Domestic combined heat and power unit

A DCHP unit with a Stirling engine, a burner and a supplementary burner. Water is heated by exhaust gas from the Stirling engine and from the supplementary burner. If a sensor detects that the water temperature has risen indicating that there is insufficient demand for the heat in the exhaust gas, a fan, which normally provides the flow of air to the supplementary burner is operated without firing the burner to generate a flow of cool air to cool the water.