with front cover off
Furnace   Operations

The following is a generalization of furnace components and operations it is not intended to describe all.

The modern forced air Furnace starts when the Thermostat calls for heat if all is in order a fire will start in a controlled environment. 

The Furnace is designed with multiple safety features to prevent the fire escaping, the fire box, or the build up of gas that would cause an explosion, or allow harmful flue gases from ruining your day.

The fire is fueled by fuel Oil or Gas (Natural or Propane) the burning of fuels creates carbon monoxide along with other flue gases that are unhealthy to breathe. These are expeled to the outside through a flue pipe.

Sequence of Operation:

The Draft Inducer is a fan blower mounted at the exhaust of the Heat Exchanger and attached to the Flue Pipe.  When a call for Heat comes from the Thermostat this fan motor should start immediately.

Failure of the Draft Inducer to run will prevent the Furnace from creating a fire.

The Control Module will not start the Hot Surface Igniter or other ignition devices because the Pressure Switch operates with a negative pressure when the inducer does not run this can’t happen.

If all is well with the Draft Inducer and Pressure Switch the Control Module has a set time to send an electrical current through the Hot Surface Igniter it will become hot and have a yellow glow, near the end of that time the Gas Valve will open and the Gas Burners will ignite.

If the Control module does not get a signal from the Flame Sensor the gas will turn off almost immediately.

If it does sense the flame the Fire will continue. A set time in the Control Module passes and a signal is sent to the Blower Motor to start, when running, it passes air from the living areas in the house across the Heat Exchanger, warm air is then sent to the registers and warms the house.

The Over Flame and Over Heat devices are in the circuit that controls the Gas Valve if either is heated to the set point they will open and the Gas Valve will close and the fire will go out.  The fire will not start if either is defective or over heated.

Not all Furnaces use Hot Surface Igniters some older Furnaces use Standing or Intermittent Pilot Lites with different control systems


Make sure the air intake is not restricted by furniture or unchanged filters.

Older furnaces may have cracked or rusted Heat Exchangers that can have Carbon Monoxide leaks. Very Dangerous!

Really old Furnaces don’t have Control Modules; they use other operational devices and safety protocols.

Acme Heating and Air Company Logo
To ACME Home


To Repair Page