A large majority of homes these days have a gas furnace like the one in the photo. In this article, I'm going to point out some of the main components, we will learn about what the components do, and I'll give you some tips on how to maintain your furnace each year.
A heat exchanger is long metal tubes that are inside your furnace. You can't really see the heat exchanger which makes inspecting them rather difficult. The flames from your furnace burn inside the tubes and the toxic gases are vented out of the home through the roof. This keeps the toxic flames and gases separated from the clean, warm air that is blown into your home. If your heat exchanger develops a crack, this can allow harmful gases like carbon monoxide into your home. Carbon monoxide detectors should be placed on every floor of the home, and also within 15 feet from bedrooms.
Gas furnaces need a way to ignite the gas to start a flame. There are mainly two different types of igniters, a standing pilot light, and electronic ignition. A standing pilot light is when the pilot light is on at all times. When the thermostat calls for heat, the gas valve opens and the burner turns on to heat the home. An electronic ignition is either an intermittent pilot light, or a hot surface igniter. An intermittent pilot light is when the thermostat calls for heat, the pilot light turns on and then ignites the burners. Most furnaces these days use a hot surface igniter. This is sometimes referred to as a "glow plug" because when the thermostat calls for heat, this igniter gets red hot and starts glowing. The gas valve will open and when it gets hot enough, it will ignite the burners. In the photo below, you can't really see the igniter, but you can see the red glow as it is heating up.
TIP: If your hot surface igniter doesn't glow when calling for heat, you may have a crack in the igniter which won't allow it to heat up and glow hot enough.
A burner is where the gas and air are combined to create a flame that will heat your home. Burners can be cleaned carefully. Dust and debris can get in your burners and cause the flames to flicker, or it can potentially cause one or more of the burner ports to not ignite. A healthy burner will have blue flames.
Gas (most commonly natural gas and less commonly propane gas) is piped into the home and into a gas valve inside the furnace cabinet. The gas valve controls the gas going into the furnace burner and will also shut off if a safety switch fails.
A thermocouple is used with a pilot light to sense whether the pilot light is lit or not. When the thermocouple heats up from the flame on the pilot light, it sends a 24 volt signal to the gas valve to allow gas to flow and keep the pilot lit. If the thermocouple is going bad or does not sense any heat, then the gas valve close and will not keep the pilot light lit.
When the igniter heats up and ignites the gas, the flame from one burner will pass directly over a small probe called a flame sensor. This sensor heats up from the flame passing over it and sends a signal to the gas valve telling it to stay on. If the flame sensor doesn't heat up or is going bad, it will shut the gas valve off so it isn't spewing gas through the furnace with no flames.
TIP: If your furnace burner will not stay on, chances are, the flame sensor is no good or dirty. If the burners ignite but then shut off right away, this is a good sign that the flame sensor is bad. These sensors can be cleaned with emory cloth, or a very fine grit sandpaper. This will clean the carbon off of the sensor and it can operate properly after. Cleaning this probe can potentially save you a few hundred dollars on a service call.
Draft Inducer Motor
When gas is burned it gives off toxic fumes that need to be vented out of the home. A draft inducer motor turns on to create a small vacuum that pushes the air and fumes up and out of the vent pipe.
A pressure switch is used to make sure the draft inducer motor is on and is pushing gases out of the exhaust pipe. If the draft inducer motor turns off or fails, then the pressure switch will not allow the gas valve to be turned on. This is a safety mechanism to ensure that all the dangerous gases are being pushed up the exhaust pipe.
A blower motor is used to blow air past the hot heat exchangers which heats up the air. This warm air is blown up through the duct work in the house and into rooms through the air vents.
Return Air Filter
I'm pretty sure we've all seen and replaced one of these before. These keep out dust and debris from entering back into the furnace. A dirty air filter can cause a furnace to not work properly and can force the blower motor to work too hard. These should be replaced frequently.
The best tip I can give you is to have your furnace inspected once a year by an HVAC professional. They will clean, adjust, calibrate, and test all the components of your furnace to make sure they are working efficiently and ready for the cold winter months.
My name is Chris Dallaglio and I'm the owner of Total Eclipse Inspections LLC. I'm a licensed Home Inspector, Radon Measurement Professional, U.S. Dept of Energy Home Energy Score Assessor, part-time blogger.