Mechanical ventilation and heat recovery units, more commonly known as MVHR, are increasingly common in the UK, but results can be variable. Lee Jackson discusses why MVHR is all-too-often seen as a tick box exercise
MVHR is about creating a controlled and efficient living environment rather than just opening a window to cool down a property or firing up a boiler to heat it up.
However, too regularly MVHR is installed not because of the potential benefits but more as a way of gaining credits towards certain regulations and Code for Sustainable Homes levels. It is as if the specifier is saying, “I’ve added MVHR which gives me extra points to meet the Code” without any real thought given to the functionality or efficiency of the home being created.
The problem is that the industry still assumes ventilation is basically being delivered by a fan on the wall. Now, faced with a technology which cannot just go straight from the shelf into the home, there is a problem in the form of multiple factors, from floor space through to air tightness, and meeting SAP guidelines.
Fans were traditionally fitted on the basis of room size and installers followed basic regulations to decide which room needed a fan and of what size. But now homes are increasingly sealed-up and insulated, MVHR requires a whole-house approach which means technology cannot just be bolted on.
One of the major problems relates to flooring design and the incorrect choice of floor joists.
The preferred approach by many is to use open web flooring joists as they have a diagonal internal structure with a lot of space that is suitable for ducts.
However, the reality is that an I-joist will often allow a simpler duct design and therefore result in a more effective and less noisy fan. This is mainly due to the fact that an open joint often means more bends are needed. As a bend adds the equivalent of an extra metre of ductwork to the calculations, this simple joist choice can have a large impact on the final results of the system.
This highlights the root of the problem which dominates the MVHR market, namely the lack of communication amongst various parties. Firstly, there is a significant hurdle to cross in getting floor joist designers and MVHR installers talking to each other. If a conversation can happen then issues around the duct design and ultimately the variable results can become a thing of the past.
This issue is compounded by the fact that MVHR does not have a specific trade which is responsible for it – falling between plumbing and electricians. These trades don’t always have access to the building designers in order to influence the design of floor joists.
Not only does the industry need to be proactive in making these dialogues take place, but it should be putting forward its own innovative solutions. By calling the SBS helpline on 0800 688 8388, the team can use plans to provide a more integrated design between the structure and ventilation systems, whilst making sure the solution achieves the correct performance. A similar approach needs to be taken industry-wide on all project sizes to ensure MVHR is delivering to its potential.
Secondly, homeowners need educating about the system, how it runs, and ultimately its purpose. MVHR units need their filters changing which many see as a hassle but can impact air purity if ignored. What’s more, the units are often turned off, particularly in housing association properties, because the tenant associates the noise of the fan with energy usage.
I also believe that many tenants still don’t believe that the units are delivering fresh air into the property and would rather just open a window. The industry and installers need to be shouting about the very real benefits that MVHR can deliver; not just air purity and energy savings, but also reduced noise pollution and increased security in comparison to an open window.
Fortunately, attitudes to air tightness are beginning to change with many housebuilders realising that it is pointless having a well built, air tight property complete with an MVHR unit which has issues elsewhere. After all, air tightness can be achieved through solid build methods alone. The best housing designs maximise the efficiency of the technologies included in the property without worrying about points totals.
The truth is that MVHR is getting a bad press, but the issue is not the technology rather poor building design. In the future, manufacturers, merchants, architects, specifiers, and contractors need to work much closer together, as a team, to ensure that this technology works efficiently within the design of the entire building.
// The author is the head of technical services at SBS //