How Does a Furnace Work?

In many cases, even the most experienced homeowner isn’t completely up to speed with all of their home’s functional parts. One of the most crucial appliances in any home is the furnace, especially when the winter months come around. But many homeowners don’t know a lot about how their furnace actually works.

Having a good understanding of your furnace and its functionality is extremely important, especially if you want to avoid any unfortunate furnace breakdowns in the dead of winter. Basic knowledge of how your furnace works can help you to troubleshoot small problems on your own and also be able to better communicate any issues to an HVAC technician in the case of a more serious repair.

The Furnace Basics: What Is It?

Originally, furnaces were fueled by harvested materials like coal and wood. Nowadays, furnaces run on gas, electricity, or propane depending on the model you choose. Each fuel type offers different pros and cons depending on your home heating needs; however, most homes contain gas-powered furnaces.

Every furnace system is made up of several parts including the control system, the internal workings, and the ventilation system. The control system is made up of the electrical controls on the unit itself as well as the thermostat, which you and your family use to turn the system on and adjust the temperature as needed. The internal workings include the burners, gas valve, blower, heat exchanger, and duct. The ventilation system allows the combusted gases within your furnace unit to safely exit your home through the flue pipe.

How Does a Furnace Work?

Regardless of the type of fuel used, furnaces are constructed on the basis of heating using forced air. This means that the furnace unit creates heat and transfers it into the air, which is evenly spread throughout your home using blower fans and your home’s ductwork. Additionally, your duct system is typically also used by your air conditioning unit in order to take up less space within your walls.

Though all furnaces rely on the forced air principle, they achieve the result differently depending on their type of fuel.

Natural gas and propane

Natural gas and propane furnaces contain a pilot light, which is an actual flame that remains burning indefinitely within your unit. When you turn on your furnace, the pilot light works to ignite the multiple burners within the unit’s combustion chamber. The heat created here then travels to the heat exchanger where it mixes with air and is slowly raised to the temperature you set on your thermostat. Once the air has reached the proper temperature, the furnace’s blowers push the air into your ductwork, where it is ventilated evenly throughout your household.

Oil furnaces and boilers

For both oil heat furnaces and boilers, the work begins with a combustion chamber, where the fuel (oil) ignites. Then a heat exchanger warms the gases or water flowing through the component. In an oil furnace, a fan (or blower) pulls in the home’s air from cold air return ducts and sends it through the heat exchanger. Heated air then goes through warm air ducts and circulates throughout the house. Boilers use pumps to propel heated water through pipes to your home’s radiators.


Electric furnaces do not have a pilot light. Therefore, their creation of heat comes from an electrical point of ignition rather than combustion. After the furnace has been turned on and the ignition has taken place, multiple elements of heating are activated using conductive metal coils. Electrical current travels throughout these coils, which provides the heat to mix with the air just before the blower stage. Once the air has risen to the temperature specified on your thermostat, the blowers within your unit send the air into your duct system to be ventilated into your home. The level of heat achieved by the heating coils depends on what temperature the thermostat was set to.

Measuring Your Furnace’s Energy Efficiency

Another factor of furnace functionality that most homeowners aren’t aware of is the energy efficiency of their system. Furnace energy efficiency is measured by Annual Fuel Utilization Efficiency, also known as AFUE, which essentially determines how much of the heat created by your furnace actually makes it into your home through your ducts and how much is naturally dispersed in the combustion process.

The typical lifespan of a furnace is between 15-20 years, and better kept furnaces tend to last longer. If your furnace is between 10 and 20 years old, it likely has a rating on the AFUE scale of around 60%. This means that only about 60% of the heat being created in your furnace is actually being used to heat your home, meaning that you may actually be losing money every time you power on your furnace.

Many furnaces on the newer side, i.e., installed less than 10 years ago, have an AFUE rating of up to 90%. This means that nearly all of the combustion taking place in your furnace is yielding tangible heat within your home, and it also means that you’re likely not overpaying for your home heating. Some homeowners will intentionally choose to install a furnace with slightly lower efficiency, such as around 80% AFUE, if they want to save money on the unit and don’t use the furnace too often. However, if you use your furnace heavily, like in the cold New England winters, you should consider investing in a high-efficiency model.

Have questions about your furnace? In addition to home fuel delivery, Atlantic Fuels offers residential and light commercial oil, propane, and natural gas heating system and equipment repair & maintenance services throughout New Hampshire’s Seacoast Region – 24/7/365. Give us a call at (603) 964-6967 or send us an email today!

Request a Delivery

Get reliable, hassle-free heating fuel delivery at the best prices in Maine & Southern New Hampshire.