Is your indoor environment too dry?

Frequently, it is challenging to keep relative humidity from getting too high as this is harmful to both human health and buildings.

But exceedingly dry air can also be a problem. If the relative humidity is too low, some will experience dry skin, dry eyes and dry mucous membranes.

Airmaster air handling units are now available with an enthalpy exchanger to maintain the level of indoor relative humidity.

Controlling the relative humidity


Now you can decide what is best for you: a standard heat exchanger, an enthalpy heat exchanger or a combination exchanger for your Airmaster air handling unit.

An Airmaster air handling unit does more than just replace the air in the room. It enables you to control the indoor environment according to several different parameters, which now include relative humidity. To reduce the discomfort of excessively dry indoor air in winter, Airmaster recommends starting out by establishing demand control of the replacement air using either CO2, TVOC and/or Airmaster’s adaptive humidity control. Being able to control room temperature to keep it from getting too high is also very beneficial. Both parameters will help maintain reasonable relative humidity indoors.

In cold, dry climates, however, indoor relative humidity can sometimes fall below 30% in winter months. Even in moderate climates, relative humidity above 30% is required for special purposes.

To help out in these situations, Airmaster is now launching enthalpy heat exchangers as an optional feature for all our air handling units.

Airmaster air handling units are now available with three different exchanger solutions:

Standard: counter-current heat exchanger

New: enthalpy heat exchanger solution

New: combination heat exchanger made up of an enthalpy heat exchanger + Airmaster’s standard counter-current heat exchanger.

When should I consider enthalpy or combination exchanger?

The enthalpy solution is particularly suited for extremely cold climates such as northernmost Scandinavia or mountainous regions such as the Norwegian interior or the Alps.

A combination heat exchanger is a cross between these two types, meaning that it has a higher impact on temperature than an enthalpy-only exchanger but a reduced effect on humidity levels.

The combination heat exchanger is ideal for inland regions of Central Europe where, although cold, dry periods can occur, the climate is not considered extreme.

The combination heat exchanger is also ideal for coastal areas with a moderate climate if you want to be certain of maintaining a higher-than-normal indoor relative humidity (over 40%) at all times.

How an enthalpy heat exchanger works

An enthalpy heat exchanger is a heat recovery unit that transfers moisture between supply air and exhaust air.

The moisture is transferred by means of osmosis through a polymer membrane with a pore structure. The polymer membrane keeps supply air separate from exhaust air, just like Airmaster’s other heat exchangers.

This prevents the transfer of unpleasant odours or contamination.

Output and efficiency

Due to the lower temperature efficiency ratio of the enthalpy exchangers, the maximum output of an enthalpy-only exchanger is lower for AM 150 and AM 300 to comply with Ecodesign Directive requirements. Otherwise, the air handling units operate normally.


Maximum output of an enthalpy-only exchanger:

  • AM 150: 100 m3/h
  • AM 300: 190 m3/h


A combination heat exchanger can maintain these normal outputs for all Airmaster air handling units.

What is the ideal level of relative humidity indoors?


  • <30%: if the relative humidity in a room is below 30%, this will increase the risk that some people will be discomforted by the dry indoor environment.

  • 30–60%: relative humidity within this interval is normal and will be experienced as healthy for humans and buildings alike. Relatively humidity will often be at the high end of the scale in summer and the low end in winter.

  • 60–70%: if the indoor relative humidity is at or above this level, it is too damp and may cause problems, such as mould fungus.

Ready for a breath of fresh air

At Airmaster, we are “decentral” through and through. We are unrivalled when it comes to creating the best indoor climate and optimal well-being because we take each room and its occupants into account – and because we constantly strive to improve on what we do.