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The PA404 is a series 40 2-wire SPST pressuretrol controller for use in 24 120 240 or 277 volt control circuits or in self powered millivoltage applications. Features The PA404A is a pressure actuated high limit safety control for steam heating boilers. It breaks an electrical circuit to shut down the burner if steam pressure rises above a safe level. The PA404B is used with suspension-type unit heaters to make the fan circuit when a pressure rise indicates the presence of steam. Each model contains an SPST MICRO SWITCH snap-acting switch. Sensitive pressure-actuated diaphragm expands or contracts with pressure variations moving a lever mechanism which operates the snap switch. A screw on top of the case allows set point adjustment without removing the cover. Differential is adjusted by means of a dial located inside the case.