Architekturbüro Sture Larsen

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Hrach House

No maintenance costs for the solar system since the year of completion in 1985. Yearly auxiliary heating demand: 15 kWh/m²a


A sustainable solar house

This house was the first of its kind in Austria, using an air collector and an un-insulated rock storage unit. The solar heated air warms up the rocks, which slowly loose their heat to the surrounding rooms by radiation.  The circulating air is in a closed loop and has no contact with the air in the rooms. 

However, the success of the house was also due to application of building-oriented design principles: a thermally well-insulated envelope combined with heavy construction material with a high thermal capacity in the interior of the building.  The ratio volume to envelope area was held as low as possible.  The rooms were arranged according to their thermal requirements.  A low-cost concept was achieved by multifunctional use of building components, i.e. the use of constructional components to store heat, the use of the collector as roofing and as a shading device.

From the south-west
Ground floor    

The Solar Air System

Active charge: A fan-forced air collector delivers solar-heated air to the rock storage unit through a closed circuit. The circulated air is only used to transport heat within the system and has no contact with room air.
Passive discharge: The uninsulated storage unit on the main floor slowly passes the heat on to the adjacent rooms by radiation. Radiant heat is also distributed from hypocausts in the first and second floors. The system only uses one fan for the charge mode, while the discharge mode is purely passive.
Auxiliary heating: A tile oven in the living room.
Domestic hot water is preheated by leading the cold water through 38 m of pipes through the warmest part of the rock storage.
Air collector: 52 m2, 60° tilt; single glazed, underflow aluminium absorber, selective surface; air flow rate: 2.700 m3/h.
Heat storage: Rock storage 12,4 m3; face area 5,3 m2; vertical air stream.
  Storage and distribution in hypocausts. Thermal insulation under the foundation slab.


Using dual- and multifunctional building components (such as the integration of constructional and solar components) was an important strategy to cut costs.  The solar air system uses a minimal amount of technical equipment: one fan, controls and a couple of one-way dampers.   The house was monitored* during a seven month period during the 87/88 season.  The remaining amount auxiliary heat needed was 2.020 kWh including 127 kWh of power for the fan.
*Vorarlberger Kraftwerke AG, Austrian Ministry of Science


Inside the sunspace    
South front of the sunspace
Mounting the air collector
Sunspace from the west