I didn't realize all these posts were going on today, for some reason, I got no notice of them. I'm sorry this has turned into a bit of a brawl but if I felt there was something beyond what is reasonable for my to personally check, I would stop. After being enlightened about checking for the motor spinning, that led us to a whole new host of options.
Bob - we don't actually have a chimney. Wasn't even an option when we built, this dumb PV was used in its place. As I understand it, any wind the wrong way with enough force can shut them down. Thankfully neither ours, nor my parents (who built a house at the same time, different builder and no chimney have had any problems). The manual for my PV gives the sequence of events on the O terminal, then "reading" the draft through the tube and activating the Y terminal by closing the prover switch. That being said, there are a couple things that can cause the prover switch not to close thus powering the burner motor. As noted in a troubleshooting section of the maintenance guide provided by the manufacturer (so they must not think it's too dangerous for Joe regular homeowner to try) was to check the prover switch for continuity when there was a call for heat. I have not yet been able to test that when it goes into lockout mode but will do that after the boiler cools down and hopefully goes to lockout (or doesn't) on next firing. I had thought about "removing" the switch from the circuit by jumpering the black and orange off the primary controller so the PV isn't intercepting the signal and running through the PV first, but we may not need to go to that extreme. I called Tjernlund today and they were helpful. He said it absolutely sounds like the switch is going fluky and asked if I knew the troubleshooting tips as far as checking for 115v on the terminals during a call, checking the draft, checking the tube, etc. I said we aren't getting 115v on the Y terminal when it gets locked out because obviously, the switch isn't closing to complete the circuit. This evening we have removed the draft tube (it was terribly gunked up on the flue end) so that we cleaned it out well with a piece of weedwhacker line. Then (at the recommendation of the Tjernlund rep) we took out the PV motor and fan set and cleaned that up well. Then we put it all back together. It made a call and fired but it had been warm so.... we also ensured while we were cleaning out the motor area in the flue chamber, we ran a shop vac up through the dampener flap. When the PV came on, I can tell it's pulling more air just by how far the dampener flapper is flipping in. So at this point, I'm waiting for cool down to force a call and see what happens. If all is right with the world, then as Tjernlund noted, the tube was likely plugged up enough to cause eratic behavior. If we do not read continuity on the prover switch during a call and 45 seconds of not firing when it should, then I will be replacing the switch tomorrow as I've already called a local supply shot and they have one in stock.