Viessmann VITODENS Operating Instructions Manual page 24

Repairs
Inspection and maintenance
Safety valve (DHW cylinder)
Check the safety valve function every
six months by venting, or have it
checked by your heating contractor.
There is a risk of the valve seat
becoming contaminated (see the
valve manufacturer's instructions).
DHW filter (if installed)
For reasons of hygiene
&
on non-back-flushing filters, replace
the filter element every six months
(visual inspection every two
months),
&
on back-flushing filters, back-flush
every two months.
24
(cont.)