Air humidification for Controlled Domestic Ventilation systems

A new system uses semi-permeable sterile membranes

Air humidification for Controlled Domestic Ventilation systems

If one’s own four walls were not only a private place of residence but also a workplace, then air humidity would have a much higher priority there. However, what is planned and technically implemented in terms of air humidification in buildings where people work has so far only rarely received attention in connection with controlled domestic ventilation. Condair has now developed a new solution for controlled domestic ventilation, which will be presented for the first time at ISH/Aircontec.

AHUs that are operated for people’s living spaces should provide a comfortable and healthy indoor climate all year round. In addition to good indoor air quality through the introduction of filtered outdoor air, this also includes ensuring an indoor temperature of 20 to 27°C with a relative humidity between min. 30% (winter) and max. 65% (summer).

People feel comfortable and perform at their best under these indoor air conditions. This is how the essential tasks of indoor air technology are summarised in “Taschenbuch für Heizung + Klimatechnik” [Compendium of heating and air-conditioning technology], which has been a standard reference for engineers and system designers in the ventilation and air-conditioning industry for generations.

Optimum indoor air humidity from a medical viewpoint

From a medical viewpoint, at indoor temperatures of 20 to 24°C, a relative humidity of 40 to 60% is optimal. Within this humidity range, pathogens suspended in the air and embedded in droplets are deactivated in a very short time.

However, in dry air below 40% relative humidity, their lifespan increases dramatically. In addition, the functioning of the mucous membranes of the respiratory tract and the associated facets of the immune system are much better preserved at a relative humidity above 40% than when the air is too dry.

However, beyond recommendations in various standards and guidelines, there are no binding statements on minimum or maximum values for relative humidity in rooms occupied by people — neither for offices nor for private living areas.

This means that home owners can decide in accordance with their own wishes what standards the indoor climate within their own four walls should meet. The issue of excessively dry indoor air in winter is well known, because it often causes problems from which many people suffer.

Dry air makes people ill

Viruses and bacteria can cause infectious diseases. The immune system protects people from attacks specifically through the cleansing function of the mucous membranes along the respiratory tract. Excessively dry indoor air disrupts this function and facilitates attacks by viruses and bacteria. Flu viruses, in particular, thrive in very dry air. It allows them to float around for hours. This is why the risk of infection of every person who is in the same room increases, the longer they remain in the air.

This is not the case when the air humidity is between 40 and 60%. At these humidity levels, viruses cannot survive and are deactivated after only a few minutes. The risk of infection is therefore significantly lower at the right humidity levels. In addition, the natural defence mechanism of the mucous membranes along the respiratory tract is not compromised and can function optimally. This is the simple reason why people catch colds much less often in summer than in winter.

Indoor humidification possibilities

For over 20 years now, homes are increasingly being equipped with a system for the controlled mechanical ventilation of living spaces (controlled domestic ventilation system). This ensures good air quality by constantly introducing filtered outside air into the living spaces and removing polluted indoor air. At the same time, an efficient heat recovery system ensures that the outside air, which is cold in winter, is heated to a supply air temperature of about 15 to 16°C. A central controlled domestic ventilation system like this one offers ideal conditions for centrally humidifying the outside air that is too dry in winter. However, this raises the question of which humidification technology can best meet the requirements in private living areas — and what technical effort is involved.

One way of transferring humidity from the extract air to the supply air in a central controlled domestic ventilation unit is by enthalpy heat exchangers. Several manufacturers offer humidifiers for their controlled domestic ventilation. The principle is based either on sorption rotors or vapour-permeable membranes. In both cases, water molecules in the exhaust air are transferred to the outside air drawn in by the controlled domestic ventilation unit, which is dry in winter, and which then flows into the living rooms as preheated supply air. However, in most cases, this moisture gain from the apartment’s exhaust air, which is usually not very humid, is not sufficient to create an air humidity of at least 40% in the living rooms.

Another possibility is to install an electric steam humidifier in the ventilation system. Humidifiers of this type are relatively easy to control and also work hygienically. However, due to the electrical energy they consume in generating steam from liquid water, they are usually not desired in efficient residential buildings.

Rooms can also be humidified individually through the operation of decentralised evaporative humidifiers. Although these require relatively less electricity to operate, they require a great deal of cleaning to ensure perfect hygiene. In addition, such systems must be regularly refilled with water.

Diffusion humidifiers for controlled domestic ventilation

A new, efficient solution used by Condair to add humidification to controlled domestic ventilation is diffusion humidification. Easy to install — it can even be retrofitted into existing systems (as shown in the illustration on page 1) — and works hygienically.

Design and technology
The technology of diffusion humidification is based on the natural diffusion of water vapour through a breathable membrane. This is designed as a sterile membrane and separates humidifying water from the supply air flow.

The diffusion capillaries in the membrane are so small that they are only permeable to water vapour molecules. Unwanted substances in the water are thus safely filtered out and do not enter the supply air of the ventilation system. For its operation, the humidifier installed in or next to the unit must be connected to a water supply system.

Water requirements
Drinking water is used as the humidifying water. Up to a water hardness of 13°dH, it does not require additional treatment. A water softener, available as an accessory, is only required at higher values. The softening cartridge is replaced annually.

Depending on the existing water quality, a descaling agent and a disinfectant are added to the humidifying water as a maintenance measure.

The cleaning cycle can be easily activated via the touch display of the humidifier and runs automatically.

Water temperature
The humidification process takes place at a water temperature of 25 to 35°C. The humidifying water must be heated to this temperature for this purpose. Heating water or domestic hot water are suitable heat sources. Optionally, the humidification water can also be heated using an electric heating element.

A significant side effect of diffusion humidification is the warming of the supply air. When the outside air is cold in winter, the supply air temperature after efficient heat recovery in the controlled domestic ventilation unit normally only reaches values of about 15 to 16°C and is blown into the rooms at these temperatures.

The indoor air is then reheated to the desired value of 20 to 22°C using radiators or underfloor heating. The diffusion humidifier, through which tempered humidifying water flows, warms the supply air coming from the heat recovery system by a further 5°C, depending on the water temperature and air flow. This effect enables higher supply air injection temperatures and eliminates the need for subsequent heating in the rooms.

Humidification capacity
The nominal humidification capacities are 1 and 2 kg/h. This allows the dry outdoor air to be humidified by about 4 to 6 g/kg. This means, for example: If 0°C cool outdoor air (absolute humidity 3 g/kg) is heated to 22°C in the controlled domestic ventilation unit, it has a relative humidity of only about 18%.

Humidification by 5 g/kg increases this relative humidity to a value of about 48%. This capacity may still be varied slightly by changing the water temperature.

The capacities of both diffusion humidifiers are designed for standard controlled domestic ventilation system supply air flows of up to 180 and 300 m³/h each. For more information on the subject of diffusion humidification for controlled domestic ventilation, please visit

Pros and cons
The risk of mould growth and increased energy consumption are often cited as downsides of active air humidification. However, relative humidity initially has no influence on the growth of mould.

This is caused solely by the availability of liquid water in the masonry. Therefore, mould growth mainly occurs as a result of inadequate building physics aspects as well as condensation caused by cold bridges and poor building insulation.

In a properly planned and sufficiently insulated building free from cold bridges, indoor air humidity in the range of 40 to 60% does not cause undesired condensation in the masonry.

There is no such thing as active air humidification with zero energy use. However, diffusion humidification offers a very efficient and energy-saving way to provide a comfortable and healthy indoor climate by using heating or service hot water for heating.