Flame sensors and ignition controls


What is a flame sensor? It may also be known as a flame rectification probe in the HVAC industry. They are found on boilers, furnaces, hot water tanks and just about any gas burning appliance where flame proving is required. The simple act of the flame making contact with this probe after the appliance ignites rectifies the heat of the flame into voltage. This is then sensed by the ignition control board and the main flame is then allowed to stay on until the heat demand is satisfied. This flame rectification probe is just one of many safety devices incorporated into today’s modern heating equipment meant to prove the existence of flame in the ignition sequence.

Should one clean a flame sensor? Typically no, and here is why. Most gas heating technicians tend to clean the sensor with emery cloth to remove the oxidization from the probe. And while this may temporarily rectify the situation, in all honesty the flame sensor needs to be replaced. In an emergency situation one can clean the sensor with a piece of leather, say a belt which tends to remove the oxidization and does not remove too much of the flame sensor’s metal. Removing too much of the metal (thinning or the probe) can result in a reduced rectified voltage being sensed by the ignition control module or board resulting in repeat failures.
The best course of action of course, is to replace the flame sensor or flame rectification probe to reduce the chance of a callback. And while this is not always possible when one does not carry flame rectification probes as stock in the service vehicles, temporarily cleaning the probe with a piece of leather may get you through until a new flame probe is found. Never assume that merely cleaning the flame probe is the ultimate fix as it is merely a temporary repair.
Flame sensors evolved from the old thermocouple or powerpiles, most commonly used on heating equipment in the past. A bad flame sensor may cause main flame shut down a few seconds after the main flame ignites. This is not always caused by a failed flame sensor and could well be a different issue altogether. Troubleshooting this is best left to a heating service technician.

The picture shows a Rauschert Flame Rod and a Fenwal Ignition control module from a Trinity series boiler.
Alberta General Plumbing



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