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
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.
50 CIBSE Journal September 2013
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 semi- detached 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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