1. In a heating system comprising an evacuated partially waterfilled tank, a service water boiler in the tank, a space heating system return conduit extending through the boiler, a gas burner line, a solenoid valve in said line and having an operator for controlling the valve, and control means connected to the operator for opening and closing the valve, said control means including a master control, sensor means in said tank and responsive to the pressure/temperature conditions in the tank and primary sensor switch means, said control means opening the valve when said master control calls for heat and said sensor means detects that the pressure/temperature conditions are below a preselected amount and closing said valve when said preselected pressure/temperature conditions are exceeded, the improvement wherein said control means includes: second sensor switch means connected to the sensor means for operating said valve for closing said valve when a given temperature lower than said preselected temperature is exceeded providEd that the master control and primary means is not calling for heat, said valve having a solenoid, said control means including a series electrical circuit through said solenoid, said master control and said primary sensor switch means, said second sensor switch means being in parallel with that part of said circuit consisting of the master control and the primary sensor switch means.