Pressure Switches

The pressure switch is a safety device that confirms your furnace's draft inducer motor is pulling combustion gases out of the heat exchanger before allowing ignition. When it fails or the inducer ...

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Switch Type

How to Choose the Right Furnace Pressure Switch

The pressure switch is a critical safety device in gas furnaces that verifies the draft inducer motor is creating proper airflow before allowing ignition. When the inducer starts, it creates negative pressure in the heat exchanger that closes the pressure switch, signaling the control board that it is safe to proceed with the ignition sequence. A failed pressure switch will prevent the furnace from lighting and typically produces a diagnostic error code.

How Pressure Switches Work

Pressure switches contain a rubber diaphragm connected to a set of electrical contacts. A small hose connects the switch to the inducer housing or collector box. When the inducer creates sufficient draft, the negative pressure pulls the diaphragm, closing the contacts and completing the safety circuit. The switch opens again if draft is lost, immediately shutting down the gas valve to prevent dangerous flue gas spillage.

Pressure Ratings

Pressure switches are rated in inches of water column (WC), typically ranging from 0.20 to 1.50 inches WC for residential furnaces. Single-stage furnaces use one pressure switch, while two-stage and modulating furnaces may use two switches with different pressure settings for each firing rate. Always replace with the exact pressure rating specified for your furnace model.

Single vs Dual Pressure Switches

Single pressure switches (SPST) have one hose port and verify draft at a single pressure point. Dual pressure switches combine two switches in one housing for two-stage furnaces, with separate hose connections and pressure settings for low-fire and high-fire operation. Some systems use two separate single switches instead of one dual switch.

Signs Your Pressure Switch Needs Replacement

The most common symptom is the furnace locking out with a pressure switch error code (often 2 or 3 flashes on the diagnostic LED). Other signs include the inducer running but the ignition sequence not starting, intermittent lockouts especially in cold weather, or a clicking sound from the switch without the furnace proceeding to ignition. Before replacing the switch, check the inducer hose for cracks, blockages, or condensation.

Finding Your Replacement

Note the part number and pressure rating printed on your existing switch. Use our Look Up by Model Number tool above to find the exact replacement. Every order placed before 3 PM CST ships same day from National HVAC Parts.

Frequently Asked Questions

Can I use a different pressure rating switch?

No. The pressure rating must match the original specification exactly. A switch with too low a rating may not close reliably, causing intermittent lockouts. A switch with too high a rating may close even without adequate draft, creating a safety hazard. Always match the inches of water column (WC) rating on the original switch.

Why does my pressure switch keep tripping?

Repeated pressure switch faults usually indicate a problem other than the switch itself. Common causes include a blocked flue or exhaust vent, a cracked or disconnected inducer hose, condensation in the pressure hose, a weak inducer motor, or a partially blocked condensate drain (on condensing furnaces). Always check these before replacing the switch.

How do I test a pressure switch?

Disconnect the hose and gently suck on it while testing continuity with a multimeter across the switch terminals. The contacts should close when you create suction and open when you release. If the switch does not close with suction, or stays closed without suction, it needs replacement. Never bypass a pressure switch, as it is a critical safety device.

What causes condensation in the pressure switch hose?

In high-efficiency condensing furnaces (90%+), the flue gases contain significant moisture that can condense in the pressure tubing, especially in cold weather. This water blocks the hose and prevents the switch from sensing draft. The fix is to drain the hose, reposition it to slope downward toward the inducer, or install a condensation trap if one is missing.

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