A gas furnace can be broken down to its individual parts. Here are the main basic components for any gas furnace.
When gas burns it gives off toxic fumes including carbon monoxide which is dangerous when breathed in. To keep the toxic gases separate from the air that is blown through a home a heat exchanger is used. A heat exchanger is simply long metal tubes that are S-shaped. Gas flames burn inside the tubes and the toxic gases vented out of the home through the roof. The blower motor forces air past the outside of the tubes heating the air as it does. This keeps the toxic flames from the clean-air that is blown into a home.
A heat exchanger should be examined on a regular basis because any cracks or damage to the heat exchanger can allow toxic gases into a home. Carbon monoxide detectors should always be used in a home to help and spot any problem if it occurs.
Igniter and Pilot lights
Gas furnaces need a way to light the gas to start a flame. Older furnaces use a pilot light that burns constantly lighting the gas when it comes on. Modern furnaces use a hot surface igniter (HSI) that will glow red hot and ignite the gas.
A burner is where gas and air are combined creating a flame that becomes the heat source.
Natural gas is piped throughout the country, into cities, and into homes. As it is piped from one stage to another it is set at different pressures. At the point it comes into a home the pressure is usually between 8-12 inches of water column. It goes through a gas valve and is step down the correct pressure for the furnace, usually 3-5 inches of water column.
Inches of water column is what is used to measure gas pressure
The gas valve controls the gas going into a furnace including shutting the gas off when a safety switch fails.
A Thermocouple is used with a pilot light to sense whether the pilot light is lit. It uses two small metal wires inside of a tube that when heated conduct electricity sending 24 Volt signal to the gas valve allowing gas to flow. If a thermocouple is bad or does not sense any heat then it was shut the gas off.
A flame sensor works as a safety to detect heat when a Hot Surface Igniter is used. If it does not detect heat it will shut off the gas.
Draft Inducer Motor
Since gas when burned gives off toxic fumes it is necessary to vent the fumes from a home through a vent. A draft inducer motor turns on before the gas is allowed to burn. This creates a small vacuum that pushes air out the vent pipe.
A furnace pressure switch is used to make sure the draft inducer motor has turned on and is pushing gases out the vent pipe. It does this by using a small diaphragm that pulls in when the inducer a motor is on and creating a vacuum. If there is a vacuum and the diaphragm pulls in it activates a switch which allows the gas to begin to flow. If it does not sense a vacuum then no gas will be allowed into the system.
A blower motor is what is used to blow air into a home.
Blower Motor Capacitor
A blower motor capacitor is used to help start a blower motor and in some cases help it to run at a steady rate of speed.
Furnace Limit Switch
If a furnace becomes too hot then a limit switch is used to turn off the gas as a safety. For example if a blower motor stops working and no air circulated temperatures will rise inside of a furnace. If this happens a limit switch will shut off the gas to the furnace until the problem is fixed.
Return Air Filter
A furnace air filter is necessary to keep out dust and other debris from entering a unit. A dirty air filter will cause a furnace to not work properly and should be replaced frequently.