Screw feed
The fuel is fed by the feed screw into the boiler from below. It is pushed up into the centre of the chamber and spreads out over the surrounding grate. The combustion air is supplied by a fan, above and below the fuel bed. The air flow can be adjusted to give the best combustion results. The quality of the fuel (moisture content, particle size, etc.) is very important in this respect. The boiler chamber is generously dimensioned so that the gases can achieve complete combustion before they leave the chamber. Regulation is achieved by switching the feed screw and fans on and off as heating demand varies, or by means of a continuously variable frequency converter.

Stepped grate
The fuel is fed into the chamber through the front of the boiler. Combustion takes place on a stepped grate, which consists of alternately tilting and fixed grates that form a “stairway”. The fuel progresses step by step down the stairway where it is dried, generates gas and undergoes final com-bustion. The combustion air is supplied partly through gaps in the grates (primary air), and partly through nozzles in the combustion cham-ber (secondary and tertiary air). The air supply is adapted to the fuel type and moisture content. As an alternative to integrated burner equipment, a separate pre-burner can be used. Wood, bark, peat and coal can be burned in this way in the form of shavings, chippings, briquettes or pellets. The boiler can also be fitted with an oil burner, for use when required.