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Unless buildings are airtight, MVHR systems won’t be effective – which poses a particular problem in leaky, existing homes. New research by Calebre reveals the extent of that problem

Installing an MVHR system in a ‘leaky’ dwelling increases the building’s energy requirements because it increases the air change rate

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T

here is a small but growing market for mechanical ventilation with heat recovery (MVHR) systems in existing homes. In 2009 it was estimated that 15,000 units – worth £30m – were sold annually in the UK, and of these, about 5% were in the retrofit sector. However, the use of MVHR in existing homes poses immediate questions. Installing any such system presents a technical challenge, as its effectiveness depends on striking the correct balance between optimum heat recovery, fan efficiency, air-flow rate and building airtightness. But in an existing home, airtightness can be problematic, as it is often difficult to access and address the source of a leak – for example, if service penetrations are located behind fixed installations or constructions. As there was no information on this, an investigation was conducted by Calebre, the low-energy technology research project funded jointly by Research Councils UK Energy Programme and energy company E.ON. The findings suggest that existing homes using MVHR must be made significantly more airtight if overall energy savings and carbon dioxide emission reductions are to be achieved.

The E.ON Retrofit Test House which is being used to trial Calebre technologies

What we did The research used the E.ON Retrofit Test House (figure 1) – a replica of a 1930s semidetached house. The case study examined the extent to which an MVHR system, fitted as part of an overall retrofit strategy, would reduce energy use and CO2 emissions. The house was simulated, using dynamic thermal modelling software, at six levels of airtightness, expressed as air permeability values: 10, 7, 5, 3, 1 and 0.63m3/(h.m2) @50Pa. The upper value of 10m3/(h.m2) @50Pa corresponds with the minimum building standards for new dwellings, and can be achieved by basic draught-proofing measures. It should be noted that air permeability values for existing homes can be higher than this. The lowest value,

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MVHR RESEARCH FOR EXISTING HOMES

What we observed Installing an MVHR system in a ‘leaky’ dwelling increases the building’s energy requirements because it increases air change rates. The extra air needs to be heated to maintain the internal temperature – although this will be partially offset by heat recovery. Figure 2 shows this increased requirement for the test house, relative to natural ventilation, at air permeability values of 10 and 7m3/(h.m2) @50Pa. With an MVHR system specified to minimum building standards, it is necessary to improve the airtightness to 3m3/(h.m2) @50Pa. At this level, the reduction in space heating energy exceeds the energy expenditure required to operate the system.

It should be noted that the MVHR system uses electricity to operate, which is more carbon-intensive than the mains gas used for the space heating system. This means that a greater reduction in space heating energy is needed to offset the increased electricity consumption and ensure an overall reduction in CO2 emissions. For the system operating to best practice standards, the researchers found that energy savings and CO2 emission reductions were achieved at air permeability values of 5 and 3m3/(h.m2) @50Pa respectively. In other words, energy savings can be made at a slightly poorer level of airtightness than with the MVHR system operating to minimum building standards. This suggests that in

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Building air permeability (m2/h.m2 @50Pa)

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Figure 1: Modelled annual space heating and auxiliary energy consumption of the E.ON Retrofit Test House

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2250 2000

Annual CO2 emissions (kg CO2)

0.63m3/(h.m2) @50Pa, corresponds to the Passivhaus standard of 0.6 ach-1 @50Pa after conversion of units. The mid-range value, 5m3/(h.m2) @50Pa, represents the measured air permeability achieved by the test house after extensive draught-proofing work. For each level of airtightness, two MVHR systems were simulated: one specified to minimum building standards, with a specific fan power of 1.5W/l/s and heat recovery efficiency of 70%; and the other to best practice standards, with a specific fan power of 1W/l/s and heat recovery efficiency of 85%. The annual energy consumption and CO2 emissions were calculated and compared with the simulated naturally ventilated test house – which has kitchen and bathroom extractor fans but no MVHR – at an air permeability of 10m3/(h.m2) @50Pa.

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Figure 2 shows that an air permeability of 1 m3/(h.m2) @ 50Pa is needed for an MVHR system specified to minimum building standards to achieve an overall reduction in CO2 emissions.

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MVHR RESEARCH FOR EXISTING HOMES

A researcher gathers data for the Calebre project at the E.ON Retrofit Test House

The simulations indicate that there is potential to reduce both energy use and CO2 emissions, but reallife savings are only likely to occur if the systems are correctly installed

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existing homes where it proves difficult to realise low air permeability values, higher performance systems are needed. In addition, it may be possible to achieve further savings by using a more complex air flow rate control strategy, provided the ventilation rate can maintain the indoor air quality. Further research on this is required. The simulations indicate that there is potential to reduce both energy use and CO2, but real-life savings are only likely to occur if the systems are correctly installed. Calebre Project Briefing Note No. 1 illustrates the need for high-quality workmanship when it comes to draught-proofing and correctly balanced installation. Quality control and training of installers is critical to optimising the operation of these systems. What we recommend As a result of the investigation, we concluded: The airtightness of existing dwellings must be improved when installing MVHR systems to maximise energy savings and carbon emissions reductions Retrofitted MVHR systems should be specified to the highest performance parameters to cope with the higher levels

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of air permeability often demonstrated by existing dwellings Further research is needed to understand the relationship between MVHR systems and airtightness levels in other buildling types, and to establish the required air change rate to maintain indoor air quality An approved installation process or standard of quality control should be exercised to ensure the optimal operation of the installed systems When carrying out airtightness improvements to properties, care should be taken to ensure an appropriate air supply to combustion appliances. CJ

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l For a copy of the Calebre MVHR briefing

email K.L.Chadwick@lboro.ac.uk More information at www.calebre.org.uk Professor Phillip F G Banfill is director of research at the School of the Built Environment, Heriot Watt University; Sophie A Simpson is a research associate at the School of the Built Environment, Heriot Watt University; DENNIS L LOVEDAY is Professor of Building Physics, and director at the Sustainability Research School, Loughborough University; and KEYUR VADODARIA is a senior research associate at the Centre for Advanced Research in Building Science and Energy (CARBSE), CEPT University, India. HYWEL DAVIES chaired the advisory group for the project.

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