MVHR - is it always the best option for domestic buildings?

Posted by: David Miles

Mechanical ventilation with heat recovery (MVHR) extracts heat energy from exhaust air. However, it carries a high capital cost and case studies revealed that many systems are incorrectly designed and operated. In the mild climate of most of the British Isles, more efficient options are available at lower cost.

MVHR requires an almost airtight building

The key limitation to MVHR is that it can only extract heat energy from air that passes through it. Any air that passes through the building fabric instead of the ventilation system takes its heat energy with it. In the UK, the standard assessment procedure (SAP) for testing the energy performance of new or renovated homes involves pressurising the building to 50Pa and measuring the air leakage. For MVHR to deliver savings, it's usually recommended that must be no more than 4m3 of air for every square metre of floorspace per hour - usually expressed as 4m3/m2/hr.

Casey Cole of Guru Systems, which specialises in low carbon heat networks, calculated that MVHR systems can be efficient in slightly less airtight buildings but most designs still need building fabric that is considerably more airtight than the UK building regulation requirement of a maximum of 10m3/m2/hr. The Future Homes consultation, ongoing at the time of writing, proposes an updated SAP with the higher standard of 8m3/m2/hr.

To put it another way, routing enough of the airflow through the ventilation ducts for MVHR to deliver energy savings requires building fabric that is twice as airtight as the UK building regulations propose, and that proposal is intended to mandate what they regard as highly energy-efficient buildings. 

Meeting such a high standard of airtightness comes with a high price tag, both for the building fabric itself and for the extensive ductwork needed to meet all of the building's ventilation needs.

Passivhaus in Britain

Passivhaus logo

The combination of MVHR and highly efficient building fabric is central to meeting the very high heating efficiency required to meet the Passivhaus standard for new buildings and the related EnerPHit standard that may be applied to renovated buildings.

To meet the Passivhaus standard, space heating demand must be below 15kwh/m2/yr. A similar standard of 15-20kwh/m2/yr was suggested by the UK government's Committee for Climate Change (CCC) in their UK Housing: Fit for the Future document, accompanied by some rather bold statements about the central role of MVHR in meeting their recommendations:

"Achieving very high levels of thermal efficiency requires increased airtightness and the use of Mechanical Ventilation and Heat Recovery (MVHR) systems."

UK Housing is repeatedly referenced in the Future Homes consultation and appears to have heavily influenced its proposals for revising the UK building regulations.

However, the Passivhaus standard was developed in Germany, where winters tend to be longer and colder than in much of the UK. In the milder climate that dominates the British Isles, Paola Sassi of Oxford Brooks University found that in the milder climate that dominates the parts of the UK where most of the population live, not only could Passivhaus standards of energy consumption be achieved without incorporating MVHR but building around MVHR actually led to poorer energy performance:

While the Passivhaus model is a robust model for ultra-low energy building in cold continental climates, the post-occupancy evaluation of a building in the mild southern UK climate designed to Passivhaus standards was shown to perform more efficiently and without compromising comfort without MVHR.

The CCC do not address the construction costs needed for MVHR to be efficient, but the Passivhaus Trust estimates that the efficiency of their standard comes at capital costs 20-25% higher [PDF] than in a building insulated to the lower standard required by UK building regulations.

Case studies of MVHR

Sassi's study only compared two houses in Cardiff, but several larger scale studies assessed the performance of larger numbers of homes fitted with MVHR. Studies commissioned by the Zero Carbon Hub and Innovate UK have been carried out in Britain while another study was conducted by the Dutch Ministry of Housing, comparing mechanical ventilation systems with and without MVHR in the Netherlands which has a similar climate to Britain.

All three studies found that MVHR rarely lives up to the potential suggested by the CCC. Many homes fitted with them had consistently poor indoor air quality and were prone to overheating.

Given that the main reason for installing an MVHR is to cut the energy costs of heating a home, it's particularly relevant that the Innovate UK study confirmed Sassi's prediction that MVHR does not deliver particularly good energy efficiency. The installations they considered delivered an average energy consumption of 40kwh/m2/yr, falling well short of the CCC's suggested target of 15-20kwh/m2/yr.

By contrast, our own case study of rented flats using electrical heaters optimised for efficiency by occupancy-based control delivered space heating at less than 12.5 kwh/m2/yr, meeting both the CCC's recommended standard and the Passivhaus standard. The houses used for the case study met the insulation and airtightness standards of the proposed update to the SAP as well as those mandated by the current building standards, but fell well short of those recommended by the Passivhaus Trust which would have entailed a significantly higher capital cost.

Design, installation and operation

Arguably, none of the three studies discussed above tested MVHR's potential fairly because few of the systems they assessed had been correctly installed in the first place, and many were not correctly operated.

In the Dutch study, over 75% of the rooms tested did not have a high enough exhaust rate to ventilate the room properly. Worse, the MVHR systems were often installed with a single inlet vent that was so close to the exhaust vent that the exhaust air was being drawn back in, making the indoor air quality worse than the outdoor air before it even entered the home.

A further problem was the noise and vibration of air moving through the ducts. Because MVHR systems require the fabric to be almost completely airtight to function efficiently, high volumes of air must pass through the ducts. It is likely that some of the buildings needed wider ducts to allow enough ventilation at airspeeds low enough to be silent.

As well as having been poorly installed, the Dutch study found that 96% of the systems were not being operated as recommended by the manufacturer. Sometimes the problem could be traced back to installation problems such as controls being difficult to access. In others, the residents simply didn't know how the systems were supposed to be operated. Whatever the reason for the poor operation, neither the air quality nor the energy efficiency were what they could have been.

The problem of poor installation suggests its own solution: expanding the building regulations to include design and installation standards for MVHR. The UK's building regulations currently set out specific requirements for most types of building services but the poor installations reported by the Zero Carbon Hub and Innovate UK studies show that MVHR falls into a gap between Part F that deals with ventilation and Part L that deals with most aspects of heating energy efficiency.

If MVHR is to achieve a significant share of the British market in building services, it will need to be installed by companies that do not specialise in it. The case studies demonstrate that a system as complex as MVHR requires careful design and skilled installation to perform well and in the absence of design guidelines and performance standards, buildings end up being designed around MVHR-equipped ventilation systems that deliver neither good ventilation nor efficient heating.

Operation and maintenance

The problem of systems not being used properly is likely to require a different solution. Mandating a commissioning process that involves training residents may help but commissioning can only involve the first owner or resident to use a system that may last for decades. A home may change hands several times during the lifespan of an MVHR system that is integral to the home, and it cannot be assumed that a home's first owner will repeat the briefing they received to whoever they sell the building to several years later.

In the Innovate UK study, many of the homes were council houses or housing associations in which tenants were not as engaged with the operation of the systems as a homeowner might be. It's easy to forget to change a filter when it only needs doing once a year and when the system seems to be working fine, it's hard to see the point of splashing out £100 on new filters. If clogged grilles were forcing the fan to run for longer and consume more electricity, the loss of efficiency happened gradually enough that no one noticed.

For someone who is used to their home's services needing little more than setting a thermostat or opening and closing a window, the operation and maintenance requirements of ASHPs and MVHRs may be dauting or simply easy to forget about.

The Atamate view of MVHR

Our view is that while MVHR may be appropriate for some of the colder parts of Britain, there are more cost-effective options available for most of the country. We have shown that occupancy-controlled electrical heaters deliver better efficiency than MVHR in Cardiff, which has a climate typical of England, Wales and much of Scotland.

The buildings in our case study did have heat pumps installed in the ventilation exhausts but rather than using recovered heat energy to heat the incoming air, they used it to heat the hot water supply. Systems like the Ecocent boiler integrate the ventilation system with water heating, which usually needs more energy than space heating in a well-insulated building. Unlike space heating, which is only needed at the colder times of the year, hot water is needed all year round.

We accept that MVHR may be the best option in some of the colder parts of the British Isles but unless it is designed and installed properly, the result is an expensive building constructed around an MVHR system that does not deliver either the comfort or energy efficiency that a cheaper and simpler design might have done.

The case studies also show that an MVHR system requires more involvement from the people who live in the home it is recovering heat into than a conventional heating system, both to regulate the heating and to perform maintenance tasks like cleaning and changing grilles.

Our view is inevitably coloured by the fact that building controls are our core product but when the authors of the Innovate UK study concluded that 'it is clear that MVHR are not fit-and-forget systems', we make no apology for seeing building controls as the solution to the problem of residents not prioritising the optimal running of their MVHR. 

If you are reading this on a screen, the solution is literally staring you in the face. The operating system of a modern computer, tablet or phone runs in the background without needing any input from us. If it does need us to do something, it sends a message stating what is required.

Similarly, providing advanced control for MVHR would ensure that its operation is constantly optimised around the indoor air quality, internal temperature and external temperature without needing input from the users. When a filter needs changing, a building control system can send a message to the user along with a breakdown of how much it would save on the electricity bills, simultaneously saving the residents from having to remember when to change the filters and giving them a financial incentive to do so.


Ask us for more

We have responded to the proposed role of MVHR in the UK government's Future Homes consultation on revising building regulations elsewhere link.

If you’d like to know more about our views on the future of housing in Britain, ask us on the form and we'll be happy to let you know.

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