How To Light A Gas Furnace With Electronic Ignition

You might have seen or at least heard about electronic ignition systems that recently replaced traditional standing pilot systems. It’s also likely that you’ve heard about the many benefits of electronic over standing pilot ignition systems.

Well, it’s true. Electronic ignition systems are a lot more efficient than standing pilot systems. They are also significantly safe. Read on to learn how they work and why you should consider getting one.

What is Electronic Ignition?

The term “electronic ignition” refers to ignition systems that rely on an electrical switch to initiate heating. Instead of a standing pilot system that stays on throughout, electronic ignition systems use transistors controlled by sensors to generate electronic pulses that ultimately ignite the flame. As a result, the electronic ignition system doesn’t need to stay on continuously.

Instead, the pilot light only comes on when the sensors determine that the thermostat is calling for heating.

How Do Electronic Ignition Systems Work?

Electronic ignition systems come in two main types – intermittent pilot systems and hot surface ignition systems.

The Intermittent Ignition Furnace

Intermittent pilot ignition systems are solid-state ignition devices that automatically light a pilot burner. They also energize the heating system’s main gas valve when the thermostat calls for heating.

How it Works

Intermittent ignition systems use a spark plug that produces a spark to light the furnace’s burners. When your thermostat calls for heat, the furnace’s circuitry passes a high voltage between the spark plug’s center electrode and ground electrode, breaking down the insulation between the electrodes and allowing current to flow in the discharge phenomenon.

This produces a strong spark that triggers instant ignition and combustion if exposed to a compressed air-fuel mixture. The spark only lasts about 1/1000 of a second and is extraordinarily complex.

The ignition happens because the spark discharge activates the fuel particles between the two electrodes, triggering an oxidation process (chemical reaction in which an atom or compound loses one or more electrons).

Oxidation produces tons of heat that instantly combusts when exposed to a compressed fuel-air mixture.  

How to Light an Intermittent Ignition Furnace 

You don’t need to light the intermittent ignition pilot. Instead, it ignites automatically when the thermostat calls for heating. The ignition process is as follows;

  1. You turn on the furnace: The intermittent ignition system cannot come on if the furnace is switched off. So, the first step is to turn it on.
  2. The thermostat calls for heating: Your thermostat will call for heating whenever indoor temperatures drop below your setting, typically 68°F to 72°F. Immediately the intermittent ignition module opens the pilot valve and simultaneously supplies a continuous spark to the electrode in your pilot burner.
  3. The main gas valve opens: The pilot flame sensor detects the pilot flame and signals the intermittent ignition system to stop the spark and open (energize) the main gas valve.
  4. Heating begins: With the main gas valve now open, gas flows to the burners and ignite. This is known as the heating cycle.

A Few Points to Note

  • In step #3, the main gas valve cannot be energized until the flame sensor detects and confirms the presence of the pilot flame. It will stay closed. If the pilot flame goes off, the gas valve closes, and ignition recurs after about half a second.
  • The heating cycle in step #4 typically runs for about 15 minutes (it could be more or less) until the home is in the thermostat setting. At this point, the gas valve closes, and the main burner (or burners) shuts down, and the furnace enters an “off-cycle” until room temperatures drop below the thermostat setting, at which point the heating cycle resumes.
  • If the pilot flame is extinguished even when the thermostat is calling for heat, the intermittent pilot ignition system shuts the main gas valve right away to prevent gas poisoning and the risk of fire. The ignition will produce another spark about 0.8 seconds later to attempt to relight the pilot. If everything goes well this time, the furnace enters a heating cycle.

The Hot Surface Ignition Furnace

Hot surface ignition systems are either silicon carbide igniters or silicon nitride igniters. Whichever the case, they operate the same way. However, silicon nitride ignitors are considered the better of the two because they are less fragile.

The ignitors typically take an “M” or folk shape. When you pass a strong current through the ignitor, it glows red hot to temperatures over 2500°F. Natural gas and propane combust spontaneously when exposed to such high temperatures.

Most furnaces pass a low-voltage but high-current electricity through the hot surface to shorten the heating process. This way, it takes only a few seconds, typically less than five, for the ignitor piece to glow red hot. Once it’s at least 2500°F hot, the ignitor requests the furnace to open the main valve to allow gas to flow to the pilot system.

Now, remember that natural gas naturally combusts (without a spark) at 1163°F while propane does the same at 920°F to 1020°F. It means that both gases will combust when exposed to the glowing ignitor piece at 2500°F, instantly lighting the pilot. The ignition system has a safety mechanism that observes the pilot for up to five seconds to ensure that it’s burning normally.

If everything is fine, it allows gas to flow to the burners. Then the pilot lights the burners, and your furnace comes on. However, if the pilot doesn’t seem to light properly, such as if the flame appears weak, the ignition system cuts the gas supply to the pilot system to prevent the risk of gas poisoning or an explosion.

Hot surface ignition systems use a thermocouple to detect the presence and evaluate the health of the pilot flame. The thermocouples have tiny sensors that allow them to pick up even the slightest electric signals. If present, the flame generates a small alternating current that the sensor converts into a direct current and sends to the control module.

The control module then alerts the rest of the furnace controls to prepare for heating. However, if the sensor cannot find any flame or the present flame is feeble, the control module will terminate the ignition process, wait a few seconds, then retry. Most furnaces try up to three times, after which the furnace enters “lockout” mode unto you reset it.

How to Light a Hot Surface Ignition System

Make sure that the furnace is plugged On and getting power from the primary circuit. The breaker must be ON. Additionally, make sure the main gas valve is open. The valve is open when the lever is parallel to the gas pipe. If it’s perpendicular to the pipe, it’s off.

After verifying both, all you have to do is press the start/ON button on your furnace, and the unit will roar to life. The ignition process is as follows;

  1. You turn the furnace ON at the switch: The furnace won’t automatically start heating when you turn ON the furnace. Instead, it waits until the thermostat asks for the same. However, it must be ON to communicate with the thermostat.
  2. The thermostat calls for heating: Thermostats request heating depending on the owner’s settings, typically 68°F to 72°F.
  3. Control board checks safety controls: It checks that everything is working and the pressure switch is closed. If all is tine, it allows closes the circuitry that allows voltage to pass to the hot surface ignition system.
  4. The ignitor glows red hot: This typically takes up to five seconds, depending on the type of hot surface ignitor.
  5. Gas flow to the pilot system: As the hot surface ignitor glows, the control board simultaneously allows gas flow to the pilot system.
  6. The pilot lights: Once the hot ignition system is red hot, the pilot gas valve opens, letting out compressed gas at high pressure, which instantly combusts upon coming in contact with the red-hot ignitor piece.
  7. Gas flows to the main burners: If the pilot flame is burning normally and there are no safety issues (there’s a flame sensor to verify this), the furnace’s control system opens the burner valve, allowing gas to flow to the burners.
  8. The pilot system lights the burners: The pilot system is strategically located within the burner area. So, barring any late surprises, the pilot will light the system as soon as gas comes out of the burners.
  9. The hot surface ignitor goes off: Once the burners are up and burning healthily, the control board cuts the power supply to the hot surface ignition, effectively killing the pilot light. Meanwhile, the burners continue to light.

The burners will continue burning and producing heat until your home is heated to the temperature setting on your thermostat. Then, the thermostat requests the furnace to cycle off, at which point the gas valve leading to the burner closes, and the flame goes out until the thermostat requests for heating again.

Electronic Furnace Ignition Benefits

Electronic furnace ignition systems come with a few important advantages over traditional pilot ignition systems.

  • Safety: Electronic ignition systems are a lot safer than traditional standing pilot furnaces. For one, there’s a reduced risk of fire. Additionally, it eliminates the risk of gas poisoning in case the standing pilot accidentally goes off.
  • Energy-saving: Although the standing pilot doesn’t consume a lot of gas, it still gobbles up a significant amount. According to the University of California, San Diego, the average standing pilot consumes $219 worth of natural gas per heating season. You’d undoubtedly find a better use for that money.
  • Reduced maintenance: Finally, electronic ignition systems save homeowners from tedious maintenance routines typical of standing pilot furnaces. For example, you no longer have to worry about shielding the pilot flame from wind or keeping an eye on it throughout the heating season.

Downsides

The only major downside of electronic ignition systems is durability and reliability. Neither spark ignition systems nor hot surface ignition devices are as durable as standing pilot ignition systems.

In particular, hot surface ignition systems tend to break down rather often, necessitating expensive repairs or replacements.

Summary

That’s all. Now you know how to light a gas furnace with electronic ignition. You also understand how electronic furnace ignition systems work. Ensure to take the best care of your ignitor to prolong its life.

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