The fluid friction heater comprises a heating chamber (7) and heat exchange chamber (8). The heating chamber contains a viscous fluid and rotor (14) shearing it to produce heat. The heat exchanger passes a circulated fluid, which accepts the heat generated. A storage chamber (10) is connected to the heating chamber, forming an auxiliary reservoir for the viscous fluid. A stirrer (13) agitates the viscous fluid in the storage chamber. Preferably the viscous fluid is a silicone oil.
