The function of the flame-sensing rod is to tell the controller that the main gas burners have ignited. If no flame is present after a certain amount of time, the controller needs to take the appropriate actions, first of which is to close the gas valves to the main burners.
The controller applies alternating voltage between the flame-sensing rod and the base of the flame (ground). The ions in the flame provide a high resistance current path between the two. Because the surface of the base flame is larger than the flame-sensing rod, more electrons flow in one direction than the other, resulting in a very small dc offset current. If a flame is present, the dc offset is detected by the controller, which tells the gas valve to remain open. If there is no current flow, the controller will close the gas valve and the system will purge itself of any remnant gas before trying to reignite or lockout.
It''s very important that the flame-sensing rod works properly. Dirt, corrosion, or bad connections in the flame-sensing circuit can cause the controller to think the flame didn''t ignite, and the gas valve will be shut down prematurely.
Measure the current in the flame-sensing rod by placing a meter in series with the rod. The meter must be capable of measuring between 1-10 microamps dc with a measurement resolution of 0.1 microamps.
Before connecting the meter to the flame-sensing circuit, determine if the system uses a separate flame-sensing rod or uses the hot surface igniter as both the igniter and sensing rod. Typically, in hot surface ignition systems, the flame-sensing rod is separate from the hot surface igniter, in which case it''s easy to connect to the circuit.
For ignition systems where the flame rod and igniter are separate, any meter with microamps can be connected in series between the controller and the flame-sensing rod. When the flame is on, there should be a measurable microamp dc signal, typically below 10 microA. If the microamp dc signal is too low, the furnace will perceive this as a "no-flame situation" and the gas valve will close. Here''s what to check:
- Make sure there is ac voltage between the flame diode and the base of the flame. Measure ac voltage from the flame-sensing rod to the base of the flame. If there is no voltage, check to see that the wires from the module to the flame-sensing rod are correctly connected, or if the flame-sensing rod or the wire connected to it is grounded.
- If voltage is present, make sure the flame rod and burner are clean and free of dust, dirt, or debris. If they are not, take them out, clean them, and reinstall. Cleaning the flame rod and burner will remove impedance from the flame current circuit and thus should increase flame current.
- If the current is still too low, check to make sure the connections from the controller to the flame diode and the connection from the controller ground to the flame rod ground is low (it should be well under 100 ohms).
- If all else fails, replace the flame-sensing rod. Now you should be able to measure the correct dc offset current.
- If the heater is still not operating correctly, the problem may lie in the controller.
However, when these systems use the same hot surface igniter for sensing and ignition, you may put yourself and your equipment in danger. During the ignition part of the startup cycle there are many amps going through the hot surface igniter. Only after the ignition cycle is finished will the hot surface igniter perform the function of a flame-sensing rod.