If the switch is not working, the furnace will not start.
What is a Gas Furnace Pressure Switch?
A gas furnace pressure switch is a safety component that makes sure gas is being vented from a furnace before starting.
The furnace pressure switch sends a signal to the furnace control board that verifies the inducer motor is working.
An inducer motor spins creating a vacuum that pulls out toxic gasses that come from the flames.
Before natural gas goes into a furnace and starts a flame, it uses the pressure switch as a safety, to know that the gasses are being vented.
How Does a Gas Furnace Pressure Switch Work?
- When there is a vacuum, the pressure switch diaphragm pulls out and closes the switch.
- If there is no vacuum, the pressure switch diaphragm does not move and the switch stays open.
- If the switch stays open, the furnace will not turn on since it thinks the inducer motor has not turned on.
A pressure switch has a diaphragm that pulls out when a vacuum is applied to it.
If there is no vacuum, the diaphragm does not pull out.
The diaphragm uses a switch connected to it that will show open or closed depending on what the diaphragm is doing.
While an inducer motor may be bad, often, a line is clogged and needs cleaning out.
How Do You Test a Gas Furnace Pressure Switch?
A multi-meter will be needed to test the switch for continuity.
Basically, The switch needs to be tested to see if it is closing when the pressure diaphragm pulls in.
If the switch has continuity and shows closed with the furnace not in operation, it is bad.
The switch should only be closed when the furnace is on, and the inducer motor is working.
When the furnace starts, the draft inducer motor will turn on and begin to blow the air out the vent creating a vacuum that pulls in on the diaphragm and closes the switch.
A multi-meter can be used to see if there is any change and if the switch closes or not.
Steps To Test a Furnace Pressure Switch
- Take the two wires off the pressure switch.
- Turn the furnace on.
- Set the multi meter to continuity and touch the two pressure switch leads.
- If the switch closes, the pressure switch is working and is good.
- If the switch stays open, the pressure switch is Not working.
Where Can You Buy a Gas Furnace Pressure Switch?
HVAC supply stores will sell you a pressure switch but usually at elevated prices. If you can wait a few days, I would buy from Amazon or eBay since they both have a wide selection.
Before changing out an inducer motor or pressure switch, keep in mind that a kinked hose, clogged flue pipe, or anything that stops the inducer motor from pushing air out of the vent will cause a good pressure switch not to work.