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Furnace Basics: How Does a Furnace Work?

October 31st, 2017

As we head into winter, it’s a good idea to learn a thing or two about the machine that will provide us with heat and comfort throughout the cold, winter days—most likely, a gas furnace. The more you know how a furnace works, the better you can troubleshoot it if something goes wrong. And since space heating accounts for roughly 45% of energy bills paid by U.S. homeowners, it’s no wonder why people want to know more about how they work (U.S. DOE).

To be clear, you may have a different heating system, such as a boiler, heat pump, or active solar heating. If your heating system heats water and you have radiators, you have a boiler. A heat pump is basically an air conditioner that works in reverse. Keep in mind that many homes have a hybrid heating system, which combines the energy efficiency of heat pumps for mild weather with the powerful heating capacity of furnaces during more extreme temperatures.

And not all furnaces use natural gas, although most do. Your furnace may run on propane, heating oil, or electricity.

Understanding how your furnace works will help you troubleshoot your furnace and potentially avoid an expensive HVAC service call. Sometimes, you can quickly solve the problem yourself. Other times, you can easily show the problem to your HVAC technician, saving time and frustration. You’ll also impress the technician.

Welcome to Furnace 101, where we teach you the basics of your furnace and how it works to provide heating to your home.

  1. What Is a Furnace?

The most common type of furnace is a gas-powered central air system, which heats air in a one area and then distributes it throughout the home via ductwork and vents.

This system is also known as a ducted warm-air or forced warm-air distribution system.

The main parts of your furnace are the control system (thermostat and electrical controls), gas valve, burners, heat exchanger, blower, and duct and ventilation system. When your furnace creates heat, combustion gases are vented out of your home via a flue pipe. 

  1. Thermostat and Gas Valve

Your furnace starts when it receives a signal from the thermostat that tells it to turn on. Depending on the temperature you set, when the thermostat detects the air temperature dropping below that number, it activates the furnace.

When the thermostat sends its signal to the furnace, the furnace gas valve opens and ignites the burner component beneath the combustion chamber. The gas valve works with the thermostat to regulate the amount of gas that flows into your furnace.

If your gas valve is working fine, but the furnace isn’t turning on, check to make sure your pilot light is on. For instructions on relighting the pilot light, refer to your manufacturer instructions or contact a professional.

You may also have an issue with your thermocouple or thermopile, which helps detect whether your pilot light is on or not. Your thermocouple is an essential safety device that makes sure your gas valve is not sending gas into a furnace without a working pilot light.

  1. Heat Moves to Heat Exchanger

The flames from the burner component heat a metal heat exchanger. The heat circulates through the looped tubes of the heat exchanger, transferring the heat to air.

This is an extremely important part. If it’s not working, you won’t receive any heat. Heat exchanger can also be dangerous if they develop any leaks or cracks.

An efficient and well-maintained heat exchanger is essential for a working furnace. During the process of heating the heat exchanger, combustion gases should be venting safely out of the house.

  1. Blower Motor and Fan Moves the Heat (Air Handler)

As the heat circulates through the heat exchanger, the blower motor and fan moves the heat through plenum, and from there through the rest of the ductwork in your home. The heat flows through the various vents in your home to provide you with heat. Once enough heat is generated, the thermostat will shut off the heater.

When there is a problem with your furnace, it is usually the thermostat (control system), gas valve/thermocouple/heat exchanger (heat source), or the blower/air handler/ductwork (distribution system). In any case, a trained and qualified HVAC technician will be able to fix the problem quickly and effectively.

  1. How is Furnace Energy Efficiency Measured?

The energy efficiency of your furnace is dependent on its Annual Fuel Utilization Efficiency (AFUE), which basically tells you how much of your fuel is used for heating and how much is lost due to combustion (usually out of your vents).

If you have a 90% AFUE furnace, 90% of the fuel is used to directly heat your home. The remaining 10% is lost. High-efficiency furnaces have AFUE percentages above 90%. Think of AFUE as the gas mileage of your furnace.

Older, low-efficiency furnaces have an AFUE rating between 50% and 70%. Mid-range efficiency furnaces around 80% to 85%, while high-efficiency furnaces have AFUE rating above 90%, sometimes as high as 98.5%.

In order to maintain an efficient furnace, annual professional maintenance is not a suggestion—it’s a requirement. AFUE will decline over the years due to ductwork leaks and inefficiencies, dirty furnace components, loose and damaged parts, and clogged air filters.

When it comes to furnaces and HVAC systems, the phrase “an ounce of prevention is worth a pound of cure” definitely applies. Manufacturers know this too, which is why they typically void any warranties if regular maintenance is neglected.

The best way to stay on top of heating and air conditioning maintenance is by signing up for a home maintenance plan. Your trained and licensed technician will be able to clean your unit and get it ready for the working season ahead. Click here for a list of professional furnace maintenance tasks.

Ask your technician for ways to increase your furnace’s efficiency.

Furnace Safety & Maintenance Tips

  • Schedule a professional tune-up every fall (before you first turn on your heating system). They will be able to test your heat exchanger for cracks, clean your pilot light, inspect the ventilation system, and much more.
  • Replace the furnace filter at least once every 90 days (set reminders to check the filter every month). Write the date of replacement on the filter itself so you know exactly how long it has been since the last filter change-out.
  • Periodically check the furnace flame color and characteristics (about every month or so when you check the filter). The furnace flame should be blue with perhaps a small yellow tip. Any other color (red, orange, green, etc.) indicates a fuel/burner problem. Contact a professional to fix the flame color. Do not try to fix it yourself.
  • Test your smoke and carbon monoxide detectors every 30 days to make sure they are working properly.
  • Use a programmable or smart thermostat to save money on utility bills.
  • If you smell gas (sometimes described as a rotten-egg smell), do not operate any electrical components. Evacuate the home and then call your utility company or fire department. Do not reenter the home until a professional has deemed it safe to do so.
  • Seal your duct connections with mastic sealant or aluminum foil tape. For better, more comprehensive duct sealing solutions such as PureFlow™, contact Service Champions.
  • Click here if you are experiencing strange sounds coming from your furnace.
  • Learn more furnace safety and efficiency tips.
  • Find out if you should invest in a new furnace.

A combination of insulation improvements, air sealing, and winterization projects will help ensure your heating system is efficient and long-lasting. The worse your home’s insulation, the harder your heating and cooling system will have to work.

Having problems with your heating equipment? Want to improve home comfort and energy efficiency?

Schedule an appointment with Service Champions, or give us a call at (925) 308-5025.

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