FURNACE OPERATION BASICS:

Your furnace heating starts when the thermostat senses that the room temperature is below the temperature you required/set. Most homes in Canada have a thermostat set at 70 degrees / 22 celcius.

The first step is for the furnace inducer motor to pull air through the unit starting the required combustion airflow. When proper airflow is established, the gas valve opens releasing the gas flow through the burners where it is then ignited by a spark / glowing heat from the ignitor or in the case of older units, the small flame from the constantly lit standing pilot.

Next, the unit senses the flame is lit, the spark or ignitor is then turned off. Normally, the gas will burn for a couple minutes before the blower is started to prevent cold air blowing out the vents at the start.

Once the required temperature is reached, the blower motor is energized and air will blow over the heat exchanger. This heats the air that is then supplied to your home through the ductwork and vents.

When the thermostat senses that no more heat is required and the desired temperature is reached, the gas valve is turned and shut off. The blower motor usually runs for another few minutes to cool off the heat exchanger. This can be controlled by either a timer or a temperature switch.