Skip to main content

Demand-controlled decentralized ventilation

Demand-controlled decentralised ventilation gives you the best possible air quality in each and every room – with the lowest possible energy consumption. There is no need to choose between prioritising air quality – and thus the well-being of those in the room – and prioritising energy consumption. You can quite simply have both.

Demand-controlled decentralized ventilation

Decentralised ventilation is installed in the actual room to be ventilated. This also makes it possible to control each room individually via individual settings. In addition to individual basic settings for each room, we can also control the ventilation automatically as needed using various sensors.

This could be a PIR sensor (motion sensor), which activates the ventilation when there is movement in the room. It could also be a CO2 sensor, which controls the airflow intelligently according to the number of people present in the room. You can also opt for a TVOC sensor, which measures the presence of “little” or “much” of volatile organic compounds. 

In sports facilities such as changing rooms, you can also choose to control the ventilation according to the humidity level, ensuring that it always remains at the desired level.

So, there are a host of options for combining different basic settings with sensors. The options for time control, CO2 sensor, TVOC sensor, PIR sensor and humidistat/hygrostat are set out below.

Time-controlled ventilation

Time-controlled ventilation

In rooms used on a regular basis, it can often be a good idea to choose time-controlled basic ventilation.

In both classrooms and offices, it can be beneficial to have the basic ventilation start up an hour before people arrive. This way, there is fresh air in the room right from the start of the day.

You can choose to control the ventilation solely according to a weekly schedule or you can opt to supplement it with a sensor. Particularly in rooms in which both the use and number of occupants vary, it makes good sense to supplement with a CO2 sensor and possibly a TVOC sensor.

This way, the air capacity automatically adjusts according to the number of occupants in the room (CO2) or according to the volatile gases (VOC) emitted.

There are seven different time programmes with individual set points as well as a night cooling programme, offering ample opportunity for you to pre-programme the time schedules used most.

Demand-controlled decentralised ventilation with CO2 sensor

Demand-controlled decentralised ventilation with CO2 sensor

The CO2 concentration has long been used in indoor settings as an indicator of air quality. In many places, there is legislation governing maximum CO2 values, partly because it is a reliable indicator of activity level and thus the need for air replacement, and partly because the COconcentration can be measured with a high level of accuracy. It is therefore both appropriate and technically possible to use CO2 levels to control ventilation.

The air we exhale contains CO2, so the more people there are in the room, or the more we move about, the higher the CO2 concentration becomes.

It therefore makes very good sense to opt for ventilation controlled according to the CO2 level in rooms with a more irregular usage pattern throughout the day. These might be classrooms, or the many different types of rooms in kindergartens and day care institutions. Other rooms where the usage varies considerably can be group rooms, meeting rooms or canteens, etc.

Time schedule combined with sensors

There are several combination options for controlling the ventilation. In schools, the basic ventilation typically runs according to a schedule, supplemented by CO2 demand control. This means that the ventilation in most of the school will stop running after home time. But there is still fresh air in the one room that is to be used for a parents’ meeting because the ventilation is also controlled by the CO2 level.

A PIR sensor can also be added, which will start the unit when movement is detected and stop it when movement in the room ceases. The CO2 sensor will then adjust the airflow to the number of occupants. The PIR sensor thus functions as a start/stop for the basic ventilation, and the airflow/air quality are controlled by the CO2 sensor.

As a new feature, we can now also demand control the ventilation by means of a TVOC sensor, combined with a CO2 sensor.

Demand-controlled decentralised ventilation with PIR sensor

Demand-controlled decentralised ventilation with PIR sensor

The ventilation unit can be set to start/stop via a signal from a PIR sensor (motion sensor). When the PIR sensor detects movement, it sends a signal to the ventilation unit to start. The unit starts normal operation with the pre-set airflow and supply temperature. When movement is no longer detected, the signal ceases, and the unit stops after the pre-set run time.

The PIR signal is often used to change the operation of the unit from basic ventilation to normal operation when people are in the vicinity of the sensor. Demand-controlled ventilation with a PIR sensor is particularly suitable for use in rooms with irregular use such as meeting rooms, libraries and changing rooms.

In rooms that are not used on a regular basis, it may make sense to leave it to the PIR sensor to control the start of the ventilation unit with basic ventilation, and then let a CO2 sensor control the airflow. With this combination, the CO2 sensor will typically control when the ventilation unit should stop again.

In premises with both regular and occasional use, it may make sense to let the ventilation run according to the time schedule and CO2 sensor during the set hours, and then let a PIR sensor start and stop it at other times. This could, for example, be a classroom that is also occasionally used for parent meetings, etc., in addition to its normal use during the day.



Humidity also plays a significant role in achieving indoor air comfort. If the humidity is too high, there is a risk of rot and mould forming in the building, with health implications in the worst case. If the air is too dry, it can cause dry eyes and dry mucous membranes, etc., which are also sources of discomfort.

A normal, comfortable humidity is in the 30–60% range, which provides the healthiest indoor climate for people and building alike. It should be noted here that many asthma and allergy associations, recommends that the relative humidity should preferably be below 45%, as high humidity can be particularly troublesome for asthma sufferers.

In Airmaster’s opinion, the first step in preventing the air in a room from drying out in winter is to establish demand control of air replaced by means of either CO2, TVOC, and/or Airmaster adaptive humidity control. It is, in addition, beneficial to have control over the room temperature and not let it become unnecessarily high. Both will have a beneficial effect in terms of achieving desirable indoor humidity.

Hvad er den optimale indendørs luftfugtighed?

<30% - Hvis den relative luftfugtighed indendørs er under 30% er der en øget risiko for at nogle vil opleve gener som følge af et tørt indeklima.

30-60% - En luftfugtighed i dette interval er en normal luftfugtighed, der vil opleves som sund for både mennesker og bygninger. Om sommeren vil luftfugtigheden ofte ligge i den høje ende, og om vinteren i den lave.

60-70% - Er den relative luftfugtighed indendørs på dette niveau eller over, er det for fugtigt og kan give problemer med f.eks. skimmelsvamp.

Adaptive on-demand control

Airmaster air handling units can be fitted with an extra humidity sensor or extended programming.

Integrated humidity and temperature sensors on supply and extract make exact calculation of absolute air humidity possible.

Automatic adaptation to weather conditions

The adaptive humidity control automatically prevents the air drying out in the winter and reduces humidity in the summer. This effective, energy-saving form of operation creates a healthy environment and a healthy energy bill.

Control by a wall-mounted hygrostat

A hygrostat registers relative air humidity, and sends either a start or stop signal to the air handling unit. Start/stop signal can be adjusted. Humidity in the air affects the length of hygroscopic man-made fibres. Depending on the humidity level, the fibres will activate a contact that triggers the signal. When the relative air humidity goes over or under the level set, the hygrostat sends a start/stop signal to the air handling unit. Hygrostats are often used to switch a unit from basic ventilation to full operation when the
relative humidity set is exceeded.