In article , Fred wrote:
Does the pressure gauge on your system fluctuate in time
with the swish?, mine does. I have the same boiler and
have been fighting an instability in the flow since
installation. My thoughts so far are that the flow from the
pump is not as smooth as it could be - pulsating somewhat.

We had this on our church Celsius boilers installed August
2001. A Keston engineer came down to fix something else under
warranty and noted this, and changed the PCB's to a later
model (no charge). I'm not sure of the exact technicalities
but the system picks up that the pump is running too fast or
slow and corrects this. It then decides that the new speed is
too slow or fast so goes back to the original. And so on ....

--
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