Cause known, working on solution
Thanks for all of the advice. I know why it is cycling, but need to work with my buddy to try to get it solved.
I borrowed a logging multimeter from work, to check the switches. [It records min and max voltages, for up to 36 hours.]
I put this on the IBS [Inducer Blower Switch] overnight. The switch is in-line [electrically in series] with the inducer blower motor. This switch is a thermal, normally closed, and goes open at 140 F. When normally closed, the switch has under a volt AC on it. In the morning though, the meter showed that the switch had 120V AC on it at some point, proving that it went open.
From what loonie and aircon said, I think the problem is not enough angle on the outlet flue. He worked things around well in a tight furnace room to use the existing hole in the wall, but the furnace is only 12 feet from the wall, and the run takes a jog around things, for 18 feet total, plus 6 elbows [ 4- 45's and 2 90's ] into a 2 inch concentric vent kit. There is a 6 foot section that runs pretty much level. So, I plan to ask him to redo the outlet flue, putting more angle back to the furnace.
Per 'beenthere's question, there is no drip tee.
From what you all have suggested, and from reading the install guide, it says condensate collecting in the flue will block exhaust flow. This would explain why sometimes it cycles at 12 minutes, sometimes closer to 20, sometimes not for much longer. It also explains why Bernie wouldn't have seen this, because he drops by late afternoon, when the humidity in the air is lower. The worst cycling occurs in early morning, on recovery from setback, and when early morning moisture is high. But a question, why can't the inducer blower overcome any water vapour that the condensate creates though????
Thanks again, JP