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Old 10-24-2011, 11:40 AM   #1
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Williamson gas furnace cycles frequently after running for several minutes


Hi,
I have questions about a Williamson forced air gas furnace model WU47-12-1D17. I may replace the unit, as itís awfully old, but Iím familiar with/curious about electronics so Iíd like to understand why this is happening.

This fall, it started up fine, but very shortly developed the following problem. It fires up fine and runs for 5-7 minutes, after which the flame shuts off and starts to cycle. Thereafter, it cycles on very short duration, but not consistently. Sometimes just on and off within seconds. Other times, on for a minute then off. When it cycles, the exhaust blower continues to run. Iíve examined or tested the following:
a) Roll-out kill switch is intact (one-time thermolabile resister)
b) Exhaust thermal protection switch is NOT tripping
c) Circulation blower temp limit is NOT tripping (peaks at about 145C)
d) Vacuum switch to a Fenwal control module is NOT tripping
e) Vacuum switch to the exhaust blower control board is NOT tripping.
f) The condensate drain is clear and vacuum hoses are clear. I donít have a sensitive vacuum guage, so I canít tell the actual vacuum drawn, but it seems enough to NOT trip the above switches.
g) I cleaned the flame sensor. When connected, voltage from sensor to ground is about 35V. When the burner is on, this voltage increases to about 85V. Immediately after the burner stops, the voltage jumps to about 90-95V, then returns to 35V. The time response on my voltmeter is not short enough to know whether the voltage on the sensor changes before or after the burner goes out, but the fact that the voltage jumps up momentarily after the flame dies suggests it is still sending an OK signal (Iím guessing the voltage increases briefly as the load drops while the Fenwal module recycles?)

It seems the problem is the Fenwal control module is receiving a fault signal (LED flashes once) and is cutting out the gas, then cycling. The Fenwal module, which controls the gas valve, has the following input/outputs:
1) 120V hot input
2) Ground wire
3) Single wire input from the flame sensor
4) Single wire 120V output to glow igniter (other wire from igniter goes to ground)
5) Single wire 24VAC output to the gas valve (other wire to valve goes to 24VAC ground)
6) Single 24VAC input that I assume is hot when the thermostat calls for heat (24VAC hot from transformer goes to thermostat, and upon returning, splits to the Fenwall module (via vacuum switch d in list above) AND to the exhaust blower control board (via kill switches a, b, c above)

Conclusion: possible faults are:
I) Gas control valve (not likely, but I suppose possible).
II) Fenwal module (but why does it work fine for 5-7 minutes, then start cycling?)
III) Fenwal module is receiving a erroneous fault signal from elsewhere (I assume the exhaust blower control board).

Questions:
1) Am I missing something else?
2) How can I test the conclusions above?
3) On a related note, how does the exhaust blower control board send a fault to the Fenwal module? There is no input from the former into the latter. The only way I can think of is that since the return wire from the thermostat sends 24VAC to both the Fenwal and to the exhaust blower control board, the exhaust blower motor board may be momentarily grounding the potential, which the Fenwal detects as a fault? If so, how should I test this?
4) An HVAC service person suggesting running a separate ground from the mounting plate of the flame sensor to the Fenwal module ground. This makes no sense to me as the flame sensor is isolated by porcelain, and the mounting plate is connected to the chassis - as it the Fenwal. Of course, it's badly rusted, but I cleaned it up and it makes no difference. Has anyone heard of this intervention working?

Sorry for the long-winded details. Fascinating to learn how this works.
Thanks!
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Old 10-24-2011, 11:49 AM   #2
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Williamson gas furnace cycles frequently after running for several minutes


The flame has an electrical signal through it, sensed by the flame sensor. Read in micro amps, a very solidly and well connected ground is not an opinion, it's mandatory for this to work.

All hvac techs on this Earth know this.
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Old 10-25-2011, 09:15 AM   #3
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Williamson gas furnace cycles frequently after running for several minutes


Thanks Doc. I was aware of the microamp current, but not having an ammeter that sensitive, I was told (by the same tech) that the voltage is a reasonable first measure. In any case, tried an independent ground from the flame sensor mounting screw to the Fenwal control module ground. Didn't help. Other thoughts?
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Old 10-25-2011, 04:37 PM   #4
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Williamson gas furnace cycles frequently after running for several minutes


the pressure switch that senses the ID discharge air into the burner might be bouncing an the next heating cycle jump it out if it shuts off with the induced draft running...if it comes on with the quick jump the air is bouncing the contacts(check the tube from the switch to the fan shroud tap for water and within the switch also) causing the main gas to shut off as it should killing 24Vs to the valve...
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