Furnace pressure switch
A furnace pressure switch is a combustion-air safety device. Its contacts change state only when the draft inducer creates the pressure condition the furnace control expects, so ignition cannot proceed when the venting path has not been proved.
Also called draft pressure switch, inducer pressure switch
What does a furnace pressure switch prove?
A furnace pressure switch proves that the draft inducer and vent path have created the pressure condition required for safe ignition. Its code reports circuit state, not which part failed.
Furnace pressure switch: how it works
The inducer starts before the burners. Pressure transmitted through a hose moves a diaphragm inside the switch and changes its electrical contacts. The control board watches both directions of that change: a switch closed before the inducer starts is wrong, and a switch that stays open after draft begins is also wrong.
Where you meet the term
- In a no-heat sequence where the inducer starts but ignition never advances to igniter glow or flame.
- In a flash-code chart that distinguishes a switch closed too early from one that failed to close after the inducer started.
- In a technician's draft diagnosis, where switch state is compared with measured pressure and the model's published setpoint.
How to check it
- Observe the startup sequence with panels installed. Listen for the inducer and note whether the sequence advances to igniter glow before the code appears. That timing is useful without exposing live parts. Owner check
- Inspect exterior vent terminals. From outdoors, note visible snow, leaves, nests, or physical damage at the intake and exhaust without inserting tools into either opening. Owner check
- Measure draft and switch state together. A technician compares a manometer reading with the exact furnace specification while confirming when the contacts change electrically. Technician
Furnace pressure switch: what it is not
- An open pressure-switch code does not prove the switch is defective. The inducer, vent, condensate path, hose, wiring, or pressure available at the switch can produce the same circuit state.
- A pressure switch is not a thermostat for the furnace cabinet. It responds to a draft-pressure condition, while a high-limit switch responds to temperature.
Furnace pressure switch questions
Can a furnace run with the pressure switch bypassed?
It must not. Bypassing the switch removes proof of the combustion-air path and can allow ignition when venting is not safe. A bypass is not an owner test or a repair.
Does an open pressure-switch code mean a bad switch?
No. It means the board did not see closed contacts at the required point. Draft, venting, condensate, tubing, wiring, and the switch itself remain separate branches to test.
Why are there stuck-open and stuck-closed codes?
The board checks the switch before and after the inducer starts. Closed too early fails the first check; open after draft begins fails the second one.
Where this definition comes from
Written from service manual material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- GMVC80 furnace installation instructionsGoodman GMVC80 furnaces: the one-flash ignition lockout sequence, the troubleshooting chart entries separating a low stage pressure switch circuit closed at the start of a heating cycle, sticking contacts and a short in the switch wiring from a circuit that is not closed, the high stage version whose symptom is a furnace that runs on low stage only, the worked total external static pressure example, and the minimum recommended circulation air filter sizes by model.
- Affinity YP9C furnace installation manualYork Affinity YP9C modulating furnaces: the seven-red-flash ignition lockout sequence, and the two-red-flash entry reading pressure switch closed with inducer pressure below the switch setpoint, described as the switch being closed when it should be open.