Learn & Understand

Standing, Intermittent, or Interrupted: The Three Kinds of Pilot Flame

The companion calculator sizes a pilot flame as a small percentage of the main burner's capacity. That sizing is only part of the story, because pilots come in three fundamentally different types, distinguished by when they are lit, and the differences carry real consequences for fuel waste and safety. The industry has been steadily moving away from one of them, and increasingly away from pilots altogether.

The Pilot's Job

A pilot is a small flame whose purpose is to reliably ignite the main burner and, in many systems, to be the proven flame that flame-supervision equipment watches. It needs to be big enough to light the main burner dependably and to be sensed by the flame detector, but no bigger, since every bit of pilot fuel beyond that is waste. That is why pilots are sized as a small percentage of main capacity. The question of type is about timing: when is the pilot on?

The Three Types

A standing pilot never goes out, burning fuel around the clock whether or not the equipment is producing heat. An intermittent pilot is lit at each startup and remains on for as long as the main flame runs, then goes out when the burner shuts down. An interrupted pilot goes further: it lights the main burner and then is deliberately turned off once the main flame is established and proven on its own, so the pilot burns only during the brief ignition period.

Why Interrupted Is Considered Safest

The interrupted pilot is generally regarded as the safest arrangement, and the reasoning is subtle but important. Once the main flame is running and being supervised directly, the pilot is no longer needed, and keeping a small pilot flame burning near the main flame could, in a fault, mask a main-flame problem or provide an unintended ignition source. By shutting the pilot off during the main run, the interrupted design ensures the main flame must stand entirely on its own to be proven, if the main flame fails, there is no pilot propping up the flame signal. It also, of course, uses the least pilot fuel. This combination of safety and efficiency is why interrupted pilots are widely preferred in industrial practice.

The Trouble With Standing Pilots

Standing pilots, once common on simple appliances, are increasingly disfavored. They waste fuel continuously, a small flame burning every hour of every day adds up over a year, and they raise safety concerns because a continuously burning open flame is always present, and a pilot that quietly goes out can allow gas to accumulate if not properly supervised. The trend across the industry has been to eliminate standing pilots in favor of ignition only when heat is actually needed.

Beyond Pilots: Direct Ignition

The logical endpoint is doing away with the pilot entirely. Direct spark ignition lights the main burner straight from a spark, no pilot flame at all, and hot-surface ignition uses an electrically heated element to ignite the fuel. These approaches remove the standby pilot fuel completely and simplify the flame path, which is why many modern appliances and burners have abandoned pilots for direct ignition. The pilot-sizing question disappears when there is no pilot.

Using the Pilot-Sizing Figure Well

Take the calculator's pilot input as the small capacity needed to reliably light and prove the main burner, no larger. Then consider the type: an interrupted pilot, lit only for ignition and off during the main run, is generally the safest and most efficient choice, while standing pilots waste fuel continuously and are being phased out. And recognize that many modern systems skip the pilot altogether with direct spark or hot-surface ignition, eliminating standby pilot fuel entirely.