It's a large, old, poorly insulated house, heated by two American Standard 105,000 BTU 80% efficient approx 10-year old furnaces. They are twinned together and I refer to them as F1 and F2.
Several days ago I noticed the furnace was running for an extended period of time (20-30 minutes) without reaching the thermostat setpoint. Upon investigation I found F2 control board flashing 3x - indicating a pressure switch issue. The good thing about having two identical units is that it allows swapping of parts for troubleshooting.
I swapped the Honeywell pressure switch - no change, then the induced limit switch - no change, and ultimately the inducer. When I swapped the inducer the error code moved with it from F2 to F1.
Initial diagnosis: inducer needs to be replaced. I ordered a replacement which should come in a couple of days.
BUT... this morning the unit was running forever again - and I find no flame in F2 and the control board flashing 3x. What???
I should also note that this is an INTERMITTENT problem. The error code will appear occasionally, and the unit with the error will have no flame. Even with the error code and no flame, it sounds like the inducer is running (though a little difficult to tell).
Each unit is piped separately, so it's not a clogged pipe. It's not logical that there would be an intermittent failure of BOTH control boards, or BOTH inducers at the same time.
What else could it possibly be?
Any/all suggestions/advice greatly appreciated!!!!
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