How far should the pilot flame engulf the thermocouple?

How far should the pilot flame engulf the thermocouple?

• A. 1/4 inch
• B. 1 inch
• C. 2 inches
• D. 1/2 inch

Answer: (D)

The heat from the pilot flame must reach the thermocouple tip to keep the gas valve open, so the flame should surround enough of the tip to heat it reliably. About half an inch of the thermocouple tip engulfed by the flame ensures it sits in the hottest, most stable part of the flame, giving a steady millivolt signal that keeps the valve energized. If the flame covers less than that, the thermocouple may not heat enough and the pilot could go out; if it covers more than that, it can overheat the tip and shorten its life. So engulfing about 1/2 inch is the standard, reliable distance.

The heat from the pilot flame must reach the thermocouple tip to keep the gas valve open, so the flame should surround enough of the tip to heat it reliably. About half an inch of the thermocouple tip engulfed by the flame ensures it sits in the hottest, most stable part of the flame, giving a steady millivolt signal that keeps the valve energized. If the flame covers less than that, the thermocouple may not heat enough and the pilot could go out; if it covers more than that, it can overheat the tip and shorten its life. So engulfing about 1/2 inch is the standard, reliable distance.