I am having some issues with my Bowen G610-3B water heater. When ignited, the flame shoots out of the inlet of the u-tube, rather than through the inlet and out the exhaust end. I have tried to push a wire through, to clear any possible obstructions, but the tube is not cylindrical all the way through. I was able to negotiate he wire through once, but there was certainly not enough room to "tie a rag onto it, and pull it through to clear the soot". Has my u-tube been damaged, or is the a part of the design? I hope the pictures will help.