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When the holiday decorations start to come out of storage and the temperature outside drops, the air conditioner switches off for the last time. Soon, you’re switching over to the furnace and enjoying the instant comfort created by having warm air flowing out from the vents around your home. When you dial your thermostat to your preferred heat setting and listen to the equipment turn on, you might find yourself wondering, “what is a furnace, anyway?” 


Because of their ubiquity in homes across the nation, forced air furnaces and their newer cousin, the condensing furnace, are often taken for granted and overlooked. However, they are fascinating pieces of technology that today can capture well over 90 to 95% of the heat energy contained within a given fuel. Let’s look at how this hardware works and some of the most important things to know.


  • What is a Furnace? The Basic Ideas of Operation


Today’s furnace is a descendant of the traditional fireplace — a method of burning fuel to heat the home. Today, most modern furnaces are highly enclosed units that rely on either propane fuel or, more commonly, natural gas. The furnace’s job is to take that fuel, burn it efficiently, and use the heat created from that combustion to warm the air throughout your home — all while ensuring that the harmful combustion byproducts escape into the atmosphere outside your home. 

  • The Parts of the Furnace To Know


The idea is simple enough, but there is a surprising amount of complexity in modern furnace units. Luckily, we can break it down into the individual parts that work together to heat your home. Let’s look at each of the critical stages of heating and how they work.


  • First, your thermostat commands the furnace to turn on and provide heat. The system inside the furnace releases fuel from the gas lines feeding the furnace, which ignite inside several burners.
  • These flames feed into a long series of metal tubes where additional fuel combustion takes place. These tubes, called the heat exchanger, become extremely hot due to the burning fuel and hot combustion gases inside of them.
  • A blower motor forces air across the heat exchanger, cooling the tubes while heating the air simultaneously. This “forced air” then travels through your home, exiting from registers and heating rooms.
  • What is a furnace with condensing technology? In condensing furnaces, exhaust gases go through a secondary heat exchanger that air also passes over as before. Water vapor from burned natural gas condenses on this exchanger and releases additional heat, driving up unit efficiency.


To recap, the furnace uses burners to combust fuel, which warms the heat exchanger, which transfers its heat to the air that reaches your home.


  • Can Furnaces Be Dangerous To Use?


Modern furnaces feature extremely safe designs that make them more reliable. Many even have built-in fail-safes to detect potentially unsafe operating conditions and lock themselves down to prevent danger. However, it’s important to remember that burning any kind of fuel produces carbon monoxide — a toxic gas that can be fatal with heavy exposure. Carbon monoxide can leak from poorly maintained furnaces and cracked heat exchangers instead of escaping to the outside air. For these reasons, you should always invest in both a personal carbon monoxide detector and routine maintenance.


  • Using a Furnace Effectively


Modern furnaces feature designs that make them highly efficient, so you can enjoy lower energy costs for the same level of comfort and heating that you typically expect. However, there are still steps you can take to ensure that you get the best value for your money from month to month — especially if you still have an older and less efficient unit. Try these tips:


  • Consider investing in a programmable or “smart” thermostat. Create a program that heats your home only during the specific hours you need warmth. Don’t heat an empty home.
  • Keep your filter clean and change it regularly. Good airflow is essential for effective heating, so don’t let your filter become clogged with dust.
  • Seal your home and minimize air leaks. Don’t let that warm air created by your furnace leak out through drafty windows.
  • Use a humidifier to maintain a level of humidity that helps you feel warmer and more comfortable. You don’t need to turn your home into a sauna, but a humidity level between 35-40% will make your space feel better.
  • Invest in routine maintenance regularly. Having your furnace checked for a tune-up before the start of the heating season provides both peace of mind and the chance to correct any potential issues before they become failure points. 


If you wondered “what is a furnace?” we hope this quick overview provided the answers you sought. Remember to contact a licensed and insured HVAC professional if you experience issues with your furnace and never attempt to service the unit independently.