Hi helpers, I'm an apprentice plumber and have a lot to do with rheem stellars, i replaced a thermocouple today on one and unit lit fine and pilot stayed on, however the customer rang and complained and said I hadn't fixed it, I have sine gone back and discoved black soot all through the hot water service and out the flue. This is now the second unit that this situation has occurred on and am wondering what's happening and how I can solve it. Any help would be greatly appreciated

Your not getting enough air mixing with the gas and that's what causes the black soot. Typically this occurs when you're using a natural gas system on propane or butane. If this is the case you need to convert it to the proper propane orifices (jets) and open the air intake. If the unit has not been converted the problem still remains the same, you're not getting enough air mixing with the gas. You have an obstruction, which can be very small, inside the air intake or mixing tube or the orifice has been opened up to large and is allowing too much gas into the system and the maximum air opening isn't enough to compensate for the extra gas. Cobwebs are a common occurance inside a mixing tube, check for that after you've verified the orifice is the correct one for the type of gas that's being used.