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Gas Furnace Hot Surface Ignitor (HSI) System Information

Gas Burner Ignition Sequence

To understand how the ignition sequence works we must first break it down into the types of systems. The first is the hot surface igniter type system (HSI) which is essentially a small electric heating element somewhat like an electric oven heating element.

The other common types are either an intermittent pilot where the pilot is lit when needed or a standing pilot where the pilot stays lit much like on a standard gas water heater. There is also one other type that is not very common on gas furnaces but is standard on most oil furnaces and that is the spark ignition.

Hot Surface Igniter
Hot Surface Ignition (HSI)

For the hot surface ignition type, after the control board has confirmed that the inducer fan is in fact running the heating system will apply high voltage to the hot surface igniter, some of these igniters are monitored by the board by checking to see if the device is actually drawing power. 

The board then switches on the main gas valve that allows gas to flow to the burner assembly.


The burner then lights and the flame should be uniform across all burner tubes and mostly blue in color where it can be seen. The Heating system control board confirms that the gas has in fact lit and is burning with a flame sensor of some kind, this usually looks like a small metal cylinder that is in direct contact with the flame itself.

After the system has confirmed that the gas is lit the heating system control board shuts off the hot surface Hot Surface Igniter inside a Gas Furnace igniter and continues to monitor the flame sensor. As long as the flame stays lit and the call for heating as explained in thermostat control article the system should maintain the gas valve open.

CAUTION - Always be very careful with electricity as it can be lethal, if you are not comfortable performing any of these steps please do not perform them, of course you are performing these steps at your own risk.

Important Note: The heating system control board is sometimes very sensitive to static electricity, avoid touching this board to prevent failure.

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