The furnace pressure switch is a safety control that is designed to prove that combustion air is flowing though the heat exchanger and combustion exhaust byproducts are vented out the flue. The pressure switch is a normally open set of contacts. The contacts will close when the pressure in the induced draft blower housing has reached a negative pressure set by the manufacture. The pressure switch is sensing the difference between the pressure in the induced draft blower housing and the pressure in the furnace cabinet.
When there is a call for heat, the furnace control board will first check the safety pressure switch to insure it is open before starting the inducer blower motor. If it is closed, the furnace control will go into a fault because the switch has failed to open when the inducer is not running. If the pressure switch is open, the integrated furnace control will send power to the inducer motor. A negative pressure should be created in the inducer blower housing when the inducer motor is running. All pressure switches have a specific trip point where the contacts close. The pressure switch in the photo will close at negative 0.30 inches of water column (wc).
If the pressure switch does not close, do a visual inspection of the inducer assembly paying close attention to the hose between inducer motor and pressure switch. Look for gaps around the inducer assembly; the gasket or high temperature silicone must make a complete seal. Make sure the connecting hose is tight, no kinks, and does not have any cracks.
To test the pressure switch, do not blow or suck on the ports or tubing connected to the pressure switch. This could damage the diaphragm in the switch and there is no way of telling what pressure the switch closes at. One of the recommended ways to test the pressure switch is with a Magnahelic, multi meter, ¼” tee, and a couple ¼” vacuum tubes. The ¼” tee and two short vacuum tubes can be purchased at most auto parts stores.
To test the pressure switch:
- Remove power from unit.
- Remove one side of existing hose that connects inducer assembly to Air Pressure Switch.
- Connect test hoses and tee to inducer assembly & Air Pressure Switch.
- Connect middle tube of “tee” to negative side of your Magnehelic.
- Note the “trip point” of Air Pressure Switch -0.16wc, -0.45 wc, -0.60 wc, etc.…
- Remove and isolate the wires connected to the pressure switch.
- Set the multi meter to test.
- Continuity with audible tone and connect leads to pressure switch.
- Apply power and call for heat.
- When the inducer motor starts, monitor both meters. If the pressure reaches the negative pressure on the switch, but the pressure switch does not close, the switch needs to be replaced. Replace with OEM or generic switch that matches trip point only.
- If the pressure does not reach the negative pressure listed on the switch, the switch is most likely not defective. There are several reasons the inducer motor does not create the negative pressure needed to close the pressure switch. Inspect the inducer motor operation, don’t forget to check the run capacitor on the combustion motor (may or may not have one). Look for obstructions in the flue, and leaks in the combustion system. Retest after corrective measures have been taken.
- At higher elevations, pressure switches for certain gas heater must be changed due to the lower density of the air. Contact manufacture to confirm.
The pressure switch is a critical part of the furnace safety system and verifies that the unit is venting properly. Improper venting can cause combustion issues, fires, cracked heat exchangers, and carbon monoxide entering a building. Never bypass a safety switch or device!