Abstract: A space heater employs a generally Y-shaped housing. A boiler within the housing contains a working fluid at subatmospheric pressure when at ambient temperature. This reduces the boiling point of working fluid. A series of heat exchangers are mounted in the branch portions of the housing and connected to receive working fluid from the boiler. A blower directs ambient air between the heat exchangers and through the front openings of the housing. A valve can be operated to release pressure from within the boiler and heat exchangers. An electric heating element heats the working fluid within the boiler to a temperature above the boiling point under normal operating conditions. During initial manufacture or repair of the unit, the heating element is activated to heat the working fluid while the valve is open, and then deactivated with the valve is closed to allow the working fluid to cool, thereby creating reduced pressure within the boiler.