Before we begin, here are some examples of recommended dMEV, PIV and Intermittent fan models in line with this article:

dMEV Fans:
Tornado ST100DMEVHT
Monsoon DMEVR100HT

PIV Units:
Envirovent LV-EVL-H
Vent Axia 479091 PureAir PIV

Intermittent Fans - Bathroom:
Tornado ST100HT
Envirovent SIL100HT

Intermittent Fans - Utility Room:
Airflow 9041499 QuietAir QT120 with Humidistat & Timer
Airflow 72687257 iCon 30 

Intermittent Fans - Kitchen:
Envirovent SIL150S
Vent Axia 454059 Silhouette Slimline Six Inch Extractor Fan

OK, let’s set the scene:

For the past 75 years or so, affordable mechanical ventilation has been a fairly common fixture in homes across the UK. Mostly found in bathrooms, kitchens, and utility rooms, these predominantly intermittent fans are still available today from a wide range of manufacturers in varying designs, sizes, and functionality - proving that intermittent fan design and function are still relevant in the 21st Century.

But wait, there’s a new contender on the scene - the dMEV continuous running fan! Boasting far better protection for your indoor air quality from humidity, stale air and pollutants, not to mention being super quiet and extremely energy efficient!

In truth, dMEV units are not that new and have been around for a while, but have become a more affordable technology in recent years, and for good reason.
 
Continuously venting spaces at a controlled low speed and boosting when required helps to keep on top of the build-up of humidity and other airborne contaminants in homes. This approach is particularly effective in controlling spaces prone to damp and mould and has become one of the best ventilation options for combatting and minimising the dangers of black mould in homes when combined with suitable heating and insulation strategies.

However, homes need a balanced airflow and in newer builds and highly insulated renovations this can become problematic. The issue here is that to make a home more energy efficient, we want to minimise the loss of heat. However, by insulating the property we are also stopping natural airflow both in and out of the property. This does retain heat, but it also retains all the pollutants and moisture, and blocks fresh air entering the home. In new builds and major renovation projects, incorporating ventilation systems such as MVHR which mechanically controls both the extraction and supply of fresh air while retaining indoor temperature by harvesting the heat rather than venting it to atmosphere is an excellent option, but attempting to retrofit these systems to houses that are not undergoing extensive renovation is not only very expensive but also very disruptive! 

It is also worth noting that there are many “new builds” from the last 20 years or so that have been designed for heat efficiency but were built before the now glaringly obvious flaw with the design became apparent, with many of these buildings not having effective ways of dealing with this airflow issue.

So what is the issue? Well, if you take a sealed container and try to extract all the air inside you are effectively creating a vacuum. Extractor fans are not designed to cope with the resistance that this would cause and will simply slow down and eventually stop working as the pressure increases. They are designed for free air movement, and while designed to cope with a small amount of resistance, they become ineffective if installed where this resistance is not managed correctly. The basis of their design is to create a limited amount of negative pressure in the room by removing air, which is replaced with air drawn into the room to replace that which has been removed by the fan. This displacement of air allows for fresh, warm and dry air to re-enter a bathroom, utility or kitchen from elsewhere in the house to effectively help dry out the room by venting the moisture vapour to atmosphere and in older homes, a fresh supply of air is drawn in through the fabric of the building to maintain a balanced airflow. 

But what if you live in a well insulated and effectively sealed modern home? Yes, you are extracting air from your bathroom and air is returning to the room past the bathroom door. But this air has come from your landing, which will need to be replaced from perhaps your downstairs hall, which will need air to replace that which has been drawn up towards the bathroom - where is the replacement air into the sealed home system coming from to maintain balance? If the house is well insulated, this can mean putting an extractor fan under undue strain over time, similar to the example of the sealed container above. 

Intermittent fans will struggle in these situations, but continuous running units even more so.

As mentioned above, in older buildings, this is less problematic as there will be enough of an airflow back into the home through the less insulated fabric of the building, along perhaps with unregulated airbricks etc. But how do we address this issue in well-insulated modern builds? 

One option is to make use of regulated air vents in modern windows, which can allow you to open and close them to control airflow. As an example, we don’t want cold air drawn into a bathroom whilst showering as this will condense the steam faster and cool surfaces which will become covered in condensation faster - we want to keep this room as warm as possible to keep the humidity airborne for as long as possible so that the fan can remove it before it can settle and cause damp issues. But we need a return supply of fresh air into the home to ensure the fan is extracting at its optimal performance level. So by keeping the passive air vents in the bathroom window frame closed we stop the return of cold air into the bathroom, but, by opening the passive air vents in the window frame of the living room or hallway, we ensure a supply of fresh air into the home, balancing the internal atmospheric pressure and allowing the inward air the ability to be warmed by our heating as it passes through the home before hitting the humid environment of the bathroom.

Some of you will be thinking, “Hang on a minute, I’ve just spent a lot of money insulating my house and you’re telling me I now need to leave a vent open to let in cold air?” Well, yes, you do need fresh air in your home, but if you are adverse to allowing cold air to be pulled into your home, there is another way to combat damp and stale air in the form of PIV units.

Passive Input Ventilation flips the script by actively forcing fresh air into a property rather than extracting. Usually mounted in lofts with a central vent placed in the ceiling of the landing allowing the introduced fresh and filtered air supply to easily travel to all areas of the home, PIV units allow a constant positive air pressure within the home, forcing the existing stale and humid air out of the building. In older builds, this makes use of the gaps and porous nature of the fabric of the building to allow the internal air to leave the building, and in newer, more insulated homes, the passive air vents in modern window frames come into use. These vents now have a constant exiting airflow from the house, rather than allowing air to flow in from the outside, and PIV units have the option for an in-built heating element to raise the temperature in colder weather to remove the chill from the incoming airflow.

However, Building regulations still require you to have effective extract ventilation in bathrooms, utility rooms and kitchens to cope with the higher volume of moisture and unwanted gasses or air-borne particulate occurring from cooking. This means that if you have a PIR unit installed, you will still require extract ventilation in these rooms to meet the current building regulations.

This brings us to the crunch question: Can I install dMEV continuous running fans alongside a PIV unit? 

In short, yes you can but some consideration needs to be taken before you do so. The consideration is whether the installation position of the dMEV fan will adversely affect the effectiveness of the PIV unit in servicing the whole house?

As an example, imagine a home which requires a PIV unit to help improve the air quality in the bedrooms on the first floor, which are struggling with condensation build-up on the windows. If a dMEV fan is installed in the family bathroom, also situated on the first floor, the constant extraction will cause a local negative pressure in the bathroom, which in turn will monopolise the airflow introduced by the PIV, negatively impacting the amount of airflow to the bedrooms, cancelling out the advantage of the PIV on these spaces as shown in the two diagrams below:

This scenario would be better served by installing a traditional intermittent fan in the bathroom, which will still extract as required when the bathroom is in use, but allows the PIV to more effectively service the two bedrooms the rest of the time when the bathroom extractor fan is dormant. It is also worth noting that the PIV unit will minimise backdraught issues through the dormant bathroom fan, which in turn will act as a passive outlet for the PIV, giving the equivalent effect of the passive air vents in the bathrooms and the same 24 hour protection a dMEV fan would provide the bathroom, but without creating an unbalanced airflow on the 1st floor of the house. 

So, where can you use a dMEV in conjunction with a PIV? Well, if your PIV is loft-mounted, supplying a constant stream of filtered air through the landing ceiling allowing the supplied air to travel throughout your home, installing a single dMEV in a ground floor utility/laundry room, WC or kitchen can help to promote the movement of this introduced air from upstairs to the lower extremities of the building.

This can effectively create a similar air circulation through a home as an MVHR system would, balancing the extraction and supply of air in the home without the imposition of cold air entering in winter months or the intrusive and expensive retro-fitting of ducting throughout the home for an MVHR system. In addition, the bathrooms and other rooms, where mechanical ventilation is required in line with current building regulations, can be serviced by the trusty intermittent fan, proving that while extraction technology improves, some designs are timeless and still relevant today in helping to create the perfect balance for your internal air quality needs