In the furnace sequence of operation, the ignition sequence happens after the safeties have been checked, the inducer fan comes on, and the pressure switch closes. The ignition sequence begins with the ignitor. This part provides the heat needed for combustion (remember: a fire needs heat, oxygen, and fuel), and the ignition process looks a little different depending on which type of ignitor is in the gas appliance at hand. Some methods take longer than others. Some directly light the burners, and some use a pilot light. We’ll take a look at the four main ignitor types we’ll see in gas appliances.

Standing Pilot

Standing pilots aren’t very common in furnaces anymore, but we do see them in some gas-fired appliances (including water heaters, some pool heaters, and even some fireplaces). A telltale sign of a standing pilot system is a gas valve with a “pilot” setting; the other ignitor types work with gas valves that just have on/off switches or dials.

Standing pilots need to be lit manually, usually with a battery-powered spark mechanism (like a grill) or a handheld match, lighter, or push-button spark ignitor. You turn the dial on the gas valve to the “pilot” mode and hold it down to allow gas to flow through. Light the pilot while holding the knob down, and then when you let go, the standing pilot will keep a very small pilot flame lit all the time as long as there is a gas supply to the assembly. 

Instead of having a separate flame rod to prove flame, standing pilots have a thermocouple or thermopile immersed in the pilot flame. When the flame makes contact with the surface, it generates a small voltage (in the millivolt scale) that both proves flame and powers the gas valve; it essentially locks in the circuit.

Since the voltage is so small, these systems can be a bit finicky. Blocked orifices, corrosion, and improperly adjusted thermopiles can all prevent the circuit from locking in. Connections need to stay clean and tight, and it’s important to make sure the thermopile or thermocouple can be immersed in the pilot flame. Additionally, while you can clean flame rods with steel wool, you don’t want to use anything even remotely abrasive on a standing pilot’s thermopile or thermocouple; if you must clean it, use a microfiber cloth. 

Direct Spark (DS)

Direct spark ignitors light the burner directly and are controlled by a module (e.g., the White-Rodgers 50D50U-843 All-Spark module by Copeland). While they are more common than standing pilots in gas furnaces, they’re still not as common as the other two types we have yet to cover. For example, Rheem/Ruud manufactures furnaces that use direct spark ignition.

Direct spark ignition, as the name suggests, creates a small arc, which lights the burner when gas flows out from the gas valve. Flame must be proven via a flame rod (or flame sensor), which sends a small DC voltage to the control board. Unlike a thermopile or thermocouple, the flame rod does NOT generate electricity. It just acts as a conductor to ground and gives the current somewhere to go.

The flame rod must be fully immersed in the flame to make good contact with it and send a signal to the board. Dirty or poorly positioned flame rods can cause the furnace not to start up. 

Intermittent Spark Ignition (ISI)

Intermittent spark ignition (ISI) shares similarities with both direct spark ignition and a standing pilot. Like standing pilots, intermittent spark ignitors (ISIs) light a pilot rather than the main burner; they also resemble a direct spark ignitor because they are controlled by a module and use a spark to provide the heat source. (Copeland’s All-Spark module works for ISI, too.) These are also more common than direct spark ignition systems, and we see them quite often on pool heaters in our market.

It’s useful to think of an intermittent pilot as a standing pilot that responds to heating calls. Like standing pilots, flame is proven at the pilot rather than the burner. An intermittent spark ignitor has a small tube from the gas valve, which provides a little bit of gas to the pilot assembly when there’s a call for heat. The spark lights the pilot assembly (just like direct spark ignition), flame is proven, and then the gas valve opens after the furnace already has a working heat source.

Intermittent spark ignitors may be a bit more reliable than their direct spark cousins, as it’s easier to light the burners from a small flame rather than a spark. It’s just easier for the burner to acquire its heat source from an existing flame than from a mere spark. 

Hot Surface Ignition (HSI)

The hot surface ignitor (HSI) is the most common ignitor type in new furnaces nowadays. Instead of using a spark and/or pilot assembly, an HSI has a metal piece that heats up enough to light the gas in the burner assembly. Like direct and intermittent spark ignition, a module controls the HSI. White-Rodgers has a universal module specifically for hot surface ignition: the 50E47U-843.

The ignitor is powered by a known voltage source. Depending on the application, the ignitor will usually be powered by 24, 120, or 240V power, with 120V being the most common for furnaces. The ignitor itself can come in one of two materials: silicon nitride or silicon carbide (pictured below).

Since it takes some time for the surface to heat up, there is a time delay associated with HSIs. This time delay usually lasts 30–60 seconds between the time the ignition sequence starts and the time the gas valve opens. As with direct spark ignition, there is no pilot assembly, so a flame rod proves flame at the burner.

Nowadays, hot surface ignition tends to be the most common in new furnaces, but the other ignition types are still around in the field. That’s especially true if your company also works on gas pool heaters or water heaters like ours. Just like any other part, the ignitor can be replaced if it fails, and there are OEM and universal replacements available.