The heating system for the vehicle uses the engine coolant circuit to heat a heat exchanger (15) in the air inlet into the vehicle. To boost the heat output from the engine, an additional heater is fitted comprising a hydraulic unit (7) which heats the coolant through a shear effect generated by a rotor. The viscous effect heater is driven by a belt drive which also drives the hydraulic servo pump (P) and the air conditioning compressor (31). A solenoid operated clutch (6) connects the viscous heater to the belt drive. The viscous heater is activated if the steering is turned slightly when the vehicle is not moving. This enables the heater to supplement to low thermal output of the engine. If the steering turned by a larger amount the control system detects the increased in torque and gives priority to the hydraulic servo circuit by disconnecting the viscous heater and the compressor of the air conditioning system. This relieves torque on the belt drive.
