Clermont, Florida, USA, 21 February 2018: One of the essential practices of preventing heat exchangers from exploding is to ensure that the flame exists as part of its gas-firing sequence, without which there is a risk of gas being unloaded into the heat exchanger and causing an explosion, said Bryan Orr, Co-founder, Owner, Kalos Services. Orr shared his thoughts on this subject in a blog post on hvacrschool.com.

Explaining the flame-sensing process, Orr said: “For flame detection, flame-sensing rods are stuck into the flame and connected to the furnace board. Once the board sends a signal to the gas valve to open, it monitors the current flow on the flame-sensing rod. It does this by generating an alternating current potential (AC voltage) at the flame-sensing terminal, which is connected to the sensor with a conductor.”

He further said that when no flame exists, there would be AC voltage at the rod and no current (amps), and if there were a flame, a small micro amp direct current (DC) would be present, as a path is made between the rod and the ions in the flame. “This small DC signals to the board that the flame exists,” Orr said. “If the board does not sense this micro amp DC within a few seconds, it will shut off the gas valve and try again.”

He further said that the board outputs this flame-sensing terminal right at the beginning of the sequence to confirm that the path is ‘open’ with no flame.

Elaborating on the safety of flame sensors, Orr added that flame sensors fail when they short out due to a cracked insulator, or if they are broken, or if they are not correctly placed in the flame, or get coated in silica (glass) or carbon.

Explaining some of the methods to test a flame sensor, Orr said, it must be ensured that the furnace is properly grounded while checking if the polarity is reading correctly, and the rod positioned in a way that it would be covered in the flame. He further suggested that the meter must read in the microamp scale with a 0.10 resolution.