Heat pumps are also well suited to Passivhaus – radiators ‘oversized’ to work at heat pump temperatures (around 45C) are not large or expensive thanks to the low heat load, and there is no risk of the heat pump being unable to meet the demand in cold weather. Thanks to the minimal heating requirements of Passivhaus it makes sense to favour air source heat pumps + radiators over the ground source + under floor heating combinations, which are a little more efficient but at high capital cost.
Biomass heating with pellet boilers is not so well suited to Passivhaus – basically the heating demand is just too small, and the boilers aren’t good at operating continuously at such low output. This could be managed with a large thermal store – but again the expense isn’t justified by the minimal heating demand.
Traditionally solar thermal has been a popular choice for Passivhaus designers. The PHPP energy model allows solar thermal input to reduce the total primary energy demand, which can allow simpler direct electric input as the back-up heat source. However big drops in the cost of solar photovoltaic (PV) generation has changed the landscape and solar thermal is becoming a rare option. Managing power from solar PV is a fast developing field – the starting point is controllers which use ‘surplus’ power to directly heat hot water, but newer systems are able to manage domestic appliances and heat pumps, running these preferentially when there is a high level of PV generation.
The Passivhaus Institut is moving towards including PV generation in Passivhaus certification. This will be in the form of new classes Passivhaus Plus and Passivhaus Premium. These standards require the same fabric standard as any other Passivhaus plus reductions in the primary energy demand compared with the existing Passivhaus standard, in order to address the problem with ‘net zero energy’ buildings, which generate a surplus of solar power in the summer and claim it back in the winter.
Excerpt from “How to build a Passivhaus: Rules of thumb”, Passivhaus Trust, April 2015