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I have a gas furnace that uses an intermittent pilot/ignition to light the pilot. The furnace itself is having problems starting the main burners, so I took a look. I found the following:

  1. When I turn on the furnace, the igniter lights the pilot flame, and the igniter eventually stops after 5-10 seconds, i.e., the flame rectification system appears to be working fine. However, the main burners never start. Measuring the (AC) voltage between the main burner output (MV) and MV/PV as well as ground on the ignition control module, it appears to be nearly 0 V. Does this sound like my ignition control went bad, even if it is able to detect the pilot flame and shut off the igniter?

  2. As a test, I unplugged the flame sensor at some point and turned on the system. I wasn’t surprised to see the igniter sparking even after the pilot flame was lit. I was, however, surprised to see the igniter never stop sparking (at least for 5 minutes), while the pilot flame continued to burn. Is this normal? It seems like the igniter and gas should have been shut off by the ignition control after at most a minute or two.

If it helps, the ignition control module’s make/model is Robertshaw SP715A. The furnace is a Williams Forsaire Counterflow Furnace model #8508332.

Thanks!

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If the flame sensor is disconnected it will keep sparking thinking there is no flame. I am not sure if that unit has a proving time limit or not. Many of the Honeywell models lock out after either 15 or 90 seconds and stay locked out until power or thermostat is interrupted or 5 minutes. As fo the main gas valve it may be a few things. Check the grounding. Check for continuity throughout the entire grounding path. Check for 24 volts between MV/PV and PV. I would expect voltage there since you have a pilot light. Check between MV/PV and MV. If there is power there but the gas valve does not open check the same terminals at the gas valve. Still power then bad gas valve. No power there and good ground then most likely a bad ignition module.

  • Hi @user76730 - I actually ended up replacing the furnace control board/ignition module last night, and everything started working again :). To answer your questions, I had previously seen 24V between PV and MV/PV (as expected). I hadn't checked the continuity of the grounding path, but the lack of power between MV and MV/PV was enough to take a risk and replace the module. I replaced it with a Honeywell model, and everything works beautifully. I also tested what happens when the flame sensor is disconnected, and this module shuts off the gas/pilot as expected. Thanks for the reply! – bacchuswng Jan 13 '18 at 19:45
  • Glad it all worked out for you. – user76730 Jan 13 '18 at 19:50

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