Bernd Hafner

Viessmann Werke GmbH & Co KG, 35107 Allendorf
DrHf@ Viessmann. com

Passive houses have proved a high potential of energy saving. The actual discussion is at which costs the new concept of thermal insulation of the passive houses can be realised. On the one hand, some components e. g. windows become more expensive. On the other hand, the low heating demand allows some economies in the installation of the heating system. For example an air heating working with a normal ventilation rate of 0.4 h-1 is sufficient to cover the heating demand of the house. An exhaust air heat pump saves installation costs as it needs only electricity and the exhaust air of the ventilation system.

In passive houses the heating demand is usually lower than the energy demand for hot water production. The heating system should take this in account. The solar system for domestic hot water production has a relatively high potential for energy saving. But the space for the solar installation, specially the storage, is expensive.

Approach

The base of the compact unit Vitotres 343 is a ventilation system with a heat recovery as it is required for passive houses. A focus is set on the efficiency of the ventilation system: a counter-flow heat-exchanger and fans with DC-motors are used.

An air heat-pump with a relatively low heating power of 1.5 kW takes the exhaust air coming out of the heat-exchanger as a source. A flow of ambient air is added to the exhaust air to maintain a constant flow-rate of approximately 300 m3h-1 over the evaporator independent of the ventilation rate of the house.

A 250 litre storage tank for the domestic hot water production is included in the compact unit. The heat-pump works on an internal hydraulic heating circuit filled with water-glycol mixture. The hydraulic circuit can be switched from air-heating to hot water production. The storage tank can also be heated by a solar collector.

One of the major challenges during the development of the compact unit were the required dimensions: height about 2 m, width 60 cm and depth less than 70 cm. Already a 250 l storage tank has a height of 140 cm. And still it is almost impossible to include two heat — exchangers with a sufficient surface for a solar collector and a heat-pump in this storage tank without neglecting the companies guidelines for this heat exchanges (no use of copper in drinking water, big diameters of the pipes to reduce furring).

An other solution was developed: Both, the heat pump and the solar collector work on the same heat exchanger in the storage tank (see figure 1). A new control strategy for the heat pump and solar collector avoid the simultaneous heating of the storage by both sources.