Except that Beetle said in his initial post that he'd just cleaned the snot out of the whole venting system, and cleaned the convection blower. In my case I've done the first repeatedly, but not the second (I have the gasket on hand, but was hoping to get to the end of the season before I tackled it). He's put less fuel through the furnace than I have, and I've put less through than the dealer I bought the gasket from, who said he'd never pulled his in two seasons running and never had this issue. Besides, the blower runs flat out at the higher heat levels. Given that the volume of air coming in is always the same, if anything I'd think that would help the vacuum situation at higher levels, not hinder it. In my case, since I mostly dump out when it's in the low teens or below outside AND I had more vertical run outside than was necessary I was heading for flue gases getting too cold and hindering venting. I've shortened my outside run but won't know if that worked until it gets cold again - we're in January thaw this week and I'm back on the stat (where it's been running pretty constantly in low, except for a couple of hours in the middle of the night). And from his description Beetle doesn't have that issue, anyway. In the absence of an actual spec from Even Temp as to how much venting the blower is equipped to handle it's impossible to know what's right there. From my days of reporting repetitive failures to engineering and trying to guess what the issue might be, I suspect the vacuum sensing system as it's set up is marginal, as is the volume of air moved by the convection blower, and in real world cold weather situations the combination can cause early triggering they might have never seen testing in balmy Nebraska. I'll be curious to see what Beetle's dealer's tech comes up with when he visits, though.