A ventilation system is designed to maintain a healthy indoor climate, but this does not mean that it needs to operate at the same airflow rate around the clock. Indoor air conditions change continuously throughout the day: the number of occupants, everyday activities and indoor humidity all affect how much ventilation is actually required.
This is why modern ventilation systems use CO₂, VOC and humidity sensors. These sensors allow ventilation to respond to actual demand, helping maintain good indoor air quality while avoiding unnecessary energy consumption. The principle of demand-controlled ventilation (DCV) is simple: airflow increases when occupancy, air quality or humidity levels require it and decreases when there is no need for intensive ventilation. This reduces fan energy consumption and limits the amount of air that needs to be heated, cooled or otherwise conditioned unnecessarily.
Why is maximum ventilation not needed all the time?
Indoor air quality is never constant throughout the day. In a bedroom, CO₂ concentration typically increases at night while people are sleeping. In living areas, CO₂ levels rise when more people are present or when occupants spend longer periods indoors. In kitchens and bathrooms, cooking, showering and other everyday activities can cause significant changes in humidity.
If a ventilation system operated continuously at a high airflow rate, it would consume more electricity even during periods when such intensive ventilation was unnecessary. Demand-controlled ventilation allows the airflow to adapt to actual conditions in the building while operating within the limits defined for the ventilation system.
How do CO₂, VOC and humidity sensors work?
A CO₂ sensor measures the concentration of carbon dioxide in indoor air and is particularly useful for estimating ventilation demand caused by occupancy. In general, the more people there are in an enclosed space and the longer they remain there, the faster the CO₂ concentration increases.
CO₂ concentration is measured in parts per million (ppm). Outdoor CO₂ concentration is typically around 400–500 ppm, while approximately 1,000 ppm is often used as a general reference point when assessing indoor air quality in residential spaces. The appropriate target level should, however, be selected according to the use of the space and the indoor climate requirements defined for the project.
When the measured CO₂ concentration rises above the selected target level, the ventilation control system can increase the airflow. As the CO₂ level falls again, ventilation intensity can be reduced accordingly. This means the system supplies additional fresh air when it is needed instead of maintaining the same airflow regardless of occupancy.
It is also important to distinguish between CO₂ and VOC sensors. A CO₂ sensor measures carbon dioxide concentration, whereas a VOC sensor detects volatile organic compounds in the air. VOCs may originate from materials, furnishings, cleaning products, cooking and other sources. The appropriate sensor therefore depends on which indoor air parameter is intended to control the ventilation system.
A humidity sensor operates according to a similar principle. When indoor humidity rises, for example during showering or cooking, the ventilation system can increase the airflow to help remove excess moisture. However, humidity-based ventilation control needs to take outdoor conditions and the season into account.
During winter, cold outdoor air contains relatively little water vapour even when its relative humidity is high. Once that air enters the building and is heated, its relative humidity drops significantly. Increasing the ventilation rate can therefore dry indoor air quite quickly during cold periods.
The situation can be different in summer. Warm outdoor air may contain a considerable amount of moisture, meaning that simply increasing the ventilation rate may not reduce indoor humidity. If ventilation alone is not sufficient for dehumidification, active cooling and dehumidification may be required. This can be achieved, for example, using a DX cooling coil or a chilled-water cooling coil, where part of the water vapour condenses as the air is cooled.
Humidity control should therefore not be understood simply as “higher humidity equals more ventilation”. Effective control needs to consider both indoor and outdoor conditions as well as the capabilities of the ventilation system.
Lower energy consumption without compromising indoor air quality
Demand-controlled ventilation helps avoid situations where a ventilation unit operates at high capacity even though there is no actual need for it. As a result, electrical energy consumption can be reduced while maintaining stable and healthy indoor conditions.
Komfovent DOMEKT ventilation units can use CO₂, VOC and humidity sensors. Based on the measured values, the ventilation control system adjusts the fan operation within the defined control limits.
For CO₂ control, the sensor range used in DOMEKT systems is 0–2,000 ppm, while VOC sensors use a 0–100% scale and humidity sensors measure relative humidity. The user can define the desired setpoints and the minimum and maximum ventilation intensity within which the system responds to changes in the measured values.
CO₂ and air-quality sensors can be installed either in rooms or in ventilation ducts, and up to two sensors can be connected to one DOMEKT ventilation unit. This allows the system to respond more accurately to conditions in different parts of a building. For example, in a two-storey house, one CO₂ sensor can be installed on each floor so that the ventilation control system receives information from more than one measuring point.
For automatic air-quality control, ventilation intensity can be regulated between defined minimum and maximum levels. Komfovent gives 20–70% as a typical control range for DOMEKT systems, while the minimum and maximum values can be adjusted according to the requirements of the installation.
If the minimum ventilation intensity is set to 0%, the unit can temporarily stop when indoor air conditions are satisfactory. It then starts periodically at a low airflow rate to check the sensor readings. If the CO₂, VOC or humidity level exceeds the defined value, the ventilation unit continues operating and increases the airflow as required. If the measured values remain within the desired range, the unit can stop again.
This makes both the selected air-quality or humidity setpoint and the minimum and maximum permitted ventilation intensity important parameters when configuring the system. Together, they determine how strongly the ventilation system is allowed to respond to changes in indoor conditions.
The main advantage of CO₂, VOC and humidity sensors is therefore the ability to connect ventilation operation with the actual use of the building. Correctly selected sensors and properly configured control limits help avoid unnecessary airflow while maintaining the required indoor air quality and humidity level. This allows a ventilation system to combine two important objectives: a comfortable indoor climate and efficient use of energy.
