How to Keep Water Out of the Low Pressure Switch on My High Efficiency Furnace


The low pressure switch on a high efficiency furnace is meant to monitor the pressure within the furnace and automatically shut down the unit if the pressure drops below safe levels. If the furnace flashes an error code for a closed pressure switch or if the furnace shuts down unexpectedly, there may be water in the pressure switch. Troubleshooting this issue can often help clear the switch and return the furnace to normal function as well as prevent water from entering the switch again.

Things You'll Need

  • Can of compressed air
  • Clean cloths
  • Turn off the furnace at the main power switch and disconnect the power cord or shut off the circuit breaker. Rotate the manual gas valve clockwise to shut down the gas supply temporarily.

  • Open the main control access panel on the furnace and locate the condensate trap next to the blower housing, which looks like a fan. The condensate trap has tubes exiting from the trap.

  • Disconnect the condensate tube and use a can of compressed air to blow out the tubing.

  • Empty out the condensate trap of debris and any water. Reconnect the condensate tube and make sure the tube is as straight as possible. This will help keep water out of the pressure switch.

  • Examine the heat exchanger and inducer, both of which are located next to the burner orifice, with a flashlight and clean with compressed air.

  • Find the pressure switch on the side of the furnace and disconnect the switch, but do not disconnect the vacuum tubing. The pressure switch looks like a round pipe connected to a plate on the furnace. Turn the switch over and clean out the tubing and the diaphragm in the pressure switch. Dry the switch and then replace the switch in the furnace.

  • Close the access panel and restore the gas and power to the furnace.

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