In short
A furnace pressure switch is the safety device that proves the furnace can exhaust before it lights. The inducer fan creates suction, the suction pulls a diaphragm closed, and only then does the control board allow ignition. A switch that stays open usually means a blocked vent, a plugged drain or a bad hose, not a failed switch. The part runs $250–$500 replaced.
The furnace pressure switch is the round disc with a rubber hose on it, mounted near the inducer fan, and it decides whether the furnace is allowed to light.
What a furnace pressure switch does before the burners light
The pressure switch proves draft. Before a cycle starts, the control board checks that the switch is open, and a switch already closed with the inducer off is treated as a fault. Then the inducer motor starts and pulls a slight vacuum through the heat exchanger, and that vacuum travels down the hose to the switch. If it is strong enough, the diaphragm moves, the contacts close and the board moves on to ignition.
The switch stays in the circuit for the whole burn. If the vent blocks or the inducer slows mid-cycle, the diaphragm relaxes, the contacts open and the gas valve closes. Two-stage and modulating furnaces usually carry two switches, one for low fire and one for high fire.
The number on the switch: inches of water column
Every pressure switch has its set point printed on the label in inches of water column, for example -0.60 in. w.c. That is a tiny pressure, a small fraction of one psi, which is why a little water in a hose or a partly blocked vent is enough to stop a furnace. A technician tees a manometer into the hose and compares the draft the inducer is making with the label.
- Draft weaker than the set point: the fault is upstream, in the vent, the drain, the hose or the inducer.
- Draft stronger than the set point with the contacts still open: the switch has failed.
- Replacement switches are matched to the set point specified for that furnace model, not chosen by appearance.
Why 'pressure switch stuck open' shows up in January
A pressure switch code in an Ontario cold snap is a venting or drainage problem far more often than a switch problem. The causes, most likely first:
- Snow, ice or frost over the intake or exhaust termination on the side wall. Clearing it safely is covered in what to do if a furnace vent is blocked by snow.
- A plugged condensate trap or frozen drain line backing water up into the inducer housing.
- A cracked, loose or water-filled rubber hose, or a blocked port where the hose attaches.
- A weak inducer motor that no longer makes full draft.
- The switch itself: a stiff or torn diaphragm, or worn contacts.
This page defines the part. The step-by-step troubleshooting for a switch that faults again and again lives in why a furnace pressure switch keeps failing.
What a pressure switch costs and what to do next
A pressure switch replacement runs $250–$500 installed. If the real cause is a plugged drain, cleaning it or replacing the trap is $150–$350, and an inducer motor is $400–$900. The diagnosis decides which of those you pay for.
Walk outside and check the vent pipes for snow and ice yourself. Do not bypass the switch with a jumper wire: it exists to stop a furnace from firing when exhaust cannot leave the house. For testing with a manometer, book furnace repair at (647) 801-1252 or tell us which code the furnace is flashing.
Questions people ask about Furnace pressure switch
Can I run my furnace with the pressure switch bypassed?
No. Jumping the switch tells the control board the vent is clear when nothing has proven it. If the vent is blocked, exhaust containing carbon monoxide stays in the furnace and the house. Many boards also detect a switch that is closed before the inducer starts and refuse to run. A technician may jumper a switch for seconds during testing, never as a repair.
Why does my furnace have two pressure switches?
A two-stage or modulating furnace runs its inducer at more than one speed, and each firing rate needs its own proof of draft. One switch closes at the lower draft of low fire and the second closes at high fire. Some condensing furnaces also use a second switch or hose connection to detect condensate backing up in the collector box.
Does a pressure switch fault mean carbon monoxide is leaking?
Not by itself. The fault means the furnace could not prove draft and refused to light, which is the safety working as designed. The conditions behind it, such as a blocked vent, are the ones that would spill exhaust if the switch were missing. Keep working carbon monoxide alarms near the bedrooms and have the cause found.