In article , Andy
Hall wrote:

One can argue that pulling the boiler apart every year is not
necessary - it depends on what the manufacturer recommends.

A Keston engineer (off the record) said that the best thing to
do with their boilers if working OK and giving the correct CO
was to leave well alone regardless of what the book says.
Downfiring condensing boilers are basically self cleansing by
design though. My (upfiring) Potterton Envoy collects all sorts
of stuff in the burner, but whether this would show up on an
exhaust gas analysis, I don't know.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser
http://www.sda.co.uk/qsedbuk.htm