Writing for Green Building Advisor, Martin Holladay calls the combined mechanical systems “magic boxes.” These are combination appliances that incorporate ventilation and heat pumps for heating and cooling. In may cases, they also include hot water heating. Because of their efficiency, “magic boxes” may offer reduced greenhouse gas emissions even in comparison with other efficient systems such as condensing furnaces or ground-source or air-source heat pumps.While heat recovery and energy recovery ventilators (ERVs and HRVs) are not yet common to most homes, they are an essential part of the mechanical systems for Passivhaus homes and other high-efficiency buildings. Most high-efficiency buildings have very tight construction, therefore mechanical ventilation is needed to bring fresh air in and exhaust stale air from the building. ERVs and HRVs transfer energy from the outgoing air stream to the incoming one to recover some of the energy that would otherwise simply be lost in the exhaust. 
