Furnace Basics

How A Furnace Works


At some point, every homeowner will need to get work done on their furnace. There’s no avoiding it.

Most homeowners would prefer to plan the work. They will schedule the annual maintenance with a trusted HVAC technician to clean and check their furnace. During this visit, they will mention any concerns, and the technician can offer suggestions and perform any necessary repairs. This is done to ensure the furnace is reliable and performing at its best. Most importantly they hope to avoid an unexpected broken-down furnace in the middle of a chilly Chicago night.

When dealing with a furnace, there’s electrical, plumbing, gas, venting, and sheet metal work involved, so most people will agree that a furnace repair should not be a DIY Project, despite the 129,000,000 YouTube videos! However, It is important to get familiar with your heating system. This will help keep you nice and warm, provide peace of mind, and ensure your system is safe.

Generally speaking, most furnaces operate the same way—barring some minor differences in furnace type and fuel source. The first step in understanding how a furnace works is to know the furnace’s basic components and how they operate.

Its Purpose

The main purpose of a furnace is to heat the air in your home. It is controlled by a thermostat. When the indoor temperature is not at your desired set temperature. (+/- 1 to 2 degrees of accuracy), the thermostat signals the furnace to turn on and off. Not only does the blower in the furnace move the warm air but it is also responsible for moving the air throughout the HVAC system for the air conditioner, the humidifier, the ventilation, the air filtration, and the purification system.

7 Basic Furnace Parts & What They Do

Here’s a list of the main parts of the furnace and a brief description:

  • Thermostat: While the thermostat is not in the furnace, it is an integral part of the heating system. It signals the furnace to turn on and off. When you set your thermostat to a certain temperature, the furnace will continue running heat cycles until the indoor temperature reaches that number.
  • Burners: The burners start the heating process by lighting the gas.
  • Heat Exchanger: The heat exchanger transfers the heat from the burners to the air passing over it.
  • Ductwork: The ductwork is a series of steel pathways—located in the walls, ceiling, floor, or basement—that distribute air throughout your home.
  • Blower: The blower is a fan that moves the air through the system.
  • Vent Pipe: The vent pipe removes the combustion gases from your home that form during the heating process.
  • Filter: The filter protects the furnace components.
8-Step Process

Take a look at the eight main steps a furnace takes to get you warm. We tried to keep it simple but it is a complicated process:

  1. The thermostat tells the furnace to start. It detects the air temperature in your home and—once the temperature gets low enough—signals the furnace to start cycling. The furnace will continue running heat cycles until the indoor temperature reaches the number you set the thermostat to.
  2. The burners ignite. Systems today have a spark ignition, however, some older furnaces may still have a standing pilot. It is important that the ignitor is kept clean, especially if your furnace is in the laundry room.
  3. The heat exchanger warms up. The heat exchanger allows the heat from the burners to transfer to the air in your home. Please note that only the heat is transferred, the air is not mixed together. The air circulating in your home and the air/gas created from the flames inside the furnace never come together, unless there is a leak in the heat exchanger. It is important that the heat exchanger is inspected regularly as the furnace ages, cracks can appear, creating a dangerous situation that can leak carbon monoxide (CO) into the home. Carbon monoxide is an odorless, colorless gas formed by the incomplete combustion of gas that can be very dangerous to your health. It is lighter than air so make sure you have a CO detector where you can hear it. Also, make sure it is in working order, most have a life expectancy of 7 years.
  4. Once the heat exchanger is warmed, the blower turns on to force the air from the furnace over the heat exchanger and into the ductwork.
  5. The room temperature air is pulled through the ductwork into the filter before entering the furnace to get heated. Pay attention to the filter, while its main job is to protect the furnace, once clogged it can be a detriment. The majority of “no heat” calls are created from dirt. There’s a dirty little secret that you need to know about…. Keep the filter clean.
  6. The warm air then fills the ductwork. Sometimes you might hear the ductwork expand or contract. If it is unusually loud, and bothersome, let us know, and we can offer a solution.
  7. The combustion gases go up the metal chimney vent to the roof or in highly efficient models may be vented to the side/back of the home in white PVC pipes. The heating process creates some residual exhaust fumes that aren’t healthy to breathe in. Remember that combustion exhaust gases should not mix with the air in your home. This vent pipe takes the exhaust from the furnace and expels it from your home. It is very important the vent pipes are not obstructed. Sometimes a bird may nest in it, or it can be damaged by storms. If your furnace vents to the side of your home in PVC, make sure they are unobstructed, especially in the case of heavy snowfall.
  8. Once the thermostat is satisfied, the furnace shuts down. Note, the system will run a little longer to complete the cycle and help cool down the heat exchanger. The furnace then waits on standby until the next call for heat.

If there is a problem with the sequence of operations, the furnace will typically try a few more times before it locks out. At this point, check your filter and as they say in IT, “Turn it off and then on again” by resetting the furnace. If there is not a switch at the furnace, you can reset it on your electrical panel. If the problem continues, call Air-Rite Heating & Cooling. We are available 24/7.

Air-Rite Heating & Cooling’s Furnace Service & Repairs

Understanding your heating system is important. While there are many different brands(i.e. Carrier, Bryant, York, Trane, Lennox, American Standard) of furnaces and types of duct systems, hopefully, this explains the heating process and adds some insight into your furnace’s operation. While the list of components above has been simplified, there are many more steps and more parts to a furnace(i.e. Circuit board, hot surface ignitor, ignition switch, inducer motor, gas valve). These parts add to both the safety and efficient operation. While, we hope you agree that repairs should be left to the professionals and when problems do arise, you can call on the experts at Air-Rite Heating & Cooling. We can provide a diagnostic and offer the solution to get your furnace up and running again. Furnaces play a key role in providing your ultimate comfort and consume the majority of the energy bill for your home. Now you know the main parts, how they work, how to keep them working properly, and who to call when they aren’t working. If you have concerns or you run into any issues with your furnace, we’re only a phone call away at (630) 966-8100.

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