| VENTILATION ventilation, as well as enabling more selective and controlled ventilation. More than half of new-build homes (based on ATTMA dataset 2015-16 data reported by Crawley6) are naturally ventilated. However, increasingly challenging conditions particularly related to overheating and air quality are encouraging designers to evaluate the need for alternative systems. Any ventilation arrangement that does not incorporate heat recovery will have an indirect operating cost for heating the ventilation air (by the buildings heating systems) when it is drawn into the building. The supporting document for the England Building Regulations Approved Document F (ADF7), Domestic Ventilation Compliance Guide,8 has extensive requirements that also provide a useful basis for best-practice installation and commissioning of the various systems. ADF categorises and illustrates ventilation in terms of four generic systems. It is currently under review as part of the development of a UK Future Homes Standard, and a new version is expected later in 2020 that is likely to require performancebased evidence that there is adequate means of ventilation for building occupants. Where there is an opportunity to include a riser stack duct through the building, passive stack ventilation may be used (ADF System 2 <1% of installed systems in new homes dataset6). This employs a combination of natural stack effect, together with the venturi effect, to draw air from inlet grilles mounted at ceiling level in selected rooms (for example, shower room, kitchen or utility room). This requires no power, is practically silent, and has no direct cost of operation, but the airflow rate will be dependent on the outdoor temperature and wind velocity. Automatic or manually actuated dampers (mounted in the stack) may be used to vary the flow. Airflow may reverse in summer or at times of high winds, which can cause smuts to be carried into the room from the (inevitably) contaminated ducting. These systems will provide limited summer ventilation this may be augmented by opening windows as appropriate. As with all natural and extract-driven ventilation systems, trickle vents or, possibly multiple, other purpose-made unfiltered openings will be required to allow the flow of the make-up outdoor air. Intermittently actuated extract fans through walls and windows, and with ducted discharges may be used where there is a specific generation of contaminant, such as in a kitchen or wet room, and the make-up, unfiltered ventilating air is drawn through EXPLORING THE IMPACT OF VENTILATION ON INDOOR CONTAMINANT LEVELS A key marker (or proxy) that has been used to infer the quality of indoor air in occupied spaces has been the concentration of CO2 in the indoor air, with the assumption that if there is sufficient fresh (outdoor) air to adequately dilute the CO2 emitted through human respiration, there will be an acceptable IAQ. The IAQ sheet on the freely downloadable CIBSE spreadsheet www.cibse. org/Knowledge/Design-Tool-for-IAQ-Analysis provides a useful tool to undertake CO2 analysis for a single zone. So, for example, Figure 1 illustrates that a space occupied from 8am to 6pm with an outdoor air supply rate of 10Ls1 per person will lead to an increase in CO2 of 555ppm (assuming approximately one person per 10m2 floor area exhaling 20 litres CO2 per hour per person). As outdoor CO2 is approximately 410ppm, this would create an indoor CO2 level of less than 1,000ppm a level that is commonly set as a reasonable limit value. CO2, however, is just one of the many pollutants in indoor air and is related to occupancy. Other contaminants, such as volatile organic compounds (VOCs) and particulates, will not necessarily have any relationship with occupancy, but models such as the CIBSE spreadsheet can be readily adapted to explore the impact of ventilation on the concentrations of other pollutants. 1,200 CO2 concentration (ppmv) CPD PROGRAMME Internal External 1,000 Figure 1: Simple modelled output of resulting CO2 level with one person per 10m2 floor area, room height 3m, with each person exhaling a typical 20 litres of CO2 per hour with 10Ls-1 outdoor air supply, occupied 8am to 6pm 800 600 400 200 0 0 4 8 12 16 20 24 Time (hours) openings, including trickle vents and fortuitous leakage around doors, windows and gaps in the building fabric (ADF System 1). Using manual or automatic control, this method may be applied as a means of purge or task ventilation, or could be used continuously. Such a simple system may be readily retrofitted, and can be very cheap to install and operate. Although simple in form and operation, however, it may not be operated by the building occupant as intended for example, because of problems with noise or a lack of understanding so it may not be an assured method of maintaining good IAQ. Providing multiple building penetrations to accommodate the outlets may be challenging, particularly in multi-storey buildings. Fans may be added to passive stack ventilation ducts (discussed above) to provide a hybrid system for use when there is more ventilation required than is given by natural means only. These can be controlled automatically for example, through humidity sensors or selective IAQ sensors or manually. To remove the uncertainty of natural ventilation and improve controllability, automatically controlled continuous extract systems may be employed (ADF System 3). These are normally ducted systems with a centralised fan unit, typically linked with radial ductwork of 100mm-diameter flexible or rigid plastic rectangular ducts to ceiling grilles mounted in areas requiring extract ventilation. Alternatively, decentralised mechanical extract ventilation (dMEV) is used, which consists of continuously running extract fans. Low-power fans are typically located in every wet room, running continuously at low trickle speeds, which are then boosted as required via automatic or manual control. As the power to move the air is supplied by the fan, any ductwork is able to take a more circuitous route (with the associated increased resistance to flow and additional fan power), and the openings to draw in outdoor air may be smaller or fewer. This system is relatively simple to satisfy 62 March 2020 www.cibsejournal.com CIBSE Mar20 pp61-64 CPD v3.indd 62 21/02/2020 15:08