DCV systems can’t be measured against
SAP as they provide variable ventilation depending on occupancy. This state of affairs has left MVHR in pole position simply because SAP has no methodology to measure DCV performance. This has put the technology on the back burner despite its many advantages over MVHR in terms of better indoor air quality, energy efficiency, cost and ease of installation and operation. Nevertheless, DCV is the system recommended in the 2009 NHBC report Indoor air quality in highly energy efficient homes – a review which says: ‘The next drive by the industry will be for advanced controls and, in particular, for demand controlled ventilation (DCV). If the energy savings resulting from the potential reduction in fan operation and heat loss are to be realised, the “building empty” and “room empty” minimum ventilation rates must be determined.’
46 CIBSE Journal September 2012
And now there is evidence showing the
impact of different ventilation systems on annual energy consumption. The house used for this comparison (see
graph) is a typical 1930s two-storey, three bedroom semi-detached property with three ‘wet’ rooms. Typical air tightness for this type of property is N50 value around 7 m3
/hr/m2 envelope area. The annual heating season is taken to
be 33 weeks long and, for the purpose of comparison, it is assumed that the property has a gas-fired central heating system. The research shows that the most energy efficient ventilation system is demand controlled passive stack ventilation, followed by MVHR and demand controlled MEV. The three worst energy consuming ventilation systems are, in the following order: Standard MEV, positive input ventilation (PIV) and finally, individual extract fans.
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