An MVHR system for a building having a roof with at least two pitched slopes facing in different directions. The MVHR system comprises a Heat Recovery Unit (HRU) for exchanging heat between a flow of ambient air and a flow of air from inside the building. The HRU is connected to two ports located on different pitched slopes, and a flow diverter is provided between the HRU and the ports. The flow diverter is switchable so as to reverse the flows of air into and out of the building via the two ports and through the HRU. The system can be combined with photovoltaic solar cells and/or an air source heat pump and/or a ground source heat pump system.