How a modern gas furnace starts (and where it fails)
Understanding the startup sequence makes furnace faults far less mysterious. Watch and listen at the furnace and you can usually tell which step it stalls on.
- 1
1. Thermostat calls for heat
24 volts reach the control board. Nothing at all happening here points to the thermostat, wiring, a blown low-voltage fuse, or no power at the furnace.
- 2
2. Inducer motor starts
A small motor clears the heat exchanger and establishes draft. A silent furnace or a rattling, whining inducer stops the sequence here.
- 3
3. Pressure switch proves draft
The switch confirms the vent is clear. Blocked exhaust, a snow-covered termination, a clogged condensate drain, or a cracked hose stops ignition at this point.
- 4
4. Ignitor heats and gas valve opens
A hot surface ignitor glows bright orange, then the valve releases gas and burners light. A cracked ignitor or failed valve means no flame.
- 5
5. Flame sensor proves flame
A thin metal rod confirms the burners actually lit. A dirty or failing sensor shuts the gas off within seconds — the classic 'lights then dies' symptom.
- 6
6. Blower starts and heat reaches the house
After a short delay the blower moves warm air. Failure here means hot furnace, cold registers, and eventually a limit trip.
Parts that commonly need replacement
| Component | Symptom | Notes |
|---|---|---|
| Hot surface ignitor | No ignition; furnace tries three times and locks out | A wear item; brittle silicon carbide cracks with age and thermal cycling |
| Flame sensor | Burners light 3–7 seconds, then shut down | Often cleanable, sometimes replaced; a weak microamp signal is measurable |
| Pressure switch / tubing | Inducer runs, no ignition attempt | Verify vent and condensate drain before condemning the switch |
| Gas valve | Ignitor glows, no flame, no gas delivered | Gas-carrying component; professional testing and replacement only |
| Limit switch | Furnace shuts off hot, blower keeps running | Usually a symptom of restricted airflow, not a bad switch |
| Control board | Erratic operation, no response, odd error codes | Diagnosed by elimination after inputs and outputs are verified |
| Inducer motor | Loud whine or rattle, or nothing at all on a call for heat | Bearings wear after years of heating-season run time |
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Gas safety comes first
If you smell gas
Leave the building immediately with everyone inside. Do not flip switches, unplug anything, use a phone indoors, or open a garage door opener. From a safe distance outside, call the gas utility's emergency line or 911. Only call for furnace repair once the property has been declared safe.
A faint, brief smell at the furnace on startup is not the same as a persistent gas odor throughout a room, but you are not obligated to tell the difference. Treat any sustained rotten-egg smell as a leak and get out.
Carbon monoxide is separate and more insidious — it is odorless. Every Buffalo home with gas appliances should have working CO alarms on each level and near sleeping areas. Soot around the burner compartment, a persistently yellow and lazy flame instead of crisp blue, heavy condensation on windows during furnace operation, and headaches or nausea that improve when you leave the house all warrant a professional inspection.
Venting matters more in Buffalo than most places
High-efficiency condensing furnaces vent through PVC pipe out a sidewall, typically 12 to 24 inches above grade. In a lake-effect snow band, that clearance disappears fast. A covered intake or exhaust will shut the furnace down on the pressure switch — by design, and it is protecting you. After heavy snow, clearing a foot of space around those pipes is one of the highest-value things a homeowner can do.
Older 80% furnaces vent into a metal flue or a masonry chimney. In Buffalo's housing stock, those chimneys are often old, sometimes shared with a water heater, and occasionally deteriorating internally. Blocked or damaged venting on a natural-draft appliance is a carbon monoxide concern and deserves inspection, not guesswork.
Condensing furnaces also produce acidic condensate that drains to a floor drain or condensate pump. In a cold Buffalo basement, that line can freeze or clog with biological growth, back up into the pressure switch circuit, and shut the furnace down. It is one of the most common non-obvious causes of a mid-winter no-heat call.