Austria: Fully Solar-Heated Commercial Buildings
 Austria: Fully Solar-Heated Commercial Buildings

Austria: Fully Solar-Heated Commercial Buildings

The entirely solar-heated production hall of Austrian company Habau Hoch- und Tiefbaugesellschaft has been Harald Kuster’s largest project to date. Since June this year, 1,410 m² of collector area manufactured by Austrian company Gasokol supply process heat in summer and heating in winter and ensure that sunlight alone heats the newly built 7,700 m² halls (see photo). Kuster is Managing Director of Austrian planning company FIN – Future is Now Kuster Energielösungen and an expert in designing and building solar heating systems which make effective use of a building’s thermal mass. He calls his solution VollSolar Häuser (translated as FullySolar Buildings). Kuster has planned 25 construction projects over the last 7 years, most of them commercial buildings. None of these commercial and residential structures required any conventional heating system.
Photo: Habau Hoch- und Tiefbaugesellschaft

“Since I became an energy consultant in 2001, I have come to realise that most heating boilers are actually oversized, and I did not want to repeat this mistake. With every project, I came closer to understanding how you can use a building’s thermal mass for heating,” Kuster explains the origins of his innovative approach. 
The integration of coil registers into the VollSolar buildings’ floor slabs has turned them into a kind of huge underfloor heating. During the heating season, solar energy is fed directly into the slabs. The solar circuit works at an extremely high flow rate to prevent the collector field from exceeding 35 °C and to reduce losses through the roof. The “activated” concrete releases heat into the rooms, providing a comfortable indoor environment by compensating for temperature fluctuations in- and outdoors. Kuster teamed up with Gasokol for most of his projects because the Austrian company produces a special collector type which specifically meets the requirements for heating by means of activated concrete elements.
Harald Kuster
Harald Kuster is the federal state of Salzburg’s pioneer of 100 % solar-heated houses without backup boilers and huge storage tanks. Kuster trained as an energy consultant in 2001, then designed and constructed his first VollSolar building in 2008. All in all, he has helped set up 25 VollSolar buildings across Austria – without using conventional heating systems. In 2009, Kuster founded FIN – Future is Now Energielösungen. 
Photo: Austria Solar
“Designing the 100 % solar heating solution down to the last detail is an extremely complicated process because there has not yet been any standard software programme for it,” Kuster confirms. “First, it is necessary to analyse the client’s behaviour and use of the building in order to estimate the indoor heat gains.” Then, Kuster and his team calculate the energy-saving potential of the floor slabs’ concrete. The last step is the stress test, during which the building simulation must show proof of temperatures not dropping even if there is a three-week period of cloudy weather in winter, without direct sunshine. Because demand for VollSolar design calculations is so great at the moment and FIN – Future is Now can no longer respond to all requests it receives, Kuster has entered into agreements with two licensees which have been trained in Kuster´s design.
What all commercial VollSolar buildings have in common is their low hot water demand: In most cases, the collector field cannot be made large enough if the building has no need for solar heat in summer. For example, the large collector field at Habau was only possible because the solar heat is used in summer for drying the precast concrete elements (see the following table). “In buildings with low hot water demand, like the agricultural training centre in Bruck, Austria, the solar collector circuit will switch to a low flow rate between 12 am and 1 pm in order to achieve the higher temperatures for hot water preparation,” Kuster explains. 

Project, town

Type of building

Collector area

In operation since

Building size

Concrete storage in floor slabs


Habau Hoch- und Tiefbauge-sellschaft, Perg

Production unit for finishing building elements

1,410 m²

June 2014

7,700 m² of floor space and 100,000 m³ of air volume

2,560 m³ of concrete with heat capacity of 10,200 kWh


Agricultural training centre, Bruck

Workshops on woodworking

70 m²

February 2011

636 m² of floor space and 2,670 m³ of air volume

254 m³ of concrete with heat capacity of 1,025 kWh

(in German only)

Kletterhalle, Saalfelden

Indoor climbing

130 m²

Summer 2012

353 m² of floor space and 6,419 m³ of air volume

184 m3 of concrete with heat capacity of 740 kWh

(in German only)

The table shows three VollSolar buildings designed by Kuster, who also supervised the construction of the innovative solar heating systems. In summer, the solar thermal installation at the climbing centre in Saalfelden supplies hot water for 20 shows in an adjacent municipal sports centre. 
Source: FIN – Future is Now
FIN – Future is Now Kuster Energielösungen GmbH
Harald Kuster
Salzburg, Austria
Mobile: +43 699 1722 5111

Baerbel Epp

Bärbel Epp is Founder and Director of the German communication and market research agency solrico and editor-in-chief of