Air heating is the defining method for Passivhaus, in theory minimising costs. In practice there are a lot of reasons for not using air heating:
• A particular room’s demand for heat and fresh air are rarely in balance – bedrooms need plenty of fresh air but little heat, bathrooms get no fresh air but need heat.
• Warm air rises to higher fl oors – this is not normally a problem but if the heat input to lower floors is strictly limited (by rate of ventilation) then it can be an issue, especially with more than two storeys.
• The air can only absorb a very limited amount of power – several times less than the minimum output of a gas boiler, so some form of buffering is needed if a boiler is used.
Radiators make for a cheap heating system, but are also an excellent match for a Passivhaus. Thanks to triple glazed windows you can put them where you like in a room as there are no downdraught comfort issues. Individual room control with Thermostatic Radiator Valves (TRVs) allows the system to respond to solar and internal heat gains, which provide a far greater fraction of the heating than in a conventional building.
Radiators can also provide buffering for the minimum output of a gas boiler or heat pump: the thermal mass of the radiators allows the boiler fl ow temperature to rise gradually even though the boiler may be generating 4 or 5 times the amount of heat the building needs. Then when the boiler stops fi ring the radiators continue to release heat to the rooms.
Warm radiators generate convective air movement, which helps move heat around a house. This and the high levels of insulation mean you don’t need radiators in every room.
Underfloor heating certainly works in a Passivhaus, but is an expensive option since it is a radiant source and you need to install it in every room you want to be warm. This then means that the system can be far too powerful for the heat load – and needs careful control to avoid overheating. If you can keep the floor temperature down to a degree or two above room temperature however, the heat output will be self limiting – once the room is warmer than the floor it will stop heating the room.