The MVHR system is an essential part of a Passivhaus. The heating demand of such extremely energy efficient buildings is only 15 kWh/(m2a). As a comparison, the average 1970’s semi-detached social housing property has a heating demand of about 500 kWh/(m2a).
If this was replaced by natural ventilation at the same exchange rate, the heating demand would be 35kWh/(m2a). From this it can be seen that the process of recovering the heat from the stale, used air and transferring it to the fresh yet cold air, has a major influence on the energy and emission savings.
Particular attention has to be paid to the choice of the HRV system. Claims of manufacturers regarding efficiencies have to be taken with a pinch of salt, as various test methodologies can influence the results greatly. The safest way is to compare the efficiencies of the official Passive House certification (see PHI web page). The tests are done in a complete different way than e.g. the ones for Appendix Q here in Britain and mirror the overall efficiency within a dwelling more accurately.
Also important is the level of insulation of the ducting. The intake and exhaust ducts within the thermal envelope are typically insulated with 25-50mm of vapour-proof insulation. The insulation needs to be taped correctly to the MVHR unit and the vapour barrier of the external walls. The specification of insulation needs to be checked with the Passive House Designer. All warm ducting (supply and extract) should normally be kept fully within the thermal envelope.
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