What is flame rectification?

Flame rectification is DC electrical circuit that sends a very small voltage (rated in micro amps) from the furnace electronic board up through the pilot electrode. When pilot flame is established the flame acts as a path (conductor) which allows the DC current to flow from the flame rod to ground completing the circuit. This completed signal tells the electronic board it's ok to allow the main gas valve to open . Now your furnace pilot, or hot surface ignitor will start the main flame . To put it simple, flame rectification is an electronic way to prove pilot flame before opening the main burner gas valve. It's a safety circuit to prevent the main burner gas valve from opening if there isn't apilot flame, or hot surface ignitor established.