# Performance Analysis of the Demand-Based Ventilation in a Nordic Apartment Building
###### Abstract  
In general, new Finnish apartment buildings are equipped with mechanical balanced demand-based ventilation. The airflow rate in the kitchen hood is boosted on demand to improve pollutant extraction during cooking. However, in practice, it has been found that the system does not work as desired. The focus of the paper was to present the simulation results from a case building equipped with a ventilation system that is commonly used in Finland. In the analysis, the airflow rates are calculated for the room, apartment, and air handling unit (AHU) levels for various ventilation mode scenarios. A significant imbalance of over 10% between the supply and exhaust airflows at the room and apartment levels was observed in the boosting mode. This imbalance creates a pressure difference over the building envelope, particularly in small studio apartments. The calculated pressure difference for future buildings with high airtightness were at the warning level of 40 Pa below atmospheric level. The kitchen hood exhaust system showed a 28% lower airflow rate in certain scenarios. A new solution to guarantee the designed airflow rates was proposed and assessed. The new solution consists of replacing the apartment level flow control damper and a new balancing method for the kitchen hood exhaust branch. The proposed design was able to stay within 10% of the designed airflow rates in all operation modes. ventilation; demand-based ventilation; apartment building; kitchen boosting: Nordic climate 2021202  
In Finland, residential buildings account for 85% of the building stock. Detached houses represent 75.6%, attached houses 5.4% and blocks of flats 4% of this number. However, the blocks of flats account for around one-third of the total floor area of all residential buildings, and buildings with four floors or more host approximately 1.3 million occupants. The ventilation systems of the buildings have a direct relation to their construction years [8]. The majority of the buildings from the 1970-1990s are equipped with mechanical exhaust ventilation. In new apartment buildings, balanced mechanical ventilation with heat recovery is the standard solution. Natural ventilation solutions have not been used in apartment buildings in Finland in recent decades because of the energy requirements [9], although they are still quite common in single-family houses. To improve the energy efficiency in the existing buildings, retrofitting is required. The optimal solution for the building retrofit depends on the existing ventilation system and climate conditions [10]. In old buildings with low thermal insulation and poor airtightness, the energy efficiency of the building envelope, together with an improved ventilation heat recovery, should be considered to meet the current European requirements for energy efficiency [11]. The standard ways for a retrofit are to replace the original ventilation system with balanced mechanical ventilation or to implement a heat pump together with exhaust airflow energy recovery [10; 12; 13]. Mechanical exhaust systems equipped with heat pump heat recovery and ventilation radiators are common solutions e.g., in Sweden [14; 15]. A retrofit with the installation of centralized balanced mechanical ventilation is often feasible but complicated for an old apartment building, so decentralized air handling units for apartments are preferable. New buildings are equipped with mechanical balanced systems with modern automation systems. The most common solutions are constant air volume balanced systems. Some ventilation systems have several operation modes with variable airflow rates, providing purging features based on the dwelling occupancy level using the variable-air-volume (VAV) system design [16]. VAV ventilation, previously utilized mainly in the office, commercial and public buildings is now also used in residential buildings with various decentralized and semi-centralized solutions despite their higher initial cost compared to standard constant air volume (CAV) systems [17].