Biomass boilers require little maintenance, but more than that of their gas and oil counterparts. There are a few things that must be done periodically in order to maximise the efficiency and life span of the installation:

Boilers should be cleaned regularly (ideally weekly) in order to keep it free of ash, though some more expensive systems are self-cleaning. Non-self-cleaning boilers will need to be shut down while cleaning takes place. The burner should ideally be checked daily and any ash and residue cleaned and removed.
When undertaking any internal inspection please ensure the boiler is switched off at the mains and has cooled sufficiently to allow safe access. Also be aware of sprung components and clean with caution.
Inspection of the feed system will have to be undertaken weekly to check and oil any barings and agitate any pellets or wood chips away from parts of the feed system that could be prone to blockage or unwanted build-up and compression
The heat exchanger and flue will also need to be cleaned in order to prevent blockages caused by soot and any other debris.
Stoves will require de-ashing after every operation but to enable the optimum efficiency of the appliance you must leave a bed of embers on which you lay the next fire.
Log boilers (if not automatic de-ashing) will require the same daily clean and will also require a bed of ashes left for the next burn, be careful to check all air vents during this process and clean the fan thoroughly once every quarter or in line with manufacturers recommendations.
It is recommended that all biomass boilers be checked annually by a qualified technician. The technician will:
Undertake a full internal clean and inspection
Replace all worn components (grates etc.)
Lubricate any applicable moving components
For appliances that automatically clean the flue a small compressor is installed with jets of air that are sent through the flue blowing the soot from the boiler. This method in more costly but increases maintenance intervals dramatically.
A well installed and maintained biomass boiler should last for well over 20 years.