Firstly I don't install boilers, I'm not a plumber, I commission, service and repair, hopefully anyway. Was called out to a Firebird Enviromax Condensing Boiler after it had sooted up. Boiler is about 3 or 4 years old. Cleaned it out, changed nozzle, checked oil pressure. Still burning black. It's a low level balance flue with a vertical flue kit fitted as it was getting blow back down the flue. What I did notice was the 2 torque screws that screw down tight onto the rubber gasket were lose. The problem is only 1 will tighten and you can actually lift it out. I'm thinking this might be the problem of the sooting up and how do you get around it. What does the actual m6 bolt screw down into or has it rung itself , I'm not sure.