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HPM September 2017

48 T R A I N I N G & T E C H N I C A L How to deal with extraction ventilation issues HPM’s technical expert, John Love, continues with his new series on domestic ventilation In last month's issue I looked at the Building Regulations requirements for domestic ventilation. This month's article looks at greater detail into how we meet those requirements in relation to extract ventilation from kitchens, utility rooms, bathrooms and WCs. All the major fan manufacturers include details of the Building Regulation's domestic ventilation requirements in their literature and advise on which of their fans will comply with them, but there are other considerations to be borne in mind. The first is in relation to fan duties and in particular the selection of cooker hoods. You may think that if you install an “off the shelf” cooker hood from any reputable manufacturer you will meet the requirement of an extract rate of 30l/s adjacent to a hob. Resistance to airflow Extract hood fans are primarily designed to overcome the resistance to airflow of the integral grease filter, which means that there is only a small margin left to cope with the resistance of any external ducting. If the hood is mounted directly on an outside wall and discharges straight through that wall to the outside, it will cope, but add on a length of ducting (particularly flat duct) and some bends, and you may well fail to meet the extract rate requirements of the Regulations. Some manufacturers state the maximum external resistance – in terms of length of duct and number of bends – that their fan will cope with, but if they do not, then ask before you buy, or you could be held liable for Building Regulations non-compliance. Similar considerations apply to the selection of other extract fans. Some manufacturers simply give extract duties at free intake and discharge. This means that the duty will drop if you connect it to Kitchens, bathrooms and toilets all have different ventilation extract rates any ducting, other than a short length of straight duct through the wall behind. Others actually give performance curves which enable you to check exactly what the fan will do, provided you can calculate the ducting resistance. If you do an internet search for 'calculate ducting resistance' you will find a number of sites where you can easily calculate the resistance and Vent-Axia, for example, has a 'System Calculator' which you can download. If in doubt then ask the fan manufacturer – all it will need to know is the length and size/type of duct and number of bends. Remember that flexible ducting has a far higher resistance to airflow than rigid ducting, especially if it is not fully stretched. The next consideration is controls. Fans can be switched manually by a remote switch or integral pull-cord, automatically switched by a built-in or remote passive infrared sensor detector, or connected to the local lighting switch, which is normal for WCs. If installed in a non-habitable room, such as a kitchen, utility room, bathroom or WC without windows, the Building Regulations require the provision of a 15 minute timed overrun. The timer is usually integral with the fan, which requires both permanent and switched live supplies. A fan can also be controlled by a humidistat (remote or inbuilt) to give on/off or high/low speed operation. Simple humidistats operate at a set adjustable humidity level, but this can cause a problem. Air has the capacity to hold moisture (water) up to a limiting level at which it is said to be saturated – the higher the air temperature the more moisture it can hold and vice-versa. The moisture level of air is defined by its relative humidity – the ratio of the actual amount of water within the air to the maximum it can hold at that temperature (its saturation level). If the air in a room is at 20ºC and a relative humidity of 55% and the extract fan humidistat is set at 65%, the fan will not operate. Relative humidity If the room temperature drops overnight to 16ºC, the maximum amount of water it can hold will be reduced. Although the actual amount of water within the air in the room may not have changed, since the maximum it can hold has reduced, the relative humidity will have increased, from 55% to 70%. This will result in the fan switching on, which could be annoying in the middle of the night. To overcome this problem, a dynamic humidistat can be used – manufacturers have their own trade names – which will automatically raise the humidity setpoint as the air temperature drops, so avoiding this problem. With reduced level of natural infiltration into houses, extract fans are vital to ensure adequate levels of ventilation and the use of humidity controlled extract fans will prevent undesirable increases in internal moisture levels. John K Love CEng, FCIBSE., FIPHE., FIDHE., MInstR., FConsE www.hpmmag.com September 2017


HPM September 2017
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