice


We have a range of electric heating equipment for use on 13 amp supplies including:-

  • Fan heaters
  • Ducted heaters
  • Oil filled convector heaters
  • Radiant heaters


We can carry a range of air moving equipment to assist in thorough drying along with accessories for safe use, including:-

  • Larger 600mm fans
  • In line duct fans
  • Smaller air movers
  • Carpet dryers