# Performance Analysis of the Demand-Based Ventilation in a Nordic Apartment Building
###### Abstract
However, the ventilation systems should be able to precisely control the indoor climate or otherwise the target values of indoor temperature or CO\({}_{2}\) concentration may not be fulfilled [18; 19]. This creates a demand to control the airflow rate in the apartments, so nowadays, the majority of demand-based (DB) ventilation systems have two airflow rates (high and low). The most common control strategies are based on relative humidity, occupancy, CO\({}_{2}\) level or total volatile organic compounds (TVOC) level control [3; 20]. Humidity-based control systems are usually introduced in bathrooms [21]. In Denmark, the ventilation system is sometimes controlled at the apartment level by the occupant presence and CO\({}_{2}\) level [22; 23]. This strategy gives a significant energy saving potential and improved indoor air quality in the occupied apartments. However, in certain conditions, it could lead to unacceptable volatile organic compounds (VOC) levels or create an undesired imbalance of supply and exhaust airflow rates with more than 10% difference due to the inaccurate airflow control either at the local or air handling unit (AHU) level [16; 24; 25]. Centralized demand-based systems with the combined control strategy of CO\({}_{2}\), TVOC and humidity are often proposed and simulated in studies. Their energy efficiency and overall performance are good, but we still have limited measured case studies where the benefits of demand-based systems are demonstrated [26]. In Finland, most newly built apartment buildings are equipped with demand-based mechanical balanced ventilation with a heat recovery of at least 67% efficiency [27]. The demand is to guarantee the pollutant extraction with the kitchen hood in cooking mode. Starting from 2018, the building code requires to boost the airflow rate in the kitchen during cooking gas, as it is the primary source of indoor pollutants in modern residential buildings. Cooking processes account for around 54% of particle matter (PM) and 53% of personal exposure to ultrafine particles [28]. PM exposure is directly linked to respiratory and cardiovascular health issues [29]. The building code requires that the kitchen hood airflow rate should be at least 25 L/s while cooking to ensure pollutant extraction, and 8 L/s in normal mode. The system is implemented with additional dampers needed to enable two ventilation modes: normal and boosting. In larger apartments, bathroom and toilet exhaust airflow rates are decreased to provide an increased exhaust airflow rate for the kitchen hood. In small studio apartments, the reduction in the bathroom exhaust is not possible due to the minimum required airflow rate of 10 L/s, and hence the apartment supply airflow is increased instead [27]. To meet energy efficiency demands, the minimum airtightness for a new apartment building should be 1.0 1/h at 50 Pa difference over the building envelope [26]. In future passive buildings, it is assumed that airtightness will be improved up to 0.5 1/h. In airtight buildings, it is required to control the ratio of supply and exhaust airflow rates accurately. Otherwise, the unbalanced ventilation could lead to an unnecessary high-pressure difference over the building envelope, and furthermore, to structural moisture damage, pollutant extraction or draught [30]. It has been found that the boosting mode in apartment buildings does not work as desired. It is not well known in which operation conditions airflow balance can be reached, and how it is possible to guarantee the designed performance. The novelty of the paper comes from the performance analysis of the demand-based mechanical balanced ventilation system with kitchen exhaust boosting, implemented in the apartment building with a high airtightness level. This type of system, unlike common modern ventilation systems, has not been assessed or simulated. To close the research gap, the kitchen boost system performance was analyzed concerning various operation conditions.