Hot Surface Ignition
The hot surface igniter (HSI) is the most commonly used electronic ignition system used today. The reason is they are reliable, they eliminate the need for a pilot, and they are inexpensive. These igniters directly ignite the burner and all they require is a flame sensor to detect the successful ignition of gas and proper flame occurring in the burners and turn off the hot surface igniter once proper ignition is achieved.
The hot surface igniter works like a light bulb filament, except here, we want the glowing red hot heat from the element, not the light. The hot surface igniter is placed in the flow of gas entering the burners. The hot surface igniter is usually made from a material like silicon nitride or silicon carbide (more fragile) and as electricity passes through the igniter, it will glow red hot.
When the thermostat calls for heat 24 volts are sent to the electronic ignition control module (furnace brains) which energizes the furnace blower and other systems and sends 120 volts to the HSI. The HSI heats up to around 1,800°F to 2,500°F. Once the proper HSI operating temperature is achieved, the gas valve is opened introducing gas to the burner which is ignited by the red-hot element of the HSI. Once a proper flame is sensed by the flame sensor, the HSI is shut down. NOTE: Some furnace designs have the HSI also act as a flame sensor (local sensing) instead of having a remote sensing rod in the burner flame (remote sensing).
Under normal conditions the HSI should last for 3-5 years. However, it will eventually crack and need to be replaced, quicker if the oils from your skin get on the element by improper handling.
These hot surface igniters are between $25 to $50 depending upon the model of furnace you have. I strongly recommend you keep a spare on hand. Just like a light bulb, the filament does not last forever and these igniters fail. As Murphy's Law will have it, the igniter will fail when it is at night or on a weekend and you can't easily get a spare.
We now need to take a look at some of the associated parts of the electronic ignition furnace.
Possible Reasons for HSI Failure
- Premature failure due to improper handling (oil from skin got onto element)
- Improper igniter
- End of normal usage life cycle (keep replacement on hand)
- Electric current in the home is too high (over 125 VAC)
- Severely dirty air filter causing high temperature limit switch to cycle furnace ignition on and off frequently.