Gas boilers are the default heat source in the UK and can easily be used in a Passivhaus. Combi boilers provide hot water on demand, and as such have high kW output so tend to need additional radiator capacity to buffer this. System boilers work with a hot water cylinder, so the boiler will generally be the smallest size available.
To use deliberately oversized radiators and yet avoid overheating, the boiler controls need to be able to run the heating system at a lower temperature than traditional, and yet still provide hot water. Combi boilers do this by default, but system boilers usually require some sort of add on diverter valve arrangement, along with the manufacturer’s ‘intelligent’ controls – all wired quite differently than conventional mid position valves.
In areas with no mains gas there is the alternative of LPG stored in tanks or large bottles. LPG boilers are essentially the same as those used with mains gas, the only downside is higher fuel cost and the space requirements for gas storage.
Air source heat pumps are a practical option for a Passivhaus. Modern pumps are able to heat hot water without needing direct electric boost (immersion heater), which is important considering that hot water forms the majority of their annual usage. This does mean that the performance may be lower than advertised, and remember that some of the heating demand is also met by hot water losses too.
PHPP now has a detailed section for heat pumps which uses the test data normally quoted by manufacturers for a range of operating temperatures, plus the heating and hot water loads of the building to more accurately estimate the electricity consumption and performance (COP) of the heat pump.
Underfloor heating is normally recommended for heat pumps but, remembering that only 25%-40% of the heat pump output is going to the heating, this may not be worthwhile. Radiators oversized to run at 45°C will be nearly as efficient, plus easier to control and cheaper.
Ground source heat pumps are also possible, though the advantage of using the ground as a low grade heat source in winter is reduced since heating is a small proportion of the total demand, and summer hot water efficiency will be lower than an equivalent air source heat pump. So the high cost of installing ground collectors is not normally justified.
Wood burning stoves can be used, but require careful selection to ensure safe operation with an airtight mechanically ventilated house. There are certified ‘room air independent stoves’ available from Germany and Austria, but they are expensive and hard to find. Alternatively a stove with an external air supply can be used in conjunction with a differential pressure switch to stop the ventilation in the event that it is depressurising the house (due to either supply fan failure or duct blockage).
A cheap, and sensible, approach is to use a stove purely as a room heater, downstairs in a small open plan cottage – that way it can heat the whole house. A hot stove distributes heat via convection very well. The cost of installing a boiler stove with thermal store, associated controls and heating distribution will never pay for itself in a typical Passivhaus, where a wood burning stove is used as a room heater, hot water is normally provided by electricity and solar energy, or a gas boiler.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015.