For installers, replacing or specifying a domestic extract fan can appear to be a relatively straightforward task. However, the choice between an intermittent extract fan and decentralised mechanical extract ventilation (dMEV), should not be based solely on price, familiarity or whether the new fan fits the existing opening. Both technologies have an important role to play, but they operate in fundamentally different ways. Selecting the right option requires an understanding of the property, the ventilation strategy and how the occupants are likely to use the system.
What is the difference?
An intermittent extract fan operates for a limited period, typically when activated by a light switch, pull cord, presence detector or humidistat. Depending on the model, a timer may keep the fan running after the room has been vacated. This provides a relatively high rate of extraction when activities such as showering, bathing or cooking create moisture and pollutants. Once the fan switches off, however, mechanical extraction from that room stops.
A dMEV fan is designed to operate continuously. It extracts air at a low background rate and moves to a higher boost rate when additional ventilation is required. Rather than waiting for moisture or pollutants to build up before extraction begins, it provides ongoing air movement throughout the day and night. It is important not to view dMEV simply as an intermittent fan that has been left switched on. It forms part of a continuous mechanical extract ventilation strategy, with background and boost airflow rates that must be appropriate for the dwelling.
Approved Document F recognises intermittent extract with background ventilators and continuous mechanical extract as different approaches to ventilating a home. Whichever strategy is selected, its purpose is to provide adequate ventilation while maintaining indoor air quality.
When can an intermittent fan be the right choice?
Intermittent fans remain an established and appropriate solution for many applications, particularly in existing homes.
A like-for-like replacement may be suitable where an existing intermittent ventilation strategy is performing effectively, the dwelling has adequate background ventilation and there is no evidence of persistent condensation, mould or excessive humidity.
They can also provide a straightforward solution for individual bathrooms, toilets, utility rooms and kitchens, especially where the fan discharges directly through an external wall. Models are available with different methods of control. Domus Ventilation, for example, offers 100mm and 150mm axial bathroom and kitchen fans, including standard, timer, pull-cord and humidistat-controlled variants allowing you to match the fan to the room and its likely use.
However, an intermittent fan is only effective when it operates for long enough and delivers the required airflow. A fan connected to the bathroom light may not run adequately if an occupant takes a quick shower during daylight hours and does not switch on the light. The same issue can arise if the overrun period is too short or the fan is isolated because the occupant considers it noisy.
When should dMEV be considered?
dMEV is particularly relevant to new build homes, where greater airtightness makes a planned and dependable ventilation strategy essential. It may also be suitable for refurbishment projects and existing properties where there are persistent moisture problems or where relying on occupants to activate an intermittent fan is unlikely to provide consistent ventilation.
Because a dMEV fan runs continuously at a low rate, moisture and pollutants are extracted as they are generated. The fan can then boost when greater extraction is needed as is the case with the Domus dMEV-NICO which comes with an integral monitoring device enabling automatic operation in response to occupant activity. This can help avoid the cycle in which humidity builds up while an intermittent fan is off and is then expected to clear the room in a relatively short operating period.
Continuous operation can also be valuable in rented and social housing, where landlords and property managers require a ventilation solution that does not depend entirely upon occupant behaviour.
Look beyond the fan itself
Choosing between dMEV and intermittent extract is only part of the installer’s job. Even a correctly selected fan will underperform if the rest of the installation prevents it from delivering its intended airflow.
The duct route is a key consideration. An axial fan may be suitable for a short, direct discharge through an external wall, but longer or more restrictive duct runs can create resistance that reduces airflow. Bends, crushed flexible ducting, poorly fitted connectors and obstructive external grilles can all affect performance. Installers should therefore check the complete air path rather than relying solely on the airflow stated on the fan packaging. The most powerful fan is not necessarily the right fan if its performance cannot be achieved in the installed arrangement.
Replacement air is equally important. An extract fan cannot remove air effectively from a sealed room unless air can enter from elsewhere. Background ventilators, suitable door undercuts and an unobstructed route for air to move through the dwelling are integral to the ventilation strategy.
Noise must also be considered. A fan that achieves the required airflow but is intrusive in operation may be switched off or isolated by the occupants. Where a continuous fan is installed, quiet operation at its normal background setting is especially important because the product is intended to run throughout the day and night.
Do not replace a fan without checking the strategy
When attending an existing property, it can be tempting to remove the old fan and install the nearest equivalent. Before doing so, installers should consider why the fan is being replaced.
If it has simply reached the end of its service life and the existing ventilation arrangement is otherwise effective, a suitable intermittent replacement may be entirely appropriate. If the call-out has been prompted by condensation, mould or repeated complaints about humidity, replacing the fan like for like without investigating the wider causes may achieve very little.
The problem could be an inadequate run-on time, insufficient airflow, unsuitable ducting, a lack of background ventilation or an occupant who rarely activates the fan. In these circumstances, dMEV may offer a more reliable solution, but it still needs to be selected and commissioned as part of a properly considered approach.
There will also be properties where neither an individual intermittent fan nor a single dMEV unit is the complete answer. Larger new build and extensively refurbished homes may require a whole-house system, such as centralised mechanical extract ventilation or mechanical ventilation with heat recovery.
The key is to avoid treating ventilation as a collection of isolated fans. Each product should contribute to the movement of air through the dwelling, from the supply of fresh air to habitable rooms through to the extraction of moisture and pollutants from wet rooms.
For electrical installers, the decision is therefore not simply whether dMEV is better than intermittent extract. It is about identifying which solution suits the property, the room and the intended ventilation strategy. Intermittent fans continue to provide an effective and cost-conscious answer in many applications, while dMEV offers dependable continuous extraction where a more consistent approach is required.
Making that assessment before installation will help deliver better performance, reduce the risk of callbacks and, most importantly, provide occupants with ventilation that works in practice rather than only on paper.
By Paul Williams, Domus Ventilation Product Manager
