There are a wide variety of different issues that can affect gas furnaces and cause them to fail. One potential issue is that the furnace igniter is faulty. Let’s take a closer look at how furnace igniters work and the ways that you can tell if your furnace igniter is bad.

How a Furnace Igniter Works

The igniter plays an important role in gas furnaces. As the name suggests, it is what ignites the gas flowing into the furnace so that it can create heat. It also controls the gas valve and detects whether or not the furnace is lit.

When the furnace first turns on, the igniter opens the valve so that gas can flow into the furnace. Once the gas is flowing, the igniter activates and lights the gas. It then uses a thin metal filament known as a thermocouple to detect whether there is a flame. The thermocouple contains a small electric charge that allows it to open and close the gas valve.

As the gas is lit, the flame heats up the thermocouple. This triggers it to keep the gas valve open. If no flame is detected, the gas valve will close, and the furnace won’t light.

Signs That Your Furnace Igniter Isn’t Working

The ignition system is what lights the gas inside the furnace on fire and causes it to combust. If the igniter doesn’t work, then the furnace will not turn on at all, preventing you from having any heat. That being said, there are also several other issues that can also cause your furnace to fail to turn on. It could be that there is an issue with your thermostat or that the gas flow is interrupted for some reason. Loose wiring and electrical issues can also prevent the furnace from running.

Standing Pilot Lights vs. Electronic Igniters

In the not-so-distant past, all gas furnaces used a standing pilot light for ignition. This type of igniter has a constant flame whether the furnace is running or not. Once the thermostat signals the furnace to start, the gas valve opens. The pilot flame causes the gas to instantly ignite. Because the thermocouple constantly detects the pilot flame, it controls the small amount of gas needed to keep the flame lit. If the flame isn’t lit or if the thermocouple cannot detect it, the gas flow stops, and the furnace won’t be able to ignite.

The main issue with standing pilot lights is energy waste. As long as the pilot light is lit, it is constantly using a small amount of gas. While the amount of gas it uses at one time is fairly minimal, a standing pilot light will still typically consume up to $5 of gas each month.

Another issue with standing pilot lights is convenience. If the gas flow is ever interrupted for any reason, the pilot light will go out. This means that the furnace won’t work until you manually relight it. Standing pilot lights can also get blown out due to drafts.

If you have an older furnace, it may use a standing pilot light. A few new furnaces also still use standing pilots, but the vast majority of modern furnaces have switched to using some type of electric igniter.

One of the most common types of electric ignition systems is an intermittent pilot light. This works almost exactly like a standing pilot in that it still uses a flame to ignite the gas. The difference is that the flame is only lit whenever the furnace is heating. When the furnace gets the signal to turn on, the pilot creates a spark that ignites the gas and turns on the pilot flame. As soon as the furnace shuts down, the gas flow stops, causing the flame to go out instantly.

You can also find some furnaces that use a direct spark igniter instead of a pilot flame. In this type of system, the igniter creates a spark that ignites the gas directly at the furnace burners. This is similar to the spark igniter used to light a standing pilot light or a gas barbecue grill.

The final type of system is known as hot-surface ignition. This igniter uses a thin metal filament similar to that found in incandescent light bulbs. When the furnace needs to start, electricity flows into the filament and causes it to almost instantly become glowing hot. The filament becomes so hot that the heat from it ignites the gas once it starts flowing out of the furnace burners.

Most Common Furnace Igniter Problems

There are two primary reasons why a furnace igniter stops working. The first is that the igniter has simply failed for some reason, and, in this case, the only solution is to have an HVAC technician replace the igniter. This usually happens due to normal wear and tear caused by the furnace turning on and off frequently.

The other reason has to do with either the thermocouple or the flame sensor. These work in basically the same way to detect the presence of a flame. If they cannot detect the flame, then the furnace will either not start or will immediately turn off once it does start. This usually occurs because the thermocouple or flame sensor is dirty and coated with dust and grime.

If the thermocouple is dirty, then the pilot flame will go out. This can also happen if the thermocouple is bent and doesn’t come into contact with the pilot flame. If there is an issue with the thermocouple, a technician will first use a voltmeter to measure whether there is a current flowing through it. If there is no current, then the thermocouple is faulty and will need to be replaced. If there is a current, this indicates that the thermocouple is either dirty and needs to be cleaned or it is damaged.

A dirty flame sensor will create a similar issue. In this case, the furnace will normally still light. However, if the flame sensor doesn’t detect that the furnace is lit within a few seconds, it will close the gas valve and cause the furnace to turn off. In this situation, the solution is usually to clean the flame sensor. If this doesn’t fix the issue, then the sensor will need to be replaced.

Richmond’s Home Service Experts

If you’re having any issues getting your furnace to light, the certified HVAC technicians at Davis & Green Services can troubleshoot your heating system and quickly determine what the issue is. We service and repair all brands and models of gas furnaces, and we also work on and install air conditioners, heat pumps, ductless mini-splits, boilers, radiant heating, and gas and electric fireplaces.

We work on both residential and commercial HVAC systems as well as plumbing and electrical systems. Our plumbers install and repair traditional and tankless water heaters, sump pumps, and garbage disposals. We also specialize in drain cleaning and repairs, leak detection, and a range of other services, including electrical repairs, rewiring, generators, ceiling fans, carbon monoxide detectors, and home EV charging stations. If you need to schedule any service in the Richmond, VA area, or if you have any questions, contact Davis & Green Services today.

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