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How Does a Furnace Work? Complete Guide for Scarborough & Toronto Homeowners
Understanding how your furnace works helps you recognize problems early, communicate with technicians, and make smart decisions about repair vs. replacement. For equipment and installation factors, visit our guide to furnace costs across Ontario.
The Furnace Heating Cycle — Step by Step
- Thermostat calls for heat — When room temperature drops below your set point, the thermostat sends a 24V signal to the furnace control board.
- Draft inducer starts — The inducer motor creates airflow to flush combustion gases before ignition.
- Igniter heats up — A hot surface igniter (replaces older pilot lights) glows red-hot (1,800°F) to light the gas.
- Gas valve opens — Natural gas flows into the burners and ignites.
- Heat exchanger warms up — The flame heats the metal heat exchanger. Critical: a cracked heat exchanger leaks carbon monoxide into your home.
- Blower fan starts — After the heat exchanger reaches temperature (about 90 seconds), the blower circulates air across the hot heat exchanger and distributes warm air through your ducts.
- Cycle ends — When the thermostat is satisfied, the gas valve closes, the burners shut off, and the blower continues briefly to exhaust remaining heat.
Key Furnace Components Explained
- Heat Exchanger — The most critical component. Separates combustion gases from breathable air. Replace the furnace if cracked.
- Blower Motor — Circulates air. ECM (variable speed) motors are more efficient than PSC motors.
- Control Board — The brain. Controls all sequencing.
- Pressure Switches — Safety devices confirming proper draft before gas ignites.
- Flame Sensor — Detects that the burner lit. Dirty flame sensors are the #1 cause of short-cycling.
AFUE Efficiency Ratings
AFUE (Annual Fuel Utilization Efficiency) measures how much gas is converted to heat:
- 80% AFUE (single-stage) — Standard. 20 cents of every dollar goes up the exhaust.
- 96% AFUE (two-stage/modulating) — High-efficiency. Saves $200–$400/year in Scarborough’s climate vs 80% AFUE.
How does a furnace work in a Canadian home?
A gas furnace works by: (1) thermostat calls for heat, (2) draft inducer starts, (3) hot surface igniter glows, (4) gas valve opens and burners light, (5) heat exchanger warms, (6) blower circulates warm air through ducts. The cycle repeats until the thermostat is satisfied. High-efficiency 96% AFUE furnaces condense exhaust gases to extract maximum heat. Call Madina Mechanical at (416) 424-4328 for furnace service in Scarborough and Toronto.
What is the most common furnace problem?
The most common furnace problems in Scarborough are: (1) dirty flame sensor causing short-cycling, (2) clogged filter restricting airflow, (3) failed capacitor, (4) igniter failure, (5) cracked heat exchanger. Regular annual maintenance prevents most issues. Call (416) 424-4328.
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Furnace Service Scarborough & Toronto: (416) 424-4328
