Like most things, we wish there was a simple answer to this. But the truth remains: it very much depends on many factors. Really, it all comes down to what you want to achieve, what you want to spend, and how effective you want your ventilation to be.

Two of the most commonly used ventilation options are Intermittent Extract Fans and Continuously Running Extract Fans- Either decentralised (dMEV) or centralised (MEV).

But how do they compare against each other? And which one is the right solution for your home, installation, or stock range?

 

Intermittent Extract Fans – The Traditional Choice

An Intermittent Extract Fan is the standard on/off fan that most people are familiar with- typically installed in bathrooms and kitchens. They’re usually activated by a light switch or a humidity sensor to extract stale, moisture-laden air when needed and usually turns off after a set period of approximately 15-20 minutes.

Intermittent extract fans are a simple solution for ventilating wet rooms. Whilst they are inexpensive and easy to install, they don’t offer the same continuous protection against damp, high humidity and poor air quality as other solutions.

So, let’s look at the pros and cons of this solution…

✅ Low cost & easy installation- One of the most affordable ventilation solutions for homes, with straightforward installation and minimal ductwork.

✅ Low energy use- Intermittent fans only run when they are triggered and therefore don’t use continuous energy like other ventilation options.

✅ Easy to replace or upgrade- If something goes wrong with an intermittent extract fan, swapping it for a replacement is very quick and easy to do.

❌ The environment is not consistently controlled- Since they are only activated when purposely switched on or triggered, the fan won’t provide constant control for moisture, humidity or odours. Over time, this can allow condensation to build up and cause damp conditions which can lead to mould problems. This obviously is not good for the health of the occupants or fabric of the building.

❌ Noise levels- The majority of extract fans on the market are noticeably louder which can be disruptive around the home.

❌ Less effective in airtight homes- As homes become more and more airtight, the intermittent approach (if not done right) can very quickly not meet desired standards. The airtight nature of the building and intermittent operation can allow humidity levels to rise, causing increased condensation, damp and poor IAQ. This is why people are more frequently looking at a continuous strategy.

 

dMEV – Decentralised Mechanical Extract Ventilation

dMEVS are becoming increasingly more popular as an alternative to intermittent extract fans, providing low level continuous extraction in areas such as bathrooms, kitchens and utility rooms.

Located in the ceiling or straight through the wall, they run 24/7 at a low speed to continuously remove stale air and ramp up when humidity levels rise, and faster extraction is needed.

The pros and cons…

✅ Better moisture control- Unlike intermittent fans, dMEV products provide a steady removal of moisture and pollutants. This constant approach reduces the risk of damp and mould issues as a result of high humidity levels.

✅ Quiet operation- As these fans run at a consistent low speed, they are usually significantly quieter than the traditional intermittent extract fans.

✅ Energy efficient- Most dMEV units (including our own!) use EC motors and are very energy efficient even though they are continuously running.

❌ Higher initial cost- In comparison to an intermittent extract fan, dMEV units are generally a little more expensive to buy.

❌ Ongoing energy use- Although dMEV fans are extremely efficient, they are still constantly running which will use a small amount of energy per year. We have a running cost calculator available to use on our website to give you a rough idea of the running costs of not only our fans, but anyone else’s too. You might be surprised though as they are not as expensive to run as you might think!

 

MEV – Centralised Mechanical Extract Ventilation

Now let’s talk about MEV systems… The centralised approach to continuous extract ventilation.

Unlike dMEV units which are installed in individual rooms, MEV systems use a central fan unit connected to multiple rooms via a network of ducting. This setup provides whole-house extraction from wet rooms using one single, continuously running unit which is typically located in a loft space or first floor cupboard.

This makes it a great solution for new builds or refurbishments where ducting can be neatly incorporated.

✅  Whole-house coverage- One central fan can ventilate multiple rooms at once, improving indoor air quality across the entire home.

✅  One fan unit- A single point of control to make life nice and simple.

✅  Aesthetics- You only see a ceiling valve/register, so if you are not a fan of seeing individual fan units on the wall and ceiling in each room, this is a great option for you!

✅  One break out point- You only need one external penetration through your roof or wall as opposed to individual break out points, like you get with dMEV. This reduces the amount of work required to install the fan unit and is much more aesthetically pleasing.

✅   Quiet & efficient- Positioned away from living spaces, MEV units are typically very quiet in operation, and EC motor options make them energy-efficient.

❌   Complex installation- An MEV unit requires planning for ducting and space for the central unit, which can be challenging in retrofit scenarios.

❌   Higher upfront cost- Equipment and installation can be more expensive compared to simpler systems.

 

So, which one should you choose?

When it comes down to Intermittent vs Continuous (dMEV or MEV), we would usually recommend a continuous strategy because of the benefits a continuous running strategy provides. Whichever one you choose though, one thing not to forget is with a extract only strategy, all need sufficient background ventilation typically achieved through window trickle vents or air transfer grilles.

There are a number of considerations when looking at the best option for your home or your customers, the truth is that a continuous fan will ultimately provide better indoor quality overall. Although the initial upfront cost is more and running costs a little higher, it is always better to prevent condensation, damp and mould within a home rather than having to fix it when it’s too late.