A stand-by heating/power supply system for an emergency vehicle, such as an ambulance, has an auxiliary electric heater and pump connected in the conventional heater coolant line adjacent the upstream side of the rear heat exchanger normally employed in such a vehicle. A control unit is connected to supply operating power to this heater and pump and also to operate the heater fan independently of the standard fan control provided in the vehicle. The control unit, in turn, is supplied with operating power upon connection with a conventional residential alternating current power source and includes timer circuits and electronic switches to delay the turning on of the heater pump unit and the fan and other auxiliary electrical power supplies for a predetermined time period after the connection with the alternating power source is made. The time delays for turning on different ones of the utilization devices within the vehicle also is varied, so that no sudden power surge takes place.