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How Do Furnaces Work?

Published by Melanie Riggs

hvac repair

Now that we here in Southeastern Virginia are experiencing weather more typical of winter, it’s important to have a basic understanding of the machinery that is keeping your home warm and comfortable while temperatures outside are near freezing. The better informed you are about your furnace – likely a gas furnace – the quicker you can identify a shift in the comfort level in your home and troubleshoot the issue likely by giving us a call. And, considering that heating accounts for roughly 45% of your home’s energy bill, homeowners tend to enjoy having the knowledge of how furnaces work

Alternatives to a gas furnace

You may have a different heating system such as:

A boiler

A boiler heats water and emits it through radiators

A heat pump

A heat pump is essentially an air conditioner that works in reverse. Check out this article to learn more about heat pumps

Hybrid heating

Many homes have a hybrid heating system, which combines the energy-efficient properties of heat pumps during mild weather months with the powerful heating abilities of furnaces during extreme weather.

Non-gas furnace

While most furnaces use natural gas, some run on propane, heating oil, or electricity.

Furnace 101

The most common furnace is a gas-powered central air system that works by heating air in one area and distributing it throughout the rest of the home via ductwork and through vents. 

The main components of the furnace are the gas valve, burners, heat exchanger, blower, ducts and ventilation system, and the control center, including the thermostat and electrical controls. 

Furnace and gas valve

The furnace is triggered when it receives a signal from the thermostat telling it to turn on. Based on the temperature you manually set the thermostat to or have programmed in, the thermostat will detect when the air in the room is below that number and activate the furnace. 

After the thermostat sends its signal, the furnace gas valve opens and ignites the burner under the combustion chamber which then works to regulate the amount of gas that flows into the furnace. Oftentimes when the gas valve is working but the furnace isn’t turning on, the culprit is an issue with the pilot light also not turning on. Your best bet is having a professional come and relight the pilot light. We don’t recommend attempting to do so yourself. 

The heat exchanger

The flames then heat a metal heat exchanger and from there, the heat circulates through the tubes of the heat exchanger where the heat is transferred into air. This part is super important because without it, no heat will pass through the house. Following this process, combustion gases should be circulating safely out of the house. 

Air handler

As the heat circulates through the heat exchanger, the blower motor and fan moves the heat through the ductwork in the home. The heat then flows through the various vents of the house to distribute heat to the rooms. Once the heat is sufficiently distributed, the thermostat shuts off the heater until the cycle starts over again. 

How is furnace energy efficiency measured?

Furnaces operate under a rating system called Annual Fuel Utilization Efficiency (AFUE), a percentage that basically equates to how much of your fuel is used for heating and how much is lost due to combustion. For example, a high-efficiency furnace has an AFUE percentage of 90% or above. That means that 90% of the fuel is used directly within the home to heat it while 10% is lost. 

Mid-range efficiency furnaces have an AFUE around 80-85% while older, lower-efficiency models typically have an AFUE rating that falls between 50 and 70%. 

The AFUE rating will decline over time due to leaks in the ductwork, dirt in the furnace components, damaged and loose parts, and a clogged air filter. This is just one reason why you should be having your furnace professionally maintained annually. The best way to keep on top of this is to sign up for our HVAC maintenance plan to ensure that one of our licensed technicians is cleaning out your unit and making sure everything is in tip-top shape. 

Furnace maintenance and safety tips

  • Replace your furnace filter every 90 days. We recommend writing the date you replaced it on the furnace itself so that you know when to replace it.
  • Test to make sure your smoke and carbon monoxide detectors are working properly every 30 days.
  • Invest in a programmable or smart thermostat. This will save you money on your utility bills plus it’s far more energy-efficient than the alternative.
  • Periodically check the color of the flame produced by your furnace (you can do this at the same time you change the filter). The flame should be blue in color, potentially with a small yellow tip. If it is any other color – likely red, orange, or green – this indicates a fuel/burner problem and you should contact a professional immediately. Don’t try to fix it DIY-style. 
  • If at any point your furnace is making strange noises, make sure to call a professional and schedule an appointment to have it looked at as soon as possible.
  • If at any point you smell gas (many have described this as rotten-egg like), do not operate any of the system’s components. Evacuate the home then call the fire department and don’t reenter the home until everything is deemed safe.
  • Find leaks in your ductwork and seal them with mastic sealant or aluminum foil tape. This will help to improve the AFUE.
  • Schedule regular maintenance appointments. At your appointment, they will test your heat exchanger for cracks, inspect the ventilation system, clean your pilot light, and take many more necessary precautions.

Find Out What Our Clients Say About Us

Read Our Trusted Reviews

Technician Rick did an outstanding job of completing my annual service check, correcting minor problems and cleaning my system prior to the summer season. What impressed me most is that he took the time to point out other issues in my attic (loose fitting at the hot water heater) and even outside (water spigot leaking) and fixed both on his own since they were easy fixes. What a great service professional and so rare to find someone honest and willing to take a few extra moments to help out. Thanks for a great experience!

- Mark B.

Technician Rick did an outstanding job of completing my annual service check, correcting minor problems and cleaning my system prior to the summer season. What impressed me most is that he took the time to point out other issues in my attic (loose fitting at the hot water heater) and even outside (water spigot leaking) and fixed both on his own since they were easy.

- Mark B.

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