A furnace pressure switch works by detecting differences in air pressure. If the proper air pressure differential is not maintained, the pressure switch prevents the furnace from operating.
A furnace pressure switch is a safety feature designed to ensure that combustion air intake and exhaust are sufficient for the safe operation of the furnace. When the furnace is started, a draft-inducer motor powers up and begins to draw air through the combustion system. The operation of the draft-inducer motor causes a large drop in air pressure on the exhaust side of the pressure switch, while a smaller drop occurs on the intake side. If the difference between the two is great enough, a flexible diaphragm inside the pressure switch moves and completes a circuit, allowing ignition.
If the difference in air pressure is too small, the diaphragm doesn’t move enough to complete the circuit and prevents the furnace from operating. Several things could cause the pressure switch to remain open, including a failed draft-inducer motor, a cracked heat exchanger or a blocked intake or exhaust vent. If the switch fails to close and complete the circuit, care should be taken to ensure proper air flow before replacing the switch due to assumed failure.