Performance evaluation of residential ventilation systems based on multi-zone ventilation models

This paper presents simulation results of the performance of different ventilation systems in 5 types of dwellings designs both with common and improved building air tightness. Dwelling types include detached houses, terraced houses and apartment flats. Two major types of ventilation systems are investigated: natural ventilation with passive stacks and mechanical exhaust systems. The systems are all sized according to the Belgian Ventilation Standard. The simulation results give an indication of differences in performance between the of systems. The multizone infiltration and ventilation simulation model COMIS is used to calculate the indoor air quality and the ventilation heat loss achieved by each system. Carbon-dioxide is used as an indicator for indoor air quality. Therefore the variation of carbon-dioxide concentration in response to predetermined occupation schedules is calculated. The paper presents the model premises and discusses the results of the simulations. The performance of the ventilation systems is related to the dwelling typology and the building air tightness. The results show that the natural ventilation systems and the systems with mechanical exhaust may not guarantee a sufficient indoor air quality in all rooms at all times. As a first step to develop performance based guidelines for residential ventilation systems, the effective performance of systems that meet the current standards should be evaluated. Of course, the performance depends on dwelling typology, air tightness, user behaviour, etc. The relation between these different parameters and the resulting indoor air quality is often analysed by means of numerical simulation (Jreijiry et al. 2007, Maeyens & Janssens 2003, Hayashi et al. 2001). Indoor air quality may be assessed by means of different criteria, depending on the pollutants to be removed by the ventilation system. An often used criterion for residential ventilation is based on occupant exposure to metabolic CO2-concentration as an indicator for increased odour intensities. This is the case for assessment methodologies developed in the Netherlands and France (Van Den Bossche et al. 2007). The European standard EN 13799 presents a classification of indoor air quality into four categories by CO2-level. This classification is well established for occupied rooms where pollution is caused mainly by human metabolism. Although the categories have been developed for non-residential buildings, the definition also applies to living rooms and bedrooms. Table 3 shows the limits between the classes. In the following the performance of passive stack and mechanical exhaust systems is evaluated by means of numerical simulation using the IAQclassification presented in Table 3. The systems are all sized according to the Belgian Ventilation Standard NBN D50-001. The analysis is done for 5 types of dwelling designs both with common and improved building air tightness. 2 REFERENCE DWELLINGS The reference dwelling designs have been developed in the framework of a research project on the optimisation of building envelope and services for lowenergy residential buildings (Verbeeck & Hens, 2007). The five dwellings are all single family houses with the same programme (family with 4 persons) and the same useful floor area corresponding to national statistical figures. All houses comprise a living room, 3 bedrooms, kitchen, bathroom, toilet, utility room, and hall way, with a total net floor area of about 150 m2. In the detached and terraced house also a study room is present. The dwellings further differ in typology and building compactness, ranging from a detached bungalow to a flat in a 6-floor appartment building, representative for Belgian construction practice. The market share of newly built dwellings during the last decade in Belgium is typically 40% detached houses, 40% apartments and 20% terraced or semi-detached. Table 4 gives a survey of geometrical characteristics of the five reference dwellings. The ventilation systems are sized according to the minimal requirements in the Belgian ventilation standard. They consist of: − Trickle ventilators in all habitable rooms sized in order to supply 3.6 m3/h per m2 floor area at a 2 Pa pressure difference. The total design flow rate for supply of fresh air is listed in Table 4. These flow rates are suffcicient to achieve a high IAQ (IDA 1) under normal occupation. − Passive stack units or extract fans, sized in order to extract the design flow rates listed in Table 1. For passive stacks the design flow rate is achieved at a pressure difference of 2 Pa. The total extract flow rate is listed in Table 4. − Transfer openings with a free section of 70 cm2 in at least one internal door per room, necessary to transfer 25 m3/h at a 2 Pa pressure difference; the transfer opening in the kitchen door has a section of 140 cm2. Since the number of rooms and the floor area of all houses is roughly the same, also the design flow rates for the 5 houses are similar. As the table shows design flow rates for supply are typically larger than extract flow rates according to the Belgian Ventilation Standard. The building site of the detached houses is supposed to be roughly open, the semi-detached house and apartment are located in an urban landscape, while the terraced house is in a closed city landscape. These assumptions define the local wind exposure of the houses, which affects ventilation performance. Table 4: Geometrical characteristics of reference dwellings Bungalow Detached Semidetached Terraced Apartment