DOMESTIC VENTILATION: EXTRACT FANS


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Five steps to selecting the right intermittent extract fan


Intermittent extract fans are a cost-eff ective, easy to install means of ventilating bathrooms and kitchens in existing and new build properties. It’s just a matter of getting it right according to Paul Williams, Domus Ventilation product manager


T


he extract fans are available in axial and centrifugal models. Axial fans incorporate impellers that force air to move parallel


to the shaft around which the impellers rotate. Centrifugal fans force air to move at right angles to the intake of the fan using centrifugal force. Duct-mounted Inline fans are also available in both axial and centrifugal formats, as well as mixed fl ow models which move air in both axial and radial direction relative to the shaft. Unlike standard fans, Inline fans are not installed within the room itself, but slightly removed, such as in the ceiling void or attic, connected to the room via ducting mounted with a decorative grille at the end. Axial and centrifugal fans have been designed to


cover most domestic extract fan requirements, but which type you opt for depends on a number of key factors. Five Steps to getting the right fan


1. Location


If the room has an external wall, then an axial fan will most likely be suitable as these work best short duct runs, typically no more than 1.5m, which connect directly to an external outlet or grille. If the room is in the centre of house, a longer duct


run will be required. Here, a centrifugal fan would be more eff ective as these produce more pressure to overcome resistance encountered in longer duct runs (generally up to a maximum of 6m). Inline fans have also been designed for longer duct runs and, because the fan itself is installed remotely, they are ideal for splash zones, such as in showers.


2. Size & power


Revisions to Building Regulations, which came into force in 2022, increased minimum fl ow rates across all property sizes. That means more powerful and effi cient fans need to be used, with a minimum extraction rate for a bathroom extractor fan being 15 l/s. Domus Ventilation fans are all Building Regulations compliant, with extract volumes up to 48.8l/s for axial fans. Consult the manufacturer’s performance curve, which shows the


relationship between the static pressure capabilities of a fan


compared to its extraction rate, plus pressure loss in ducting, to determine the fi nal extraction rate. The physical size of the fan also matters,


with two to select from - 100mm and 150mm. Bathrooms that are greater than 9m², and/or are


subject to heavy usage, should be fi tted with the larger size fan.


3. Energy Effi ciency


Building Regulations Part L states that Specifi c Fan Power (SFP) must be no higher than 0.5 Watts per litre per second (W/l/s). With extract volume rates up to 48.8 l/s but SFP down to 0.38W/l/s, Domus Ventilation’s latest Axial fans combine high performance and energy effi ciency.


4 Control Options


Intermittent extract fans are designed to operate on demand, so you will need to decide which method of control is right for your customer. If cost is the determining factor, then a basic timer and pull cord model which relies on the inhabitant activating the fan remains a solution. The next step up is PIR sensor-based controls that activate the fan on the detection of movement, but these are less popular than they used to be. Becoming one of the most popular control options, is the humidistat or combined humidistat & timer that react to a signifi cant change in humidity. These are at a higher point than the other options but are the most eff ective.


5. Noise


If a good quality intermittent extract fan is used, that has been correctly chosen and installed, noise should not be an issue. Start by checking the fan’s decibel rating and ensure it operates down to 32dB(A). If you are looking to fi t a fan in an open plan kitchen/diner/lounge, or in an en-suite, then a Centrifugal Fan or Inline fan should be considered as these are generally quitter than axial fans. A further recommendation is to use good quality, rigid ducting, rather than fl exible ducting which creates air resistance and the subsequent rattling can be heard. Extract fans are not an expensive item, but


they are an essential one for making a home a comfortable safe place in which to live, so be sure to pick wisely


30 April 2025


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