Maybe it was never set properly, or shifted in shipping,,,,I've seen some weird stuff on some furns,,,,one that comes to mind, had a furn that one burner would only light intermittently 4 of us were over there, I checked for level on the furn,,,,wasn't level? Go figure,,,,why would the thermocouple change with the bottle size cmp ? i almost think the valve on the 100lb tank is bad not getting enough flow through it