Many folks trying to fix a furnace that won't start may find to their surprise that they don't have a "pilot light" at all. So if you're asking the question "Do all furnaces have pilot lights?" the answer is No. In fact, most furnaces these days don't have pilot lights, or at least standing pilot lights—the kind that burns all the time and has to be relighted when they go out. A pilot light is a type of ignition system, and your furnace is likely to have one of the following three types.
Standing Pilot Ignition
You're probably familiar with a standing pilot light. It's a little blue flame that sits in front of one of the furnace burners and burns continuously. Hot water heaters, gas fireplaces, and old gas stoves also often have similar standing pilot lights. If your furnace has a little round knob on the gas valve with the words OFF/ON/PILOT, you have a standing pilot ignition. Here's how to re-light gas furnace standing pilot.
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 it lights it 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. The pilot, in turn, 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 furnaces with electronic ignition have a hot surface igniter or HSI.
This is a simple part consisting of a ceramic fork or tongue 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 if 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 a problem with the furnace's electronic control board.
What's Wrong With Pilot Lights?
The main problem with standing pilots is that they waste gas, to the tune of about 4 to 5 therms per month. A therm is the unit of measure found on most people's natural gas bills. At the national average of around $1/therm, that works out to $4 to $5 per month or $480 to $600 over a 10-year period. The other problem with standing pilots is their tendency to go out, usually when it's windy or when the orifice on the pilot supply line is blocked.
Sometimes they seem to go out for no reason other than to complicate your day. And that's what's wrong with pilot lights.