Before you beat me up too bad, I'm pretty new to the service side of things and typically only work the construction side so please bear with me. I installed this UFT-80W this summer for a 2500 sq ft, 2 zone radiant floor in a bank and started it up, checked combustion, etc. All was well. I went back when it first got cold just to check on it, it was performing perfect and combustion was still spot on. Fast forward a month, they call and say the floor is cold. I drive up there and there is an error 11, ignition failed 5 times. Gas pressure is 11", intake and exhaust are clear, ignitor tests good and is clean, 1/8" gap, gas valve is magnetized as it should, everything the manual says checks out but it WILL NOT fire. Unplugged the unit and went to lunch. Came back, same thing. 5 or 6 cycles, no fire. Spark is clear as a bell but no fire. I removed the crossover tube from gas valve to mixer and it was clear, removed the mixer and it was clear, put the mixer back on and started a new cycle, sucking indoor air into the mixer through the gas inlet port. Once I assured the gas valve was opening, I replaced the crossover tube and it fired up immediately. Watched it for a bit then shut it down and waited an hour and started a new cycle from cold. Worked like a charm. Okay, fluke deal. Clear error history and go home. We had a warm week and this is coupled with forced air so they likely didn't notice, but a week later (yesterday) I get another call saying they had another error 11 on the screen. Drove back up there yesterday after work to find the same story. Everything checked out but it would not fire. Removed the crossover tube again just for grins and let it suck indoor air for a few seconds, tightened the nuts back up and away it went. Error history showed just a single er11 since cleared the previous week. I spent 2 hours on hold with tech support while there and nobody ever picked up so I left a message, but I just now called the bank and she says it's working fine today. Again, I'm no service tech and I'm still learning these UFT units so it may be something painfully obvious but my little brain is at a loss. Some sort of airlock in the gas feed tube from valve to mixer? Doesn't make sense that it wouldn't get gas until that nut is loosened. Anyone have some expertise to offer on the subject? Please?