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Troubleshooting a High Efficiency Condensing Furnace

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Testing a Pressure Switch
Troubleshooting a High Efficiency Condensing Furnace

Furnace pressure switch examples

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A furnace pressure switch is a safety device designed to sense the negative pressure created by the draft inducer at start up (to make sure the inducer fan is running) and to shut down furnace ignition if proper combustion air flow is not maintained.

As with conventional furnace, sometimes the pressure switch on a condensing furnace will fail or get stuck in an open position. To test for proper functioning of the furnace pressure switch, you will need a multimeter (voltmeter/ohm-meter), a thermostat at a setting calling for heat, and the switch’s power wiring leads disconnected from the pressure switch terminals.

  • For each of the switch’s two terminals, the pressure switch ohm meter should read 0 or close to 0 (no resistance) when the ohm meter leads are placed on each of their respective switch terminals.
  • A resistance reading of infinity or a large number means pressure switch failure.
  • Another method is to use a volt-meter where the voltage should read between twenty-four to twenty-eight volts DC between each terminal and ground.
  • You should also check for cracked or broken tubes running from the pressure switch which can cause a tripped switch (failure) condition.

TIP: A bad pressure switch will not function even when getting proper voltage. If the pressure switch does not work, there are a couple of tricks you can try in an emergency situation to get the switch functioning until you get the switch replaced (such as when you must have heat before the furnace can get repaired). You can also try this if you do not have a volt meter.

  • A trick to get a stuck pressure switch to function is to give it a hard flick with your finger.
  • If that does not work you can disconnect the tube at the end opposite of the end that that feeds into the pressure switch (from the burner box, do not disconnect at the pressure switch itself), gently blow and suck a few times until you hear the diaphragm / plunger in the switch move.
  • The burners should then ignite if the thermostat was calling for heat. Since the pressure switch is a safety device, it must be replaced or the root cause problems fixed as soon as possible.

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