furnace work

Furnace 101: How Does Your Furnace Work?

Your thermostat is likely the first thing you check when your home gets cold, but it’s not your thermostat that produces the heat. That important job is done by the furnace, usually situated in the basement. But how do furnaces work and what are some of the common reasons they can break down? To help give you a basic understanding of this essential home appliance, we’re going to look at some of the most important components, explain how they work, and where issues can arise.

Why do we use gas furnaces?

Because of our climate, our abundant natural resources, and our home building standards, nearly all residential in Alberta furnaces operate on natural gas. The benefit of gas furnaces is their reliability, relatively easy maintenance, and superior efficiency. Most new gas furnaces have an efficiency rating between 90% to 98%. What does that mean? A furnace with a 98% efficiency rating will transfer 98% of the heat produced during the combustion of natural gas to heat in your home, (while only 2% is lost to the exhaust). In comparison, upwards of 30-50% can be lost on older, mid-efficiency, and standard-efficiency furnaces. Today’s high-efficiency furnaces not only keep your home consistently warm, but they also help reduce your gas bill.

How do gas furnaces work?

Your thermostat serves as the brains of the operation. You set it to the temperature you want in your home and the thermostat signals to your furnace when your home drops below that set temperature. When that happens, your furnace kicks into action. It works by first drawing in fresh combustion air into the burner box and through the heat exchanger, then igniting the pilot (or ignitor), then opening the gas valve delivering natural gas to the burners. Once the furnace is ignited, the blower motor is engaged, circulating air through the furnace, and then through the vents located throughout your home.

Now that you have a high-level idea of how a furnace works, let’s get into how all the parts work in that process, and the ways they can break down.

Furnace parts: how they work and where they go wrong

A lot like your car, your furnace is made up of all sorts of small parts, each of which serves an important purpose to ensure things run safely, efficiently, and smoothly. But over time these parts can wear out or fail and require occasional repair, maintenance, or even replacement. Here are some of the key components that are essential to your furnace’s performance.

Blower Motor

What it is: The blower motor is what distributes air through the furnace (and air conditioner) into the home. Its job is to blow the hot or cool air through your duct system and out the vents to heat or cool each room.

Common issues: When overworked, blower motors can overheat causing bearings to wear out. Blower motors are wear items, meaning eventually they will fail, but certain factors can accelerate the wear on this part. Most commonly, this happens from poor airflow due to clogged or dirty filters. Once a furnace’s blower motor begins to make noises on startup or during normal operation, it’s a sign that this part is on its way out and should be looked at by a service technician. If the blower motor fails, your furnace will not heat or cool your home.

Inducer motor & blower

What it is: You might not know what this part is, but you may be familiar with its sound. This important component is made up of a small, electrical motor, a blower wheel, and an inducer housing assembly. The inducer motor is effectively a fan that circulates combustion air into the furnace heat exchanger. You may recognize this part by the gentle whirring noises you often hear right before your furnace turns on. This is your inducer motor purging air and gas from previous cycles prior to starting the new ignition cycle. The inducer runs whenever the furnace is producing heat and is responsible for bringing in the correct volume of air to the furnace for complete, clean combustion.

Common issues: Like all motors, this component can wear out over time. One of the easiest ways to tell if there is a problem is to listen carefully to your furnace. A burnt-out or worn, inducer motor will often be noisier than normal and produce a higher frequency sound. The same can be said of the blower itself. When it is off-balance or worn, the blower will be louder than normal. A general rule of thumb is that if your furnace is making unusual noises, it is worth calling a professional and scheduling an inspection.

Igniter & pilot light

What it is: The gas that fuels your furnace needs to be ignited to create a flame, which keeps the burner running (think of this like a gas, or a camping, stove). This combustion of natural gas is what creates heat in the furnace’s heat exchanger. At one time, gas furnaces were kept lit by a pilot light (a constant flame inside the furnace burner assembly). However, today all high efficiency, natural gas furnaces do not have pilot lights and instead use electric igniters.

Common issues: Pilot lights can go out due to drafts – meaning you occasionally have to relight them. Also, thermocouples (a safety mechanism that ensures the pilot light stays on) wear out over time and occasionally need to be replaced. Igniters are more reliable but can fail as often as every 5 years if not sooner. Regularly changing your furnace filters can improve the life span of your igniter by keeping it from getting dirty and dusty.

Flame Sensor

What it is: This component is present on mid-efficiency and high-efficiency furnaces and takes the place of a thermocouple. The flame sensor is a safety feature designed to ensure natural gas is always being burned. If the ignitor fails, or the natural gas fails to ignite when the gas valve opens, the flame sensor will alert the furnace to shut off the supply of gas to the burners from the gas valve.

Common issues: Flame sensors rarely break, but they can certainly get dirty from carbon built-up. This reduces the effectiveness of the sensor, causing it to send a message to the furnace that there is no flame present even when the burners have ignited. This can be prevented through regular filter replacement, and annual maintenance or tune-up appointment to clean the sensor.

Note: All homes should be equipped with carbon monoxide detectors. Like a fire alarm, they will beep and alert you if there is a natural gas leak in your home. 

Gas Valve

What it is: The gas valve controls the flow of gas released to the burners for ignition by the pilot light or igniter. In older furnaces, there are two parts: the pilot tube valve which controls the release of gas to the pilot light, and the main valve which releases gas to the burners.

On newer, mid-efficiency, and high-efficiency furnaces there’s only one valve that allows gas to flow to the burners when there’s a call for heat. Higher-end furnaces offer two-stage and modulating gas valves. These vary the flow of natural gas, allowing the furnace to vary the amount of heat output.

Common issues: There are a number of reasons these valves can fail. The most important thing to know here is that a professional service person needs to be brought in if you suspect your gas valve has failed. They will be able to determine if it’s the gas valve or another component that has failed and will be able to safely replace it while preventing possible gas leaks.


What it is: The furnace burner is where the combustion air mixes with the natural gas and is ignited to produce the heat that gets transmitted throughout your home.

Common issues: Burners rarely fail but do get dirty which can cause incomplete combustion of natural gas and reduced energy efficiency. This can be corrected (and prevented) through regular professional furnace tune up’s.


What it is: The furnace filter is a component built to clean the air entering your furnace of dust, dirt, pet hair, and any other particles. This is important to ensure that these particles don’t damage your system through buildup. Some filters also work to improve home air quality depending on your furnace and filter type.

Common issues: Filters are not meant to last forever. They need to be replaced every 30 to 90 days (depending on the filter) or they will quickly become clogged and ineffective. Luckily, filters are quite easy to change out yourself and are relatively inexpensive. Learn more.

A final word of advice

As do-it-yourself YouTube videos become more popular, many homeowners are more empowered than ever to tackle projects in their homes. This is fantastic for some basic wear and tear issues; however, we strongly caution you to call a professional for your annual maintenance check and repair work. In some cases, performing your maintenance may void your warranty.

A yearly check-up can significantly increase the life of your furnace, so be sure to schedule in with a professional to avoid a costly replacement sooner than necessary.

It takes some training and serious know-how to technically understand each part of your furnace, but it’s good to have the general knowledge to keep an eye out for issues. If you suspect something may be wrong with your furnace, or if you’d like to schedule general maintenance, reach out to us today. We’re always happy to help.

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