Draft Inducer and Pressure Switch

For homeowners

The draft inducer is the small fan motor that pulls combustion gases through the heat exchanger and pushes them out the vent. It runs from the moment the furnace starts a heat call until just after the burners shut off. Two reasons it exists:

Active ventilation. Older furnaces relied on natural buoyancy — hot flue gas rises, cold combustion air enters at the bottom, fire burns in between. This is “atmospheric draft.” It works but is sensitive to wind, vent length, and temperature. Modern furnaces use an inducer to actively pull flue gas out, which makes combustion controlled and predictable regardless of conditions.

Sealed combustion. With an inducer, the whole combustion path can be sealed — combustion air comes in through one duct, combustion gases leave through another. The home’s interior air never mixes with combustion air. Safer, more efficient, more controllable.

The pressure switch is the safety that proves the inducer is actually working before the furnace is allowed to fire. A small silicone hose runs from a port on the inducer housing to the switch. When the inducer is running, it creates negative pressure inside the housing, which travels through the hose and pulls a diaphragm inside the switch. The diaphragm movement closes an electrical contact, telling the control board “inducer is moving air, safe to ignite.”

No inducer running, no negative pressure, no closed switch, no ignition. This is the single most important safety on a gas furnace because without it, a furnace with a blocked vent would happily ignite and dump carbon monoxide into the house.

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For technicians

The inducer motor. A small (typically 1/30 to 1/8 horsepower) PSC or shaded-pole motor driving a squirrel-cage impeller. Mounted in a housing bolted to one side of the heat exchanger cabinet, with an outlet that connects to the flue vent pipe.

When the motor runs, the impeller pulls flue gas through the heat exchanger from the combustion side, then pushes it out the vent. This creates a slight negative pressure inside the heat exchanger cells (typically -0.5 to -1.5 in. w.c.) and a slight positive pressure in the vent line itself.

Most inducer motors are PSC type — single-phase induction motor with a run capacitor. The capacitor is typically 5–10 µF, 370V rating, mounted near the motor. Some condensing furnaces use ECM inducers for variable-speed combustion control — staged firing rates with adjusted inducer speed to match.

The flue vent.

• 80% AFUE furnaces vent through a metal flue (typically 4” galvanized) that exits through the roof or a side wall. The flue gas is hot (350–500°F) and dry — no condensation.
• 90%+ condensing furnaces vent through PVC (typically 2” or 3” diameter) that can exit through a side wall horizontally. The flue gas exits at 100–150°F and contains liquid water from condensation. PVC handles this; galvanized steel would corrode rapidly.

A condensing furnace also has a combustion air intake pipe — typically PVC, running alongside the vent pipe — that draws outdoor air directly into the inducer housing rather than using indoor air. Two pipes run together from the furnace to the outside termination.

The pressure switch. Mechanically simple device — a single SPST switch with a pressure-sensing diaphragm.

Internal construction:

• A flexible diaphragm separates two chambers
• One side connects via silicone hose to the inducer housing (low-pressure side)
• The other side is vented to atmosphere
• A spring pushes the diaphragm toward the atmosphere side (default position is “switch open”)
• When negative pressure develops on the inducer side, the diaphragm flexes toward that side, overcoming the spring
• A mechanical linkage from the diaphragm operates the electrical contacts
• Contacts: normally open, close when sufficient negative pressure is sensed

The switch has a calibrated operating point — typically rated to close at -0.40 to -1.50 in. w.c. depending on the model.

Conditions that prevent the switch from closing:

1. Inducer not running — failed motor, failed capacitor, no power, control board not energizing inducer
2. Blocked vent — bird nest, ice plug, debris, wasp nest in the termination
3. Blocked combustion air intake — debris, snow, ice, insect nest
4. Disconnected pressure hose — silicone hose fell off the tap or the switch
5. Cracked or pinched hose — leaks atmospheric pressure into the negative-pressure side
6. Failed switch diaphragm — torn or stuck, won’t move with pressure
7. Failed switch contacts — diaphragm moves but contacts don’t make
8. Vent termination flooded — water in the vent pipe blocks airflow
9. Condensate trap on condensing furnace blocked — backpressure prevents proper draft

All of these result in the same symptom: ignition sequence fails at the pressure switch step. Diagnostic is to verify each potential cause in sequence.

Diagnostic procedure.

1. Start a heat call. Watch and listen — does the inducer start running? If no, the problem is upstream (control board, inducer motor, inducer capacitor, power supply).

2. If inducer is running, check the pressure hose. Pull the hose at the switch and feel for suction at the open end. If no suction, the hose is plugged or disconnected at the inducer end, or the tap port is clogged.

3. Check the vent for obstruction. Cycle power off, remove vent cap, inspect for blockage. Common culprits: spider webs in vent termination, mud daubers, ice (cold-climate failure).

4. Measure switch operation. With inducer running, use a multimeter on continuity setting across the switch terminals (control wires disconnected). Switch should read continuous (closed). If open with inducer running and vent clear, the switch is bad — replace.

5. Measure inducer suction directly. Tee a manometer into the pressure hose. Verify negative pressure during operation. Significantly less negative than spec means the inducer is weak or there’s a leak.

Failure modes specific to the pressure switch.

Stuck closed. The switch contacts weld shut, or debris holds them mechanically closed. The board sees “proof of vent” even when there isn’t one. Dangerous failure mode because it defeats the safety. Detection: measure switch continuity with the furnace off and the hose disconnected — should read open. Continuity in that state means stuck switch.

Stuck open. Diaphragm torn, contacts corroded open. The board never sees proof. Furnace locks out with pressure switch fault code. Replace.

Hose deteriorated. Silicone hoses age out — they get stiff, crack, develop pinhole leaks. Symptoms: intermittent operation, “sometimes works, sometimes doesn’t” calls. Replace the hose preemptively at 10+ year service intervals.

Tap port clogged with condensate. On condensing furnaces, the pressure tap port on the inducer housing can fill with condensate that runs back from the vent. Blocks the pressure signal. Disconnect hose, blow out the port, reconnect.

Replacement. Pressure switches are typically rated by their activation pressure, expressed in inches of water column. Replacement must match the rated activation pressure exactly — a switch rated to close at -0.50 in. w.c. won’t reliably close on a furnace designed for a -1.00 switch, and vice versa. The replacement part number is stamped on the switch body.