Repairman vacuuming inside of a gas furnace during a cleaning.

How a Furnace Works

It’s unrealistic to expect homeowners to be able to work on their HVAC systems on their own. Some home projects are great for DIY and a nice way to learn a new skill, such as painting a room or pressure washing siding. That being said, when it comes to electrical, plumbing, or HVAC, for safety (and warranty) reasons, it’s best to call the professionals.

Of course, it never hurts a homeowner to know just how their furnace works. After all, they will have to change the filter (sometimes as frequently as every 30 days) and a familiarity with how the system operates helps to know if the furnace is running as it should or if a technician should be called. Being comfortable with the ins and outs of the furnace helps to diagnose problems and zero in on the fix when contacting an HVAC repair company. Some of the basics of the furnace function include:

Step One : Thermostat Recognition

A thermostat’s job is to keep heat the same ( Greek “thermos” = heat, “statos” = status quo), which is why it turns on the furnace when the temperature drops too low and turns it off when raised back up to the set number again. When the temperature does drop below the set level, the thermostat sends an electrical signal to a relay in the furnace which effectively “orders” heat.  

Step Two: Safety Checks

When the furnace receives the order for heat from the thermostat, it runs a safety check to make sure the components are ready to operate. There are various sensors that communicate with the circuit board, which first checks to make sure every single safety switch is in the correct position before proceeding (and also immediately triggers an emergency shut-off if a switch is tripped).

Step Three: Inducer Motor

After a run through by the circuit board, the inducer motor then starts and removes any leftover gas or debris that may remain in the heat exchanger from the last cycle. The inducer motor essentially sucks air from the heat exchanger out the combustion flue and creates the draft necessary for the flame to burn; therefore, it runs during the entire heating cycle.

Step Four : Gas Ignites Burners

After a few seconds of the inducer motor running, the gas valve opens and fuel travels past the pilot light which ignites the burners. The burners are located in a combustion chamber and burn a mixture of air sucked through by the inducer motor and fuel released by the gas or oil valve.

Step Five : Heat Exchanger Warms Up

The burners fire up so that the heat exchanger can warm up to actually heat the air. The heat exchanger is a looped group of coils that create immense heat on their interior, which transfers safely to their exterior.

Step Six:  Flue Removes Dangerous Gases

Gas and flame create the heat inside the heat exchanger coil as they flow through the loops. This concoction is also very toxic, which is why the gas is removed through the flue after it cycles through the loops.

Step Seven : Blower

After about 30 seconds of the burners warming up the heat exchanger, the blower motor kicks in to send air throughout the home. The blower motor sends air first over the exterior of the heat exchanger coils where the air is warmed and then through the duct work of the house where it is delivered into individual rooms.

Step Eight : Thermostat Triggers Shutdown Sequence

Once the rooms have warmed back up to the thermostat settings, the thermostat sends a similar electrical current to trigger a shutdown. The gas valve closes, which turns off the burners and cools the heat exchanger. The blower and inducer motors slowly wind down and the pilot light stays lit until called upon next cycle.

It’s important to recognize the furnace as a cohesive unit – if one of the steps required for operation fails, the whole unit usually shuts down (or doesn’t start). Instead of a nuisance, this is actually a very important safety feature which also undermines the importance of annual furnace inspections to make sure each working part is, in fact, working.

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