Abstract: A combined heat and power (CPH) system for a building has an electrical generator (12) For supply power and a gas engine (14) which drives the generator, contained in a housing (30). Recirculating air is drawn in through an inlet (32), over the electric generator, gas engine and its cooling and exhaust systems (38, 36) where it is heated by waste heat. It is then mixed with controlled amounts of fresh air to regulate the air temperature before being passed into the building air recirculating system. The gas engine can be run at a constant speed which is not dependent on the required temperature of the recirculating air.