Pressure Switches — Draft Proving

A pressure switch on a gas-fired appliance proves one thing: the inducer is pulling draft. Which really means: the flue isn’t blocked, the inducer is actually running at design speed, and the negative pressure at the pressure switch’s sensing port is strong enough to pull the switch contacts closed.

If any of those conditions isn’t met, the switch stays open, the ignition module never advances past its draft-prove step, and the furnace refuses to fire. Pressure switches are simple, reliable components — most diagnostic attention goes not to the switch itself but to the draft path around it.

How the switch works

A diaphragm inside the switch body is exposed on one side to atmospheric pressure and on the other side to the pressure in the inducer housing (or a designated point in the flue path) via a small rubber sensing hose. When the inducer runs, it creates negative pressure at the sensing port, which pulls the diaphragm and closes (or opens) a set of contacts inside the switch.

The switch’s trip point is printed on the switch body in inches of water column (” WC). Typical residential values are 0.45, 0.60, 0.80, or 1.00 ” WC. The switch closes when vacuum exceeds this value; it opens when vacuum falls below it minus the switch’s internal hysteresis (typically 0.1–0.2 ” WC).

Almost all residential draft-proving switches are normally open — the default position (no draft) is open, and the switch must be actively pulled closed by suction. A stuck-closed switch defeats the safety entirely; that’s why most ignition modules also verify that the switch was open before starting the inducer, and throw a “pressure switch stuck closed” fault if it wasn’t.

Two-stage switches on condensing furnaces

A 90%+ condensing furnace has a small positive pressure on the combustion side of the heat exchanger and negative pressure on the vent side. Many two-stage condensing furnaces use dual pressure switches — one proves low-fire draft, a second proves high-fire draft. This lets the control board verify that the inducer can produce the increased draft required for full-fire operation before it allows the gas valve to go to high fire.

Failure modes:

• Low-fire switch stuck — furnace fires on low but won’t go to high.
• High-fire switch fails to close — furnace lights on low, tries to go high, fails to prove, drops back to low or locks out.
• High-fire switch stuck closed — module thinks high fire is proven even when inducer hasn’t ramped up. Safety hazard if the inducer can’t actually deliver the draft for high fire.

Sensing hose problems

The small rubber sensing hose between the switch and the inducer housing is a surprisingly common failure point:

• Age-related cracking. Rubber deteriorates near heat sources. A cracked hose leaks its vacuum signal and the switch never sees enough pressure differential to close.
• Water / condensate in the hose. On condensing furnaces, condensate can migrate up the sensing line and form a column of water that blocks pressure transmission. A switch that closes when cold but opens as the furnace runs and condensate accumulates is often hose-water.
• Soot blockage. On older non-condensing furnaces, soot can partially occlude the hose. Symptom: slow switch closure (takes longer than normal after inducer starts) and chattering on marginal draft.
• Kinked, pinched, or disconnected. Obvious when found. The hose can work loose from vibration.

Check the hose by removing it from both ends and blowing through it gently. Any restriction or blockage is a replacement condition. Don’t straighten a kinked hose and call it good — the rubber will fail again at the same spot.

Diagnostic procedure

When a pressure switch won’t close:

1. Verify inducer is running. Watch and listen. If inducer isn’t running, the problem is upstream of the switch.
2. Connect a manometer to the pressure switch sensing port (or tee into the sensing hose). With inducer running, measure the actual vacuum. Compare to the switch’s trip spec.
3. If vacuum is in spec but switch is still open: switch is mechanically failed. Replace.
4. If vacuum is below spec: problem is in the draft path — blocked flue, weak inducer, hose leak, condensate in hose.
5. Walk the draft path: inspect hose, inspect inducer wheel and motor, inspect flue for blockage or disconnection.

Replacement notes

When replacing:

1. Match trip value — the number on the side of the switch must match.
2. Match electrical configuration — normally open or normally closed, SPST or SPDT.
3. Match physical mount — some switches are panel-mount with specific mounting holes; others use a bracket.
4. Replace the sensing hose while you’re in there if it’s showing age. It’s a $2 part and removing the switch is the labor.
5. Verify after install with a manometer. Confirm trip point matches the new switch’s spec and actual inducer draft.

From the field

A boiler with a draft pressure switch that “needed” replacement every year for three years. The installer kept putting in the same spec switch; the homeowner kept calling back. I found the hose running uphill from the inducer housing to the switch, then across the top of the inducer motor. The motor’s waste heat was cooking the hose, accelerating cracking. Worse, condensate vapor from the flue was recondensing in the cool end of the hose and forming a water column over weeks.

Rerouted the hose to run downhill from the sensing port with a drip trap at the low point, and clipped it away from the hot motor surface. Switch has been working fine for two years since. None of the prior switch replacements were actually solving the problem; the new switches just worked briefly until the same hose failure recurred.

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Check your understanding

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01A furnace's pressure switch closes on inducer start, then opens about two minutes into a heat call, and the furnace locks out. On the next call, it repeats. Most likely cause?

ASwitch is mechanically failing and will soon fail to close at allBCondensate is migrating up the sensing hose and breaking the vacuum signalCInducer motor is dyingDGas valve is intermittent

02You measure 0.8" WC vacuum at the sensing port of a pressure switch rated for 0.45" WC trip. The switch is not closing. What should you do?

ACheck for a higher-rated replacement switchBReplace the pressure switch — the diaphragm or contacts have failed internallyCCheck the gas supply pressureDIncrease inducer speed

03A two-stage condensing furnace fires on low fire, then tries to step up to high fire and locks out with a 'high-fire pressure switch' error. What's being signaled?

AThe inducer has failedBAt high fire, the inducer isn't producing enough additional draft to close the high-fire switch — inducer could be weak, or flue could have restriction revealed at higher flow ratesCThe low-fire switch is stuck closedDThe gas valve is over-firing

A pressure switch is a sentinel for the draft system. When it refuses to close, the thing it’s telling you is almost never “I’m broken” — it’s “the draft system isn’t producing what it’s supposed to.” Measure before you replace, and most of the time you’ll find the real problem upstream.