How do you clean a boiler?

Boilers can be quite complex and it may be desirable to find a trained professional, at least for the first time maintenance is done on the boiler so the process and specific stems used for the boiler can be watched.

But if this option is not chosen, the following was stated in the Routine servicing section of the a Baxi Luna HT 330 manual (this is a modulating fully-condensing boiler, but the advice is probably applicable to most boilers): after turning off the gas and opening up the boiler, "chuck for any dirt in the combustion chamber. Use a vacuum-cleaner for this cleaning operation", "check that there is no dirt in the siphon", and "check that the gas valve is calibrated correctly".

Further information about boiler cleaning is desired.