My hot water heating boiler is covered with heavy coating or black soot inside the boiler casing. I would like to know what is causing all this soot. There is hardly any soot built up in the exhaust flue.

it is a natural gas, propane or oil burner? If oil burner, this is just the way it is, they are messy, if natural gas or propane, you have incomplete combustion. Let me know and i will try to help.

Running the boiler to cold or having to cold a return water temperature will cause the boiler itself to accumulate the soot.