In the not-so-distant past, all gas furnaces relied on a standing pilot light for ignition. A standing pilot light has a constant flame that instantly ignites the gas coming into the furnace whenever the heating system turns on. Nowadays, it is quite rare to find any furnaces that still use a standing pilot light. Unless your furnace was installed before 2010, it most likely instead uses either an intermittent pilot light or some form of electronic ignition. The primary reason that furnace manufacturers have moved away from standing pilot lights is to improve energy efficiency. Standing pilot lights waste quite a bit of energy since they need to constantly burn a small amount of gas to keep the flame lit.
How Much Energy Does a Pilot Light Waste?
The average standing pilot light uses somewhere around 500 BTUs of energy per hour, while some older, less-efficient pilot lights can potentially use more than 1,500 BTUs every hour. This means that the average pilot light will use more than 14,000 BTUs every day and more than 430,000 BTUs every month.
Natural gas is typically measured using therms, and one therm is equal to 100,000 BTUs. The average cost of natural gas in the U.S. is around $0.95 per therm, which means that running a standing pilot light will typically cost just over $4 per month. If the pilot is much more inefficient, it could potentially use more than $10 worth of gas each month.
If your furnace runs on propane instead of natural gas, then your costs will be much higher. Propane is always measured in gallons instead of therms, and every gallon of propane is equal to 92,000 BTUs. Running a pilot light for a month will typically use around 4.7 gallons of propane, which means that you would end up wasting more than 50 gallons a year if you kept the pilot on all year long. The average price of propane in the U.S. is around $2.34 per gallon, which means that a standing pilot light will usually cost you more than $11 a month.
In addition to improving energy efficiency, the other main reason that most manufacturers no longer use standing pilot lights is that they aren’t all that reliable. A standing pilot light can easily get blown out by a draft and will then need to be relit before the furnace can run. The pilot light will also instantly go out if your gas supply is ever interrupted. Modern furnaces avoid these issues by using either an intermittent pilot light that only ever produces a flame when needed or an electronic igniter.
Standing vs. Intermittent Pilot Lights
Many furnaces now use what is known as an intermittent pilot light. An intermittent pilot light still produces a flame that is used to ignite the furnace burners, but this type of pilot light only works on demand. Intermittent pilot lights do still use a small amount of gas, but far less than a standing pilot where the flame is constantly lit.
When the thermostat sends the signal for the furnace to turn on, the pilot gas valve will open so that gas begins flowing. The pilot light will then produce a spark to ignite the gas. Once the pilot flame is then burning, the pilot’s thermocouple will signal the furnace to open up the main gas valve so that gas begins flowing out the burners and the pilot flame will instantly ignite the gas.
Most intermittent pilot lights will stay lit the entire time the furnace is running to ensure it stays lit. The reason for this is that it is the pilot light’s thermocouple that detects whether the furnace is lit. The thermocouple is a metal probe that works to sense the heat from the pilot flame.
A small electrical current runs through the thermocouple, which enables it to both detect the flame and open and close the furnace’s gas valve. As soon as the thermocouple detects the flame, it signals the gas valve to open. If it doesn’t detect the flame for any reason, the gas valve will remain closed and the furnace won’t light.
Direct-Spark Ignition
You will also find many furnaces that don’t rely on a pilot flame at all and instead simply use a spark to ignite the gas at the burners. This type of ignition uses two metal electrodes. When the furnace needs to ignite, an electrical arc is created between the electrodes, producing a spark that instantly ignites the gas burners. A direct-spark igniter is more energy efficient than an intermittent pilot light since it only uses a tiny amount of electricity and not any gas.
Hot-Surface Ignition
Some furnaces instead use what is known as hot-surface ignition. This type of igniter is basically a metal filament that is quite similar to the filament in an incandescent light bulb. Electricity flows into the filament, which almost instantly heats the filament so it is red and glowing. The filament produces enough heat that it instantly ignites the furnace as soon as gas starts flowing out of the burners.
How the Flame Sensor Works With an Electronic Igniter
All furnaces that use an electronic igniter instead of a pilot light have a flame sensor that controls the gas flow and detects whether the burners are lit. The flame sensor works in essentially the same way as a thermocouple except that it detects the flame at the burners instead of at the pilot light.
The flame sensor sits directly inside the burner flame. An electrical current passes through the flame sensor, which enables it to detect the flame from the burners. If the burners don’t light or the flame sensor can’t detect that they’re lit, the sensor will automatically trigger the furnace’s main gas valve to close. This is an essential safety feature that ensures that the furnace works correctly and that no gas can flow if the furnace isn’t lit. Without this feature, the furnace could potentially fill up with gas if the electric igniter fails, creating a risk of explosion.
Flame sensors can sometimes fail. When this happens, no electricity will flow through the sensor, preventing it from working. In this situation, your only option is to have the sensor replaced. That being said, the more common reason that a flame sensor stops working properly is that it is covered in dirt and soot. This can interfere with the electrical current’s ability to detect the flame and lead to the furnace only ever running for a few seconds before the gas valve automatically closes and the unit shuts down. A dirty flame sensor is one of the easiest problems to avoid as all you need to do is make sure you have your furnace professionally maintained every fall.
If you need any furnace service in the Jacksonville area, you can trust the team at McGowan's Heating & Air Conditioning for help. We specialize in heating repairs, and we can also maintain your heating system to improve its efficiency and ensure it is working as effectively as possible. When it comes time to have your furnace or other heating unit replaced, we can help with that as well. We also offer a full range of cooling, indoor air quality, air duct cleaning, attic insulation, and other HVAC services for both residential and commercial customers. For more information or to schedule a service call, contact McGowan's Heating & Air Conditioning today.
