Compared to a conventional house, a high performance house has a very small heating demand (ca. 15 kWh/m2a), a micro peak heating capacity (typically under 10 W/m2) and shortened heating period (i. e. November — March in the moderate EU climatic area). The limit temperature for heating demand achieves 12 °C. This defines a new set of requirements for domestic technical systems. The heat production plant can be small and inexpensive, as also the heat distribution system. The very slow rate of heat loss means that the timing of the heat supply can be very relaxed. However, the auxiliary heat supply must shut down very quickly, e. g. when the sun shines, to avoid overheating and comfort problems.

To produce the needed remaining heat, 31 % of the projects rely on a small heat pump,

10 % burn biomass — primarily wood pellets and 14 % are connected to district heating.

The remaining 37 % of the houses have a fossil heating system (fossil fuel fired). Some projects have a combined heat and power system, increasing the overall efficiency. Most of the heat supply systems are combined with a solar thermal system to produce domestic hot water. In total, 67 % of all buildings use active solar supply for hot water production and 22 % for combined space and domestic water heating. 35 % of all buildings have a photovoltaic system to produce electricity. Figure 4 illustrates the ratio of solar thermal collector gross area to the net heated floor area of the building. This ratio averages 0.05 m2 collector area per m2 net heated floor area. For the PV-Systems the mean value is about 0.13 m2 module area per m2 net heated floor area.

Ventilation loss must be decreased while assuring best indoor air quality. In 68 % of the investigated buildings a mechanical ventilation system with heat recovery is installed. In some buildings (about 28 %) the mechanical ventilation is combined with an earth-to-air

heat exchanger (eta-hx). An eta-hx in high performance housing is a part of the strategy to increase the degree of ambient or renewable energy use. Its benefits in order of importance are:

• prevent freezing in the air-to-air heat exchangers of heat recovery systems,

• increase the temperature of the supply air,

• improve indoor comfort in summer by cooling the supply air.

The relative importance of these three functions varies with the general building concept and the climate. In the demonstration buildings with an eta-hx, the mean value for the specific surface area of the eta-hx per volume of air flow was 0.14 m2/m3h.

The benefit supplying air tempered by the eta-hx does, however, reduce the efficiency of the heat recovery from the heat exchanger. In some passive-houses the supply air is further heated, up to 50 °С, eliminating the need for radiators in all but perhaps the bath room. The mechanical ventilation rate in the buildings is usually designed to supply between 0.4 and 0.5 room volumes per hour.

Another method to reduce ventilation heating demand is by using an exhaust ventilation system. Fresh air can be preheated by solar air collectors ( in 11 % of the projects) or by using a sunspace (13% of the projects).

With this new ventilation strategies the specific ventilation losses can be reduced in the average to 0.30 W/rnF K, which is lower than the transmission losses. Figure 5 shows the relation between transmission and ventilation losses for the projects.

Most of the apartment and row buildings have transmission losses less than 0.4 W/rnF K (facade area) and ventilation losses lower than 0.6 W/rnFK (net heated floor area). These buildings are very efficient (passive-houses) and have heat recovery systems with earth-to air heat exchangers. The higher transmission and ventilation losses of the semi-detached buildings with less efficient building designs and exhaust air systems are also evident if the graph.