Testing A Pressure Switch

The purpose of a pressure switch is to prevent a furnace from running when there is an abnormal condition within the furnace system. When a furnace provides a PSW code, most technicians will replace the PSW, because it’s the least expensive item on the furnace. However, a PSW should not be replaced until it has been tested.

Bad Practices

Installing an adjustable pressure switch in a unit and adjusting it to a set point that allows the furnace to come on is unsafe and will void the manufacturer's warranty.

A snubber (see item 1 below - click to enlarge) is used to prevent the pulsing of the PSW upon the ignition of the burners. If the snubber is removed, it will cause more PSW codes.

Testing Methods

There are multiple ways to test a PSW.

  1. One way is with a PSW tester; a PSW tester is a manometer with an internal air pump and a light indicator. When the PSW closes, the light will come on.
  2. If you do not have a tester, another way to test a PSW is to install a Tee into the vacuum tube of the switch and run a vacuum tube to a manometer (see item 2 below - click to enlarge). Then place the voltmeter leads across the pressure switch terminals (see Item 5 below - click to enlarge). Energize the inducer and watch the voltage change from 24 volts to 0 volts. Pay attention to your manometer pressure when the voltmeter goes to 0 volts the switch has been made (see item 6 below - click to enlarge).
  3. (See item 2 below - click to enlarge) is a single point switch. It is measuring pressure at the point the tube installed.
  4. (See items 3 and 4 below - click to enlarge) are differential switches. In these two examples, they are measuring across the HTX. A very popular misconception of this setup is that a plugged HTX will cause a PSW code, this is NOT true.

**Remember when there is a PSW code it’s usually an indicator that there is a problem somewhere else other than a defective switch.

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