Oil Boiler Maintenance/Cleaning

[Shallot]

Our 8-unit co-op uses an oil boiler for heating.  While we've had maintenance performed on it regularly, that apparently did not include cleaning of the boiler and the vents/pipes that conduct air from it.  Our regular service provider does not clean those vents and we are looking for someone who could do that.

Does anyone have a boiler cleaner that they'd recommend?  (Or recommend we stay away from?)

Thanks!