Passive-house-suited low power wood-furnace

Log-fired furnace with low energy performance, optimised to be used in passive house constructions. The desired energy output, ranging from 1 kW to a max-peak of 3 kW, is achieved by an "energy storing unit" (energy-storage material covered by an insulation layer) in combination with valves to control the energy release.

Short Description




The project consists of developing a wood burning stove with a small capacity optimized for use in passively heated homes. Different measures such as insulating the stove encasing, reducing heat loss via the front glass panel, reducing the combustion chamber, etc. in conjunction with thermal masses is to achieve the desired output range of <1 kW in the area in which the stove is installed.

Passive houses are defined by the characteristic energy value (<15 kWh/m²a) as well as the heating load (<10 W m²). The general definition states that a building is a "passively heated or low energy house" if it can be heated to max. 45-50°C solely by heating the supply air while an air renewal rate required by hygiene principles (depending on design between 0.3 to 0.5 times of the air renewal) applies. Heating with air is based on a cost argument: Heating with heat recovery is necessary in any case. The existing distribution line thus should also be used for heating purposes. However, practical applications have shown that even passive houses have a limit when it comes to heating with air, in most cases this applies to single-family homes larger than 130-150 m². To reduce the risk of lower than acceptable humidity levels, it is recommended to reduce the air renewal rate, especially during the coldest parts of the year, which also reduces the energy distribution via air. Numerous end customers and architects also ask for cost-effective heating units with biomass. Pellet systems are for the most part already well-suited for these types of applications. Systems using small wooden logs, however, are still in need of some further development. The low consumption of wood and the relatively short heating period in the passive house provide a good user comfort even without a fully automated system.

The basis is an energy consumption and heat distribution simulation in a typical passive housing unit. Buildings in three energetic categories were used for the calculation (15, 30, and 45 kWh/m² a), as well as lightweight and solid buildings.

A plain sample stove was designed based on this data. The stove functions based on the natural draft principle and a patented combustion process and is to deliver the main share of heat to an approx. 500 to 1000 liter accumulator. The energy is to be supplied with gravity-based circulation to save pump energy and a thermal discharge safety device. In the solid building, the strategy of using solely the thermal building masses for heating and a 300 to 500 liter hot water accumulator is examined as well. One heat distribution variant targets the supply air heating of the ventilation system. The heat generation system is supplemented with a complete range of equipment such as simple control and distribution systems (structural component heater, compact wall heater).

Combustion tests were followed by a first series of laboratory measurements of the achieved output, heat distribution per area/accumulator, etc.

Based on these results, the product was technically improved and a new design prepared, and a heating system was built and dimensioned in the laboratory. The measurements have concluded positively and the stove now meets all required standards. The goal of building a wood-burning stove with the smallest output was achieved.

After naming the new stove "gastiny," the R&D project was completed and marketing of the stove commenced.

Project Partners

Project management

Rudolf Vazansky
Gast - Metallwaren GmbH & Co KG

Project or cooperation partner

  • DI Peter Holzer
    Donau Universität Krems
  • DI (FH) Martin Huber
    Bau.Energie.Umwelt Cluster NÖ


Rudolf Vazansky
Gast - Metallwaren GmbH & Co KG
Ennser Straße 42
4407 Steyr