I have a thirty year old forced air furnace which uses a White-Rodgers 36C84 gas valve and a mercury flame sensor. I had a variety of problems before last winter which I successfully solved. I'll give a quick overview before I get to my current problem:
I replaced the mercury flame sensor due to a broken capillary tube, but found that the furnace worked erratically--pilot would start up fine, but furnace would sometimes drop out when transitioning to the main burner, and start the pilot cycle all over again.
Furnace repair guy discovered that wiggling the mercury flame sensor where it electrically plugs into the valve caused the valve to operate correctly, and diagnosed a bad valve, which he declined to replace, and suggested that I do it myself. I diagnosed a poor connection between the infamous White-Rodgers resistor board and the flame sensor pins. I took care of that and the furnace worked fine all winter.
Fast forward to this winter: Pilot valve intermittent, but a slight tap with a hammer cause furnace to act normally. Thinking the furnace repair guy might have been on to something, I order a new 36C84 gas valve. While I wait for the arrival of the new valve and the time to install it, the furnace exhibits a new problem: Starts and runs fine, but when pilot valve is energized, it will not move unless about 20 minutes have passed since last shutdown.
I have now installed the new valve without the resistor board as White-Rodgers says the new valve (36C84-921) does not need the resistor. Same problem as before, but here's some interesting parts:
1. If I call for heat and then shut down before the fan starts, it can successfully restart.
2. If I call for heat and let the fan start, and then shut down and again call for heat, the pilot will energize, but not move.
3. BUT, if I unplug the mercury flame sensor, and wait a second for an audible click, pilot will operate normally when I call for heat.
Other random clues or red herrings:
A. ignitor seems to click about once every 10-20 seconds even when thermostat is
B. I swapped in a honeywell ignitor board for the W-R 5059, and saw quicker recovery when unplugging the flame sensor, but still not normal operation.
C. When troubleshooting before I bought the valve, I soldered the resistor board to the flame sensor pins. I since pulled the resistor from the board.
D. Local utility installed a new gas meter at the house mid summer.
So, what do the experts think?