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 as a way of gaining credits towards certain regulations and Code for Sustainable Homes levels rather than because of the potential benefits. It is as if the specifier is adding MVHR for the extra points without any real thought given to the functionality or efficiency of the home that is 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 that 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 as well 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 that homes are increasingly sealed-up and insulated, MVHR requires a whole-house approach and this means that 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 because 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 that dominates the MVHR market – namely the lack of communication between 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 does happen, issues around the duct design and, ultimately, the variable results will become a thing of the past.
This issue is compounded by the fact that MVHR does not have a specific trade that is responsible for it – falling between plumbers 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. For example, SBS have a bespoke helpline that can help to provide a more integrated design between the structure and ventilation systems, while also 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, the homeowner themselves need educating about the system, both how it runs and its main purpose. MVHR units need their filters changing, which many see as a hassle, but this can impact air purity if ignored. What’s more, the units are often turned off, particularly in housing association properties, because the resident associates the noise of the fan with energy usage.
There also seems to be a belief among many tenants still that the units are not delivering fresh air into the property and they 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 as many housebuilders realise that it is pointless having a well-built, air tight property complete with an MVHR unit that 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 of the matter is that MVHR is getting a bad press. However, the issue is not the technology, it’s poor building design. In the future, manufacturers, merchants, architects, specifiers, and contractors need to work much closer together to ensure that this technology works efficiently within the design of the entire building.