How Do Furnaces Work?

An essential component of your HVAC system is your heating system (the “H” in “HVAC”). We’ve discussed before the different types of heating systems, but today we’ll be focusing on hot air heating and what makes that possible: the furnace. 

But before getting into the nitty-gritty details of how furnaces work, we need to talk about a hugely important factor, and that is the efficiency of a furnace.

No one enjoys paying for things they never use. Would you be pleased if you bought a new vacuum cleaner, only to find out it picks up only 50% of the dirt on the ground? Of course not!


“AFUE” stands for “annual fuel utilization efficiency.” This means that for all the fuel you put into your furnace, the AFUE is the percentage of that fuel that gets converted into heat. You want a high AFUE; for all the money that you spend on fuel, you want to be certain the highest amount possible is used for heating and not simply lost in the process. 

Older, less efficient furnaces have AFUE of between 50% and 70%. Mid-range furnaces have levels between 70% and 85%, and the newest, most high-tech models of furnaces are above 90% (some go all the way up to 98%). So of all the fuel you put into your furnace, 90% of that is turned into heat. Not too shabby.

Professional maintenance is essential for newer high-efficiency models. Because efficiency will decline over the years from leaking ductwork, old and broken furnace components, clogged filters, etc., your furnace needs proper maintenance so you can nip any of these problems in the bud before they turn into big issues. 

Now that we are all familiar with the different efficiency levels of furnaces, it’s time to talk about how furnaces work.


Short explanation: furnace transfers heat to air. Blower fan sends hot air through ductwork.

Want more detail? We got you covered!

Steps in Furnace Heating Process:

  1. Temperature inside the house is lower than the temperature on the thermostat (or your desired home temperature). This will signal the furnace to start creating and blowing out hot air. 
  2. With a natural gas furnace, a pilot light (a burner continuously burning used to ignite larger burners when needed) ignites series of burners inside a combustion chamber. The gas valves are opened. 
  3. Heat from the burners then enter the heat exchanger. From here, heat is transferred to air.
  4. Blowers move the air through ductwork and throughout your home.
  5. Indoor temperature will rise to temperature set on thermostat.

There are also propane furnaces (which are heated similarly to the natural gas furnaces), and there are electric furnaces. The heating process for electric furnaces is a little bit different.


Because there is no pilot light with an electric furnace, an electrical ignition, not a gas flame, starts the heating process. This ignition activates heating elements that contain conductive coils. As the current passes through these coils, air is heated. This air is then pushed through ductwork via blowers. The higher the temperature on the thermostat, the more heating elements will turn on. 

It’s important to remember that during the process of heat exchange, noxious gases, including carbon monoxide, should be vented safely out of the house. While electric furnaces don’t have the same concerns about noxious gases, there is a concern for high voltage shock. 

Now that we’re all familiar with how furnaces work, it’s important to be aware of maintenance tips and common problems for furnaces. Awareness of both helps you keep your furnace in great shape, while also keeping you aware of any malfunctioning or problematic warning signs


  • Remember: before doing any work in or near your furnace, make sure power is switched off!
  • Schedule a visit from your HVAC contractor every year in the fall, before you begin using your heating system for the winter. We can check for any potential issues that might affect your system that year.
  • Replace furnace filter once every one to three months. You don’t need anything fancy for this, a cheap fiberglass filter will work!
  • Check your furnace flame color. The flame color should be blue or blue with a yellow tip. Any other color indicates there is a problem. You should call an HVAC professional for help. Do not try to fix this yourself, as the potential for burns and gas leaks are high. 
  • Test smoke and carbon monoxide detectors every 30 days to ensure they’re working properly. We know the squeaking is annoying, but it’s worth it, we promise. 
  • Turn off the furnace power switch and vacuum the burners. Vacuum everywhere you see dust, and with a flashlight look for signs of soot, as these suggest poor combustion.
  • Vacuum and brush blower blade to make sure no dust or debris got on them.
  • Use drinking straw to blow dust off the pilot.


  • Lack of maintenance: understandably, having little maintenance performed on your furnace is a recipe for disaster. This will aggravate any potential issue and make it happen quicker and more intensely, almost guaranteeing you spend more money trying to fix it. 
  • Dirty filters: considering you can buy $1 furnace filters at many stores, be sure to keep track of replacing these. Set a reminder to check or replace your filter every 90 days.
  • Malfunctioning thermostat: malfunctioning thermostats or thermostats with dead batteries are the culprits of many common heating problems. If you suspect this may be the issue, try changing your thermostat’s batteries. If the problem still isn’t solved, call an HVAC repairman to help.
  • Circuit breaker tripped: if your furnace is not producing any heat and you’re sure it’s not a thermostat problem, trying checking the circuit breaker. This can easily be tripped without anyone noticing and is an easy solution to fix.
  • Electric ignition or pilot control problems: if you aren’t getting any heat into your home and you’re sure it isn’t the thermostat or the circuit breaker, it may be a problem with your pilot or electrical ignition. Best to call a professional to handle this one. 
  • Keep area around furnace clean and unobstructed: sometimes houses get crowded. In this case, make sure your furnace and its surrounding area is clear. Debris can be a fire hazard, as well as make the heating process more difficult.
  • If you smell gas, do not operate any electrical component. Do not turn on any lights. Leave your home with the doors open, and call the fire department or utility company. Do not reenter home until given the “okay” by a professional. 

And that’s it! Now not only do you know how furnaces work, but you understand their energy efficiency standards, furnace maintenance tips, and common furnace problems. 

We hope you feel even more confident around your furnace now! Comment below if you have any questions that we missed and we’d be happy to answer!