3 Common Furnace Ignition Methods

An electronic furnace

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Many homeowners trying to fix a furnace for the first time are surprised if they don't find a traditional pilot light (also known as a standing pilot). It's often expected that any appliance that burns natural gas or propane will include a tiny flame that constantly burns and ignites the gas when the appliance's controls deliver fuel to the main burners.

In reality, pilot lights are largely a thing of the past for gas-powered furnaces, as well as for most other gas-burning appliances, such as stoves and water heaters. Today's furnaces ignite the gas through different technologies, known collectively as electronic ignition systems.

Depending on its age, your furnace might have one of the following three types of ignition methods.

Standing Pilot Ignition

You're probably familiar with a standing pilot light, the little blue flame that sits in front of one of the furnace burners. If your furnace has a small round knob on the gas valve with the words OFF/ON/PILOT, you have a standing pilot ignition.

This is relatively simple technology, though not very fuel efficient. A small heat-sensing device known as a thermocouple makes sure gas flow is delivered to the main furnace burners only when it is warmed by the pilot light. Occasionally a standing pilot light will need to be relighted, or a thermocouple will need to be replaced. But these are easy and infrequent repairs.

Intermittent Pilot Ignition

If you have a newer furnace (or one with an AFUE rating above 80), it most likely has an electronic ignition system. One type of electronic ignition is an intermittent pilot. This system uses a pilot flame, but the flame lights only when there's a call for heat from the thermostat.

The furnace's electronic control board activates an electric igniter that uses a high-voltage spark to light the pilot. In turn, the pilot lights the gas for the burners. Once the burn cycle is complete, both the burners and the pilot turn off. You can identify an intermittent pilot by its heavy-gauge wire leading to a device with a metal tip that is positioned next to the end of a small gas line, which supplies the pilot flame.

Hot Surface Ignition

Most modern furnaces with electronic ignition have a hot surface igniter, or HSI. This is a simple part consisting of a ceramic fork attached to a squarish plastic base with two wires. The igniter sits in line with one of the gas burners. When there's a call for heat from the thermostat, the fork heats up until it is red hot. The gas valve then opens, and the gas is ignited by the glowing igniter.

Hot surface igniters burn out over time, but they are easy to replace. You can usually check to see whether a hot surface igniter is working simply by looking through the louvers of the front cover when there is a call for heat. Igniters glow brightly. So if you don't see any light, something is wrong. On the other hand, if the igniter never stops glowing, there's likely a problem with the furnace's electronic control board.

Why Pilot Lights Were Discontinued

The main problem with standing pilot lights is they waste gas to the tune of about 4 to 5 therms per month. (A therm is the unit of measurement found on most people's natural gas bills.) At the national average of around $1 per therm, that works out to $4 to $5 per month wasted—or $480 to $600 over a 10-year period—just to run the standing pilot light.

The other problem with standing pilots is their tendency to go out if there's a draft blowing on them or the orifice on the pilot supply line is blocked. Electronic ignition systems, on the other hand, can go many years without any need for attention.