Quick answer
A furnace pressure switch that keeps tripping is usually doing its job. The switch only closes when the draft inducer proves it can move exhaust, so repeat failures normally mean a blocked vent, a cracked or water-logged sensing hose, a plugged condensate trap, or a tired inducer motor. On this call, the switch itself is the part replaced least often.
A pressure switch is a rubber diaphragm with a set of electrical contacts, and it has exactly one job: refuse to let the gas valve open until the draft inducer has pulled a real vacuum. So when a furnace keeps failing on that switch, the first assumption should be that the switch is telling the truth and something in the draft path is wrong. Fitting a new switch to a furnace with a blocked vent buys you about a week.
What breaks the draft path
- A blocked outside termination — snow, ice, a wasp nest, dryer lint or landscaping fabric across the pipe end. Clearing a snow-blocked vent is the winter version of this.
- A sagging exhaust run. Condensing furnaces make water inside the pipe, and the run must slope back toward the furnace at roughly a quarter inch per foot. A dip holds that water, the water blocks the pipe, and the switch drops out mid-cycle.
- A plugged condensate trap. Same water, different place: the trap under the inducer or collector box fills with sludge and the inducer can no longer breathe.
- A cracked, kinked or wet sensing hose. The small rubber tube between the inducer port and the switch hardens with age, splits at the barb, or fills with condensate — and a hose full of water reads as no vacuum at all.
- A narrowed port on the inducer housing. Combustion debris and rust flakes choke the tap the hose connects to, so the vacuum never reaches the diaphragm.
- A worn draft inducer. Bearings drag and the wheel loads with dust, so the motor still spins but no longer pulls what the switch is rated for.
How a technician proves which one it is
The switch body has its trip point printed on it in inches of water column — something like -0.60 IWC. A technician puts a manometer on the inducer port and compares the vacuum the furnace produces against that printed number while it runs. If the reading falls well short of the rating, the fault is upstream in the vent, the trap or the inducer, and no replacement switch will change that.
If the vacuum is healthy and the switch still refuses to close, then the switch is at fault: the diaphragm has stiffened or the contacts have burned. That does happen, and it happens most in cold garages and unheated crawlspaces, where a stiff diaphragm shows up only on the coldest mornings and behaves perfectly by noon.
Why a pressure switch fault comes and goes
Faults that come and go usually trace back to water or to wind. Water, because a trap or hose only blocks once enough condensate has collected — so the furnace runs beautifully all evening and quits at four in the morning. Wind, because an exposed sidewall termination facing open ground gets gusted, and one hard gust can momentarily kill the vacuum. Houses on the exposed escarpment shoulder in Waterdown and up on the Hamilton Mountain meet the wind version far more often than sheltered infill streets in Westdale.
Acidic condensate is the slow killer behind many of these. The water a high-efficiency furnace produces is mildly acidic, and over a decade it works away at traps, hose barbs and the inside of the inducer housing. A furnace whose trap has never been cleaned is the normal patient on this call, not the unusual one.
What a pressure switch repair costs in the GTA
- Cleaning a plugged trap, port or hose, or re-sloping a section of vent: often the whole fix, and normally handled inside a $90–$150 diagnostic plus modest labour.
- Pressure switch replacement: $250–$500 installed.
- Draft inducer motor: $400–$900, depending on the furnace and whether the part comes as a bare motor or a complete housing.
- Repeat failures on a furnace past 15 with a rusting collector box: read repair versus replacement before spending twice.
The test to insist on is the manometer reading, because it separates a $250–$500 part from a venting problem in about 10 minutes. ZK Mechanical brings that gauge to no-heat calls and will show you the number on it before quoting anything. Read furnace venting types for the background, or call (647) 801-1252 for furnace repair in Burlington and across Halton and Hamilton — book online here.