Paul Williams, Domus Ventilation Product Manager, provides five steps to selecting the right intermittent extract fan
Did you know, indoor air pollution can be up to five times higher than external air pollution? Combine that with the fact that we spend by far the majority of our time indoors (up to 90%), it’s easy to see that indoor air quality (IAQ) is fundamentally important to our health. Poor IAQ can exacerbate respiratory conditions such as asthma and, over the long term, affects the lungs, heart and blood vessels which can lead to stroke, heart attack and lung cancer.
There are measures we can take to considerably improve IAQ. Whilst you have no control over residents’ activities, such as the types of cleaning products they use, how they dry their laundry etc., you can make a difference through ventilation, in the form of extract fans.
Intermittent extract fans are a cost-effective, easy to install means of ventilating bathrooms and kitchens in existing and new build properties. They provide rapid local extraction of stale, damp air, which in the case of the latter reduces condensation and the opportunities for mould to grow.
Axial and Centrifugal Extract Fans
Intermittent extract fans are available in axial and centrifugal models. Whilst both perform the same task of ventilation, they operate in different ways. 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 flow 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.
5 Steps to Getting the Right Fan
- Location
Both Axial and Centrifugal fans are suited to bathrooms and kitchens, but the location of those rooms within a property will help determine which you select. 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 effective 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. However, because of the additional installation work required, they are mostly restricted to new builds and major refurbishment projects.
- Size & Power
The majority of intermittent extract fans in our homes are not powerful enough to provide adequate ventilation. Revisions to Building Regulations, which came into force in 2022, acknowledged this and increased minimum flow rates across all property sizes. That means more powerful and efficient 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 final extraction rate.
The physical size of the fan also matters. Thankfully there are only two sizes to select from – 100mm and 150mm. Bathrooms that are greater than 9m², and/or are subject to heavy usage, should be fitted with the larger size fan.
- Energy Efficiency
Building Regulations Part L states that Specific 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 efficiency.
- Control Options
Intermittent extract fans are designed to operate on demand, rather than continuously. As a result, 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 significant change in humidity. These are at a higher point than the other options but are the most effective.
- 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 fit 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 the inexpensive flexible ducting which creates air resistance, and the subsequent rattling can be heard. You can also insulate the ductwork on longer duct runs to reduce noise. A number of manufacturers provide installer-friendly duct kits which are definitely worth using.
A Word of Warning
One aspect I haven’t covered here is the cost of fans. Axial fans are the lowest cost option, but you shouldn’t determine which fan type to use based on cost as they have been designed for different scenarios. If you select an axial fan for an internal room, it will not deliver sufficient ventilation rates. 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.
For further information on Domus Ventilation visit vent.info@domusventilation.co.uk.
