How to Test a Furnace Pressure Switch Safely and Accurately

The furnace pressure switch ensures the draft inducer is working and that combustion gases vent properly; a faulty switch often prevents the furnace from starting. This guide explains step-by-step testing methods, tools needed, common symptoms, troubleshooting tips, and safety precautions to help homeowners and technicians diagnose pressure switch issues accurately.

Item	What It Shows	Why It Matters
Multimeter	Continuity/Voltage Readings	Verifies switch contacts and control signals
Manometer / Vacuum Gauge	Pressure Readings (in H2O)	Measures inducer vacuum to confirm activation threshold
Visual Inspection	Cracked Tubing/Wiring	Common cause of false open/closed states

What A Furnace Pressure Switch Does

The pressure switch monitors the draft inducer fan and flue pressure to confirm safe venting conditions before the ignition sequence proceeds. When the inducer creates adequate negative pressure, the switch closes a circuit that tells the furnace control board it is safe to energize the gas valve and ignition system. Failure To Close prevents furnace startup; Failure To Open can indicate a stuck contact or incorrect pressure range.

Common Symptoms Of A Faulty Pressure Switch

Recognizing symptoms helps prioritize testing. Typical signs include repeated ignition attempts with shutdowns, error codes on the control board related to pressure or trial for ignition timeouts, the inducer motor running but burner not lighting, and intermittent operation. Visual signs include damaged vacuum tubing and corrosion on switch terminals.

Safety Precautions Before Testing

Testing involves live electrical components and gas systems; safety is paramount. Turn off power at the furnace switch and the breaker before inspecting wiring or tubing unless voltage measurements are required. If performing live electrical tests, keep hands clear of moving parts and avoid tools contacting live terminals. If uncertain about gas or electrical work, contact a licensed HVAC technician.

Tools And Equipment Needed

Accurate diagnosis requires the right tools. Essential items include a digital multimeter with continuity and AC/DC voltage functions, a manometer or vacuum gauge capable of measuring in inches of water column (in H2O), an insulated screwdriver, replacement vacuum tubing, needle-nose pliers, and safety gloves and glasses.

How To Identify The Pressure Switch And Tubing

The pressure switch is typically a round or square device mounted near the inducer housing with two low-voltage electrical terminals and a small plastic vacuum hose. The hose connects to a vent/sensing port on the inducer or heat exchanger area. Confirm model and specifications on the switch label for expected activation pressure (commonly between 0.2 and 1.5 in H2O for residential units).

Step-By-Step Testing Procedure

1. Visual Inspection

Start with a close visual check. Look for cracked, collapsed, or disconnected vacuum tubing, blockages at the inducer port, water or debris in the tubing, and corrosion or loose wires at switch terminals. Replace damaged tubing before electrical testing because a simple tubing fault often causes failures.

2. Check For Error Codes And Monitor Operation

Observe the control board LED indicator or fault code display during a call for heat. Note codes related to pressure switch open or inducer operation. Run the furnace through a heat call while observing whether the inducer motor starts and whether the pressure switch is attempting to close.

3. Test Continuity With Power Off

With the furnace switched off and power isolated, remove low-voltage wires from the switch terminals and use a multimeter set to continuity or resistance. At rest (no inducer vacuum) most switches will be open. When common on a closed-type switch, continuity should be present only when proper vacuum is applied.

4. Use A Vacuum Gauge Or Manometer

Attach a manometer to the pressure switch port or inline with the tubing. Initiate a heat call to start the inducer. Record the vacuum value at which the switch closes. Compare to the switch’s rated activation pressure on the label. Accurate Measurement confirms whether the switch is within specification or drifting out of range.

5. Test Switch Contacts With Power On (Low Voltage)

For an active check, restore power and use the multimeter set to AC or DC voltage as appropriate for the control circuit (typically 24VAC in residential furnaces). With wires reconnected, measure across the switch terminals during inducer operation. When the switch closes, the meter should show the control voltage indicating a closed circuit to the board.

6. Bypass Testing For Troubleshooting Only

Temporarily shorting the switch to simulate a closed state can confirm whether the furnace will proceed to ignition, which helps isolate the issue to the switch versus downstream components. Never leave a bypass in place because it defeats a safety device and creates a hazard. Use bypassing only briefly and with full awareness of risks.

Interpreting Test Results

Several outcomes guide next steps: If the inducer creates appropriate vacuum but the switch fails to close, the switch is likely defective and should be replaced. If the switch closes below or above the rated pressure, replace with correct spec. If pressure never reaches activation level, check inducer motor speed, flue obstructions, blocked vents, or a restricted heat exchanger.

Common Causes And Fixes

Typical issues and resolutions include: damaged tubing — replace tubing and clean ports; clogged vent or condensate traps — clear blockages; weak inducer motor — repair or replace inducer assembly; corroded switch terminals — clean or replace; aging or out-of-spec switch — replace with exact pressure rating.

Choosing The Correct Replacement Pressure Switch

Selecting the correct switch requires matching the activation pressure, electrical terminal type, mounting style, and compatible HVAC model. The switch label specifies the closed pressure value (e.g., 0.5 in H2O) and the contact orientation. Using an incorrect rating can lead to nuisance lockouts or unsafe operation.

Preventive Maintenance Tips

Routine checks reduce pressure switch failures. Inspect vacuum tubing annually, clean vent termination and condensate traps, ensure the inducer motor runs smoothly without unusual noise, and schedule yearly furnace tune-ups to catch early signs of restriction or wear. Keeping components clean preserves proper pressure differentials.

Troubleshooting Flowchart

Symptom	Initial Check	Likely Cause	Action
Inducer Runs But No Ignition	Pressure switch remains open	Open switch, bad tubing, weak inducer	Inspect tubing, measure vacuum, test switch continuity
Intermittent Operation	Loose wiring or corroded terminals	Intermittent contact	Clean/secure terminals, replace switch if needed
Error Code: Pressure Switch Open	Check venting and condensate	Blocked vent or condensate trap	Clear obstruction, verify vacuum

When To Call A Professional

If testing reveals complex issues such as a failing inducer motor, suspected heat exchanger cracks, persistent pressure anomalies, or if the homeowner is uncomfortable with electrical testing, a licensed HVAC technician should be called. Professionals can perform advanced diagnostics, hazardous gas checks, and safe replacement with correct parts.

Tips To Avoid Common Mistakes

• Do Not Assume The Switch Is Bad Without Measuring Vacuum: Vacuum measurement distinguishes between tubing/vent issues and a failed switch.
• Avoid Permanent Bypasses: Never leave a bypassed switch in place; it disables a safety control.
• Match Replacement Ratings Precisely: Pressure rating, terminal type, and mounting must match OEM specifications.
• Perform Tests In Sequence: Visual inspection, then passive continuity, then live vacuum and voltage checks for accurate diagnosis.

Glossary Of Key Terms

• Inducer Motor: The blower that creates draft for proper venting before ignition.
• Inches Of Water Column (in H2O): Measurement unit for low pressure used by pressure switches.
• Bypass: Temporary short of the switch to simulate closure—only for testing.

Reference Data And Typical Values

Common residential pressure switch activation values range from 0.2 to 1.5 in H2O depending on system design. Typical control circuit voltage is 24VAC. Continuity on a closed switch appears as near-zero ohms; an open switch shows infinite resistance. Consult the furnace schematic and switch label for unit-specific values.

Replacement And Installation Checklist

1. Confirm switch model and activation pressure on existing label.
2. Turn off power and gas before replacing components.
3. Replace vacuum tubing with rated HVAC tubing of same diameter.
4. Reconnect terminals securely and verify wiring matches schematic.
5. Restore power, call for heat, measure vacuum and verify switch operation.

Further Resources

Official HVAC manufacturer manuals, local building codes, and EPA/NATE-certified technician guidelines provide authoritative instructions for complex repairs and safety compliance. Homeowners should consult these resources or a licensed technician before attempting major repairs involving gas or heat exchanger access.