Heat recovery ventilation became an unavoidable element of a passive or nearly zero energy building in Northern and Central Europe countries. Airtightness standards became very tight so that the building is compatible with this ventilation system. As frosting of heat recovery unit consumes a lot of electrical energy, a buried pipe system to smooth air temperature variations became also a necessary system in order to avoid defrosting. All these systems cost a lot of money and grey energy to get the benefit of 60 to 70% global net heat recovery of ventilation losses and some freshness in the summer. Southern countries, copying energy saving practices from the north, sometimes without a serious critical approach, adopted this practice and some of them even impose it for low energy consumption buildings.
This article presents the results of a life cycle assessment, taking into account all the consumed primary energy of the system, integrating also the grey energy of the machines and ducts. Heat recovery becomes a questionable principle, in climates where temperature differences and heat recovery potential are low. In cases where high-energy performance heat and cooling production, is used like heat pumps of COP higher that 4, the study shows that unfortunately the system consumes more electricity, than the electricity needed to produce the recovered heat or coolness. The study shows also the limited potential of the buried pipe technique compared to natural ventilation.
The analysis shows that heat recovery in mild climates can be considered as an energy saving measure only where mechanical ventilation is imposed for other reasons (very high external noise pollution, special uses needing high standard controlled ventilation, etc).