Types of Furnaces

How Does a Furnace Work

How Does a Furnace Work in Regina?

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When the temperature indoors falls below a certain point, the thermostat alerts the furnace which combines air and fuel and the mixture gets ignited by an electronic igniter.The combusted, hot gas begins to rise through the heat exchanger situated above the burner and heats the air which is circulated throughout the

Although furnaces can differ from model to model the operate on similar principles. The heating capacity of a furnace is measured using a British Thermal Unit or BTU. Most furnaces are rated between 40,000 and 130,000 BTU. Forced air furnaces can be installed to send heated air up, down or sideways to accommodate which way the furnace is to be installed.

Gas burners, fuelled by either natural gas or propane are used to generate the heat. Modern gas furnaces have two separate airflow systems, which operate while the furnace is running.

The first system uses a draft infuser fan motor to draw air into the burner assembly. The air allows the burners to warm the heat exchanger. The air within the burner assembly is then exhausted to the outside of the home, which is necessary as this air contains toxic fumes.

The second air system uses a blower fan to draw air from the return vent, blow it past the heat exchanger and force it through the homes venting. The room air is then drawn back through the return and the air flow cycle continues. Once the room air has warmed sufficiently the thermostat signals the furnace to turn off until the room temperature decreases.

This process can be broken down into 4 main factors:

- Temperature Control
- Ignition
- Combustion
- Air Circulation


Temperature Control

The temperature in the home is regulated by the wall thermostat. The thermostat can be powered by the furnace control board or by batteries. The thermostat must be in a central location the home, at eye level and away from direct sunlight for proper temperature regulation. When the thermostat detects a decrease in temperature it switches on the heat circuit and the furnace control board begins the ignition process.



When the furnaces power switch is turned on and heat is needed, the furnace goes through a timed ignition sequence. The sequence begins with the control board sending 120V of alternating current to the draft inducers fan motor. The inducer fan motor draws air into the burner assembly. Once the fan reaches it's maximum speed, a vacuum switch is closed, which ensures gas will not enter the furnace, unless the toxic fumes can be exhausted. If the inducer motor does not turn on or runs to slow, the vacuum switch will not close and the ignition sequence will stall. Once the switch vacuum switch closes, 24V will travel through the switch and one more limit switches. Allowing the control board to continue to the start up. The control board now sends 120V to an igniter. A defective igniter will halt the ignition sequence and prevent the furnace from heating. Be aware some models will use a spark igniter instead. Spark igniter will also receive voltage from the control board.



After a set period of time. The control board sends 24V to the gas valve. Opening it and allowing gas to flow over the igniter to establish the flame. Once the gas is ignited, a flame sensor detects whether or not a flame is present. If the burners do not light or do not stay lit, the flame sensor may need to be cleaned with a fine abrasive pad or the sensor may need to be replaced.

Mounted on the burners are one or more flame rollout limit switches, that ensure the flame is drawn through the burners. If the inducer fan motors can't draw through the burners, the rollout limit switch will open, and the control board will shut off the voltage to the gas valve. One the combustion process is complete, the control board will send 120V to the blower fan motor. The blower fan draws air through the return vent and blows it past the heat exchanger and forces it through the homes venting.


Air Circulation

The most important factor in furnace operation is air circulation. To ensure proper air circulation, the furnace filter should be checked monthly and replaced as needed. There are several types of filters available and range from 1 to 5 inches thick. The filter can be installed into the return or in an air cleaner. Mounted near the heat exchanger is a safety limit switch that monitors the air temperature. If the filter is clogged, or the blower fan motor fails, the heat exchanger will overheat, the limit switch will open, and the control board will shut of to the gas valve to prevent damage to the furnace. The blower fan motor can run at multiple speeds to improve efficiency. Lower speeds are used when the furnace is heating and higher speeds are used when the furnace is used for air conditioning.


- Furnace does not heat: igniter, flame sensor, draft inducer motor, control board
- Furnace is loud: blower wheel, blower motor, draft inducer motor
- Furnace runs for a few minutes: flame sensor, limit switch, control board
- Furnace blower does not run: blower motor, capacitor, control board
- Furnace blower runs all the time: control board, limit switch, wall thermostat


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