Pressure switch

The plastic orifice shown in the image above is a snubber. Sometimes the appliance will experience sudden pressure changes such as wind against the exhaust or the burners igniting. The snubber is designed to only allow a designed amount of pressure to be pulled against the switch at one time.

The insert should not be removed to make the appliance function. If you remove the insert and the appliance works, you have another larger problem such as a plugged intake, plugged exhaust or drain.

Pressure switches open and close due to reacting to pressure. You will usually see them labeled as PF (pressure fall) or PR (pressure rise).

Example: A 0.65” WC PF would mean if pressure drops below 0.65” WC, the pressure switch will open and stop the furnace from running.

Pressure switch errors could be the reason for most service calls made.

The pressure switch is a NO (normally open) switch used as a safety device to ensure combustion blower operation with negative pressure through the heat exchanger(s).

Upon a call for heat, the combustion blower will startup. This is pulling a negative pressure through the air intake, heat exchanger, and a positive pressure through the exhaust venting. This negative pressure closes the normally open contacts to complete the safety circuit to start the combustion process.

Possible causes for a pressure switch error:

  • A blocked drain could backup condensate from the combustion process into the secondary heat exchanger causing the pressure switch to fail open
  • A broken combustion blower wheel
  • A blockage in the exhaust venting or combustion air
  • Sagging vent pipe will not allow condensate to drain out causing smaller venting. You’ll hear a gurgling sound when this happens
  • Undersized or oversized venting
  • Too many PVC elbow fittings

Leave a Reply

Your email address will not be published. Required fields are marked *