Understanding the Functioning of Pilot Lights

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Even though pilot lights aren’t the most efficient, they are still commonly used.
Eric Pearle/Getty Images

If you own an older model furnace or water heater that runs on natural gas or have a set of gas logs in your fireplace, you’re probably familiar with the small blue flame known as the pilot light. You may also have had the experience of relighting the pilot light when it goes out. Let’s explore how the pilot light works.

The basic concept behind a pilot light is simple. It is designed to provide the flame necessary to ignite the gas coming out of the primary burner. When the furnace is turned on, a valve releases gas into the burner and the pilot light ignites the gas.

The creation of a pilot light is also straightforward. It is generated by allowing a small quantity of gas to pass through a small tube from the gas pipe. The gas is ignited at the end of the tube and burns constantly.

However, the pilot light poses a safety risk. If the pilot light were to blow out, the gas would still continue to flow from the tube. If the gas were to accumulate inside your home and ignite, it would result in an explosion. To address this issue, the pilot light tube includes a valve that cuts off the gas to the pilot light in the event of a blowout.

The valve is a remarkable little device. It must be able to detect whether the pilot light is on or off, and for safety reasons, it must be able to do so without relying on external power. How do you create a pilot light sensor that functions reliably for many years without any electricity?

The solution to this problem is ingenious. The way to accomplish this is by using the heat generated by the pilot light flame to produce electricity using a thermocouple.

Thermocouples generate electricity directly from heat. They take advantage of an electrical effect that occurs at junctions between different metals. For instance, take two iron wires and one copper wire. Twist one end of the copper wire and one end of one of the iron wires together. Repeat the process with the other end of the copper wire and the other iron wire. If you heat one of the twisted junctions with a flame and attach the two free iron wires to a volt meter, you will be able to measure a voltage.

In a pilot light, one of the junctions of a thermocouple is located in the pilot light’s flame. The electricity generated flows to a small electromagnetic valve and keeps it open. If the pilot light blows out, the thermocouple rapidly cools down. It stops generating electricity and the valve closes.

To relight the pilot light, you must press a button that manually opens the valve. Then, light the pilot light and wait for the thermocouple to heat up (about 30 seconds). Once it is hot, the thermocouple generates the electricity required to keep the valve open. You can then release the button.

The issue with pilot lights is that they use up a significant amount of gas, leading to wastage. As a result, modern appliances no longer incorporate pilot lights and instead use a piezoelectric spark to ignite the burner.

Originally released on: Feb 17, 2006

Pilot Light FAQ

What is the function of a pilot light?
A pilot light serves the purpose of producing a small flame that ignites the gas from the main burner. It was commonly found in older furnace and water heater models, but newer ones use a piezoelectric spark instead.
Can I relight the pilot light by myself?
Yes, you can, but you have to exercise caution. To relight the pilot light of your furnace or water heater, you have to manually open a valve by pushing a button. Afterward, you need to ignite the pilot light and wait for 30-40 seconds for the thermocouple to heat up so that you don’t have to hold the button anymore.
Is relighting a pilot light hazardous?
Relighting a pilot light can be dangerous as the gas may continue to escape without igniting if the flame blows out. This could result in an explosion if the gas accumulates in your room. Although pilot tubes have a cut-off valve to address this issue, they are not immune to malfunctions.

Additional Information

Related HowStuffworks Articles

  • Appliance Quiz
  • How Home Thermostats Work
  • How Water Heaters Work
  • How Tankless Water Heaters Work
  • How to Choose a New Water Heater
  • How To Replace a Thermocouple
  • Is it possible to generate electricity directly from heat?



1. What is a pilot light?

A pilot light is a small flame that burns continuously in gas appliances to ignite the main burner when needed. It is commonly found in older gas furnaces, water heaters, and stoves.

2. How does a pilot light work?

A pilot light works by using a small amount of gas to keep a flame burning continuously. When the appliance needs to be turned on, the gas valve opens and the pilot light ignites the main burner with its flame.

3. What is the purpose of a pilot light?

The purpose of a pilot light is to provide a source of ignition for the main burner of a gas appliance. It ensures that the appliance can be turned on quickly and easily when needed.

4. Can a pilot light be turned off?

Yes, a pilot light can be turned off. This is often done during the summer months when the appliance is not in use to save on energy costs. To turn off a pilot light, the gas supply should be shut off and the flame should be extinguished.

5. How do you relight a pilot light?

To relight a pilot light, the gas valve should be turned off for several minutes to allow any gas to dissipate. Then, the pilot light valve should be turned to the “pilot” position and the igniter button should be depressed while holding a flame to the pilot light. Once the pilot light ignites, the valve can be turned back to the “on” position.

6. What happens if a pilot light goes out?

If a pilot light goes out, the main burner of the gas appliance will not ignite. This can be caused by a number of factors, such as a draft or a malfunctioning component. It is important to relight the pilot light as soon as possible to ensure the appliance can be used safely.

7. Can a pilot light be replaced?

Yes, a pilot light can be replaced if it is malfunctioning or damaged. This may require the assistance of a professional technician who can identify the cause of the issue and recommend the appropriate replacement part.

8. What are the benefits of a pilot light?

The benefits of a pilot light include quick and easy ignition of gas appliances, as well as a constant source of heat for appliances that require a continuous flame. However, newer appliances have replaced pilot lights with electronic ignition systems that are more energy-efficient.

9. Are pilot lights safe?

When maintained properly, pilot lights are generally safe. However, they can pose a risk if they are not relit properly or if gas leaks occur. It is important to have gas appliances inspected regularly to ensure they are functioning safely.

10. Do all gas appliances have pilot lights?

No, not all gas appliances have pilot lights. Newer appliances often use electronic ignition systems that do not require a pilot light. These systems are more energy-efficient and safer than traditional pilot lights.

11. Can a pilot light contribute to energy waste?

Yes, a pilot light can contribute to energy waste if it is left burning continuously when not in use. This is why many people choose to turn off their pilot lights during the summer months when the appliance is not needed. Additionally, newer appliances with electronic ignition systems are more energy-efficient and do not require a pilot light at all.

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