HVAC Technician Academy
Learn how to diagnose a pressure switch that will not close, appears stuck closed, opens intermittently, or reports a real draft, venting, condensate, or inducer problem.
No. An open pressure switch often means the furnace has not established the required draft condition. The root cause may be a blocked vent, blocked intake, condensate restriction, damaged tubing, blocked pressure port, weak inducer, incorrect venting, or an actual pressure-switch failure.
The pressure switch is not there simply to "turn the furnace on."
It is part of the furnace safety and proving sequence.
Its job is to verify that the inducer has created the pressure condition expected by the furnace design.
If the pressure switch does not prove, the furnace normally should not continue to ignition.
Common directions include:
One of the strongest tests is to measure the pressure being applied to the switch with an appropriate manometer.
Compare the measured pressure to:
If the furnace is not developing sufficient pressure to meet the switch requirement, replacing the switch will not correct the underlying problem.
Pressure switches are commonly marked with a pressure rating in inches of water column.
The exact interpretation depends on the design and whether the switch is sensing negative or differential pressure.
Do not substitute a pressure switch based only on physical appearance.
Use the correct manufacturer-approved part and rating.
Voltage testing can quickly show whether the switch is open or closed in an energized control circuit.
A simplified interpretation:
| Measurement Across Switch | Typical Meaning |
|---|---|
| Most or all control voltage | Switch is open in the energized circuit |
| Very small voltage drop | Switch is closed and conducting |
Exact circuit behavior depends on the control design, so understand the schematic before interpreting any reading.
With power removed and the switch isolated where appropriate, continuity can show whether the switch contacts are open or closed.
But continuity alone does not tell you whether the furnace is actually creating enough draft.
Many furnace boards expect the pressure switch to be open before the inducer begins.
If the switch is already closed at the beginning of the heat call, the board may generate a "pressure switch stuck closed" fault.
Possible causes include:
Pressure-switch tubing is simple but critical.
Check for:
The small pressure port on the inducer or collector box can become restricted.
If the port cannot transmit the true pressure condition to the switch, the furnace may fail to prove draft even though the inducer is running.
Follow manufacturer procedures when inspecting or cleaning any pressure-sensing port.
Condensing furnaces produce water.
A blocked drain, trap, collector box, or condensate path can interfere with pressure-switch operation.
Symptoms can include:
The pressure-switch circuit can expose venting problems such as:
Verify venting against the exact furnace installation manual.
An inducer can run and still fail to create sufficient draft.
Investigate:
"The inducer is spinning" does not prove proper draft.
A furnace may prove initially and then lose the pressure switch during operation.
Possible causes include:
Measure the pressure while the failure actually occurs whenever possible.
Technician Case File
Inducer: running normally to the eye.
Pressure switch: remains open.
Technician considers replacing the switch.
Manometer measurement shows draft is weaker than the switch requirement.
Vent inspection reveals a partial restriction at the termination.
Lesson: The pressure switch was correctly refusing to prove an unsafe or inadequate draft condition.
Furnace starts normally.
Burners ignite.
Several minutes later: burners shut down.
Pressure-switch fault appears.
Inspection finds condensate slowly backing up into the collector system.
The pressure switch is responding to a changing furnace condition, not randomly failing.
Thermostat calls for heat.
Board immediately reports pressure switch closed when it expects open.
Tubing is removed after proper shutdown and a trapped water condition is found.
The sequence fault occurred before the inducer even had a chance to establish draft.
1. Read the furnace fault code and sequence.
2. Verify inducer operation.
3. Inspect intake and exhaust venting.
4. Inspect pressure tubing and sensing ports.
5. Check condensate system where applicable.
6. Measure actual draft pressure with a proper manometer.
7. Compare measured draft with switch/manufacturer requirements.
8. Verify electrical state of the pressure switch.
9. If draft is correct but the switch does not respond correctly, evaluate the switch itself.
10. Retest the complete furnace sequence after repair.
| Pattern | Possible Direction | Next Test |
|---|---|---|
| Inducer runs + switch stays open | Insufficient draft or switch issue | Measure draft with manometer |
| Draft below requirement | Venting, inducer, condensate, tubing problem | Find source of low draft |
| Draft meets requirement + switch stays open | Switch/tubing/port issue | Verify switch and pressure transmission |
| Switch closed before inducer starts | Stuck switch / trapped pressure / wiring issue | Inspect switch state, tubing and control circuit |
| Switch opens after furnace runs | Marginal draft changing during operation | Monitor pressure during failure |
The pressure switch reports a pressure condition. Measure that condition before deciding the switch is defective.
1. What does the furnace pressure switch prove?
2. Does an open switch after inducer startup automatically mean the switch is bad?
3. What instrument can measure actual draft pressure?
4. Why can condensate problems cause pressure-switch faults?
5. What should be checked before replacing a pressure switch?
1. It proves that the furnace has established the required draft/pressure condition.
2. No. The furnace may not be developing adequate draft.
3. An appropriate manometer.
4. Water can interfere with the collector, venting, pressure ports, or tubing and change the pressure seen by the switch.
5. Venting, intake, inducer performance, tubing, pressure port, condensate system, actual draft pressure, and electrical switch behavior.
Return to the heating and combustion hub.
Furnace Sequence →See exactly where the pressure switch fits in the heating sequence.
Heating Not Working →Review common furnace no-heat symptoms.
Furnace pressure-switch faults can indicate real venting, combustion-air, condensate, or draft problems that may create unsafe operation. Diagnosis should be performed by qualified personnel using manufacturer procedures and appropriate instruments. Never bypass a pressure switch as a permanent repair.
Pressure switch faults can come from venting, inducer performance, condensate, tubing, control wiring, or the switch itself. Measure the draft condition before replacing parts.
Furnace Diagnostic Service HVAC Technician Academy