[mebaskin]

Hello

I own a vintage Bryant #394GAD036100 furnace that is working just fine accept for one little problem. The unit cycles and maintains the desired temp setpoint (heating mode) but after the main recirc blower shuts off it restarts momentarily one time approx 2 sec later,
And again approx 2 sec later.
I currently have an electrostatic furnace filter and a humidifier(media wheel type external) hooked up to the auxiliary pilot contacts. Originally convinced that I was dealing with a coasting blower wheel back EMF issue,I applied 130v metal oxide varistors at various places about the control circuit board(across relay contacts,txfmr etc.),still no relief.I then disconnected the aux circuit loads (humidifier and EP filter) same problem.
I am about ready to drag out my oscilloscope and monitor the PCB for any stray spike activity. I believe that the onboard operational amplifier is simply being used as post gas valve shutoff time delay.
Does any of this make any sense,and has anyone else ever experienced this problem previously?
Thank you and I look forward to any suggestions or help. If I had a schematic on the PCB I could fix it in a snap.

Mark

[labman]

Could be a dirty or bad upper limit switch.

[mebaskin]

A defective n.c. motor relay was the culprit.The contacts had eroded so badly that arcing noise was being close coupled back into the diff comparator delay circuit.
I removed the 13vdc coil relay and replaced it with a PC mount spdt 12vdc hi sensitive coil relay.The new relay is simply used as a buffer logic inverter that then feeds an external 20A 3-32vdc input solid state relay.Works great!
Nice thing about the SSR is that it is zero voltage turnon and zero current turnoff.No more nasty mid sine wave arcing.
Please let me know if anyone is interested in this solid state conversion.No more arcing or burnt out contacts.