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

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

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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There 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 MVHR system presents a technical challenge, as its effectiveness depends on striking the correct balance between heat recovery efficiency, 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 prior 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 (CO2) emission reductions are to be achieved.

What we did

The investigation used the E.ON Retrofit Test House (figure 1) –a replica of a 1930s semi-detached house – as a case study to examine 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, 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 E.ON Retrofit 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 one specified 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 to the simulated naturally ventilated E.ON Retrofit Test House – which has kitchen and bathroom extract fans but no MVHR – at an air permeability of 10m3/(h.m2) @50Pa.

What we observed

Installing an MVHR system in a ‘leaky’ dwelling increases the building’s energy requirements because the MVHR system increases the air change rate. The extra air needs to be heated to maintain the internal temperature – although this will be partially offset by the heat recovery. Figure 2 shows this increased energy requirement for the E.ON Retrofit 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 of airtightness, the reduction in space heating energy exceeds the energy expenditure required to operate the MVHR 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 MVHR system operating to best practice standards, the investigation 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 lowerslightly poorer level of airtightness than with the MVHR system operating to minimum building standards. This suggests that in 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 emissions, 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 MVHR installation in order to avoid undermining the energy-saving efforts. Quality control and training of installers is critical to optimising the operation of these systems.

What we recommend

As a result of the CALEBRE investigation at the E.ON Retrofit Test House, we have come to the following conclusions:

  • 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 of air permeability often demonstrated by existing dwellings
  • Further research is required to understand the relationship between MVHR systems and airtightness levels in other dwelling types, and 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.

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Authors: PFG Banfill, SA Simpson, DL Loveday and K Vadodaria (details?)

Professor Phillip F G Banfill, Director of Research – School of the Built Environment, Heriot Watt University, UK

Sophie A Simpson, Research Associate – School of the Built Environment, Heriot Watt University, UK

Professor Dennis L Loveday – Professor of Building Physics, Director – Sustainability Research School, Loughborough University, UK

Keyur Vadodaria – Senior Research Associate - Centre for Advanced Research in Building Science and Energy (CARBSE), CEPT University, India