Pressure Switch Codes: Proper Testing is the Key

By: Peter Kirtschej, Service Coordinator, ABR Wholesalers, Inc.

September 13, 2013

With the summer coming to an end and the colder fall nights approaching, I have already had questions about pressure switch codes on furnaces.  Many of the issues relate to improper testing of the switches.  In our industry, we use pressure switches as safeties.  The instinct for some contractors is to jump out the switch and replace it.  Since the pressure switch is used as a safety to prove drain and vent, they should never be jumped out without proper testing.  The best course of action when dealing with pressure switch codes is to test the switch and the rest of the furnace to find the root cause of the failure.

To test a pressure switch, tee in a magnehelic or a digital monometer and check the negative pressure being applied to the switch.  Here are possible scenarios.

  • If you are checking a vent pressure switch that is rated for 2.0 inches of water column (iwc) and you are only pulling in 1.0 iwc, there is a problem with the vent.    
  • If you are checking the drain side of a switch and the water column reading is less than the rating on the switch, there is something wrong with the drain or trap, or something is causing the furnace to not drain properly.

Pressure switches are rated in inches of water column and there is a window of tolerance.  For instance...

  • If an inducer draft blowe pressure switch is rated at -1.30 iwc, the max make point is -1.48 iwc.
  • If the switch set point is -1.50 iwc, the max make point is -1.63 iwc.
  • A drain pressure switch that is rated for -40.0 iwc max make point is -0.55 iwc.
  • You can also ohm out a connection with a meter.  Zero ohms is an ideal connection.  Anything above three ohms is weak and could get worse in the future.   

Taking time to properly test the pressure switch and the rest of the furnace will help you identify the root cause of the failure.  Of course, if you encounter a switch that has water, sediment, or debris in it, then at that point the switch needs to be replaced.  If that occurs, you will also need to find the cause of the water or debris to eliminate the issue.  We stock an adjustable pressure switch that can be used in place of jumping if the switch is defective and an OEM switch is not available in your truck.  Every service truck should have one!

  • Adjustable Pressure Switch model no.: 8021203090
  • UEI Electric Monometer model no.: EM151

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