OPINION
VENTING FRUSTRATION: THE MVHR EXPERIENCE SO FAR
Although a low carbon solution, MVHR is a complex system that can be costly if wrongly installed. Nigel Ingram explains the pros and cons
As we strive to build energy- effi cient, relatively airtight homes,
the question of how to ventilate them effectively continues to be a challenge faced by many British housing developers. Mechanical Ventilation Heat Recovery (MVHR) has become a
commonplace solution, with around 25%1
of new homes having it installed.
At the Joseph Rowntree Housing Trust (JRHT), the system has been used in the fi rst phase (tendered in 2010) of 64 homes at Derwenthorpe, a sustainable community being developed on the outskirts of York. Looked at on paper, why wouldn’t
you use it? The system brings benefi ts – low energy performance, both for the resident through a reduced energy bill, and for the developer by helping to reach higher levels of the Code for Sustainable Homes. Combined with its ability to provide a healthier indoor environment in the home, the technology would appear to be a mainstream solution. We considered its pros and cons
through building and commissioning two prototypes – known as the TAP scheme2
– for Derwenthorpe in 2009, and monitored them in 2010/11. It was
then recognised that the M&E supply chain delivering this system had ‘kinks’
in it, and we were about to embark on 64 homes with a commercial housing developer. The necessity of MVHR at Derwenthorpe was based on the effect of ‘thermos fl ask’ principles, combining a fabric-fi rst approach with a low air- permeability target of 3.0 m3
/(h.m2 )
or below. At this level of airtightness, MVHR was considered essential to achieve healthy ventilation rates. The challenge of installing MVHR
became apparent early on in the fi rst phase, with the large volumetric properties at Derwenthorpe requiring many ducts. This led to coordination issues in the fl oor void, and the complexities of trades working around each other. Using small duct sizes, proposed at the design stage, with small MVHR units, caused the fans to operate at above acceptable noise levels. Combining this with relative industry inexperience of these challenges – in terms of setting the system up through balancing air fl ows in the commissioning process – meant we were left with variable outcomes on
A relative inexperience of the
industry to these challenges meant we were left with variable outcomes
quality of work. We strived to put this right through re-commissioning and, in some cases, replacement of MVHR units. The cost of these additional works,
the time it took, and the disturbance to residents, made us less confi dent about its suitablility for a further 425 homes being commissioned at Derwenthorpe. To overcome this, for the future
phases we set a relaxed air-permeability level of 4.0 m3
/(h.m2 ), still better than
current Building Regulations, but at a level which meant that MVHR was not an essential requirement. When developer David Wilson Homes was appointed for the remainder of the development, it chose to incorporate a mechanical extract ventilation system (MEV). This created a simpler system and is expected to make the process easier, quicker, cheaper and less risky. But let’s not forget the benefi ts that
MVHR can bring in delivering carbon savings and improving indoor air quality. That’s why at Derwenthorpe we have commissioned the Building Research Establishment (BRE) to monitor the MVHR systems, assessing the energy effi ciency, indoor air quality and, most importantly, the residents’ perception, with fi ndings to be published in 2015. There are many other challenges around the coordination of works on site – from installation to commissioning – that are critical to the success of MVHR systems. JRF research will support the NHBC in formulating a set of rigorous standards for MVHR design and installation. For now, we continue to install MEV
systems at Derwenthorpe. But if BRE, NHBC and others can help overcome the issues faced in the fi rst phase, then MVHR could become a practical solution once again.
References 1 NHBC 2013
2
http://www.jrf.org.uk/sites/fi les/jrf/energy- effi cient-homes-report.pdf
The fi rst Derwenthorpe phase had 64 homes
● NIGEL INGRAM is director of development at the Joseph Rowntree Housing Trust
30
CIBSE Journal March 2014
www.cibsejournal.com
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