A heater comprises a combustion chamber and a jacket extending about the combustion chamber. There is a fan having an output which communicates with the combustion chamber to provide combustion air. There is also a fuel delivery system having a variable delivery rate. A burner assembly is connected to the combustion chamber. The burner assembly has a burner mounted thereon adjacent the combustion chamber. The burner receives fuel from the fuel delivery system. There is an exhaust system extending from the combustion chamber. An oxygen sensor is positioned in the exhaust system to detect oxygen content of exhaust gases. There is a control system operatively coupled to the oxygen sensor and the fuel delivery system. The control system controls the delivery rate of the fuel delivery system according to the oxygen content of the exhaust gases