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Hello and thanks for using this service, if you have been waiting do not worry or get upset I am just one of the techs on here and I try to get to as many customers as I can and I just picked up your question now. Please give me time to respond sometimes I get your replies late so there may be a significant delay in responses. Please be patient and often I will ask you questions to narrow down the issue to get is diagnosed correctly.
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Sorry for the delayed response. It sounds like the heat exchanger (the area above the burners probably cast iron) is clogged. The jacket and draft hood will need to be removed as well as the burners (which should also be cleaned) and the inside of the boiler brushed and vacuumed out. This is potentially a hazardous condition and you really shouldn't use the boiler until you can get this professionally cleaned. Gas boilers should never produce soot. It's an indication that there's not enough air for combustion. This could be a matter of opening the shutters on the ends of the burners or you may need to get some fresh air into the boiler room