Abstract: This invention relates to a boiler installation for the combustion of solid fuels, for example forest waste material in the form of e.g. bark and/or chips, peat pellets, coal etc., which installation comprises a furnace located in connection to a heat medium portion and a grate located in the furnace. The grate feeds fuel from a fuel charge opening through the furnace while the fuel is being combusted, from which furnace the flue gases flow to the heat medium portion and continue to a chimney. The installation also includes at least one intake for controlled supply of combustion air in the form of at least primary and secondary combustion air.