In an earlier contribution to this discussion,
dbroms wrote:

my system has just been converted by installing a diverter valve to
hopefully allow me to select heating OR ho****er.
I have had fitted a worcester bosch condensing (non combi hence
system) boiler. The pump has a tee piece straight after it, one pipe
goes to an open pipe just above the water line in a header tank in
the loft and the other goes to AB connection on the 3 way valve.
It is definately vented , no fill loop or pressure vessels.

do u require anymore info?

AIUI, not being a combi doesn't automatically make it a 'system' boiler -
'conventional' may be a better word.

So there's a vent pipe connected *after* the pump, which loops over the top
of the F&E tank. There must also be a feed pipe running from the bottom of
the F&E tank. Where does that connect into the boiler flow pipework?

You have pretty certainly got a case of pump-over - where water is being
pumped up the vent pipe into the F&E tank - to be replaced by water flowing
down the fill pipe, and back into the system. This is *bad*. Not only is it
wasting heat, it is also introducing trapped air into the system - which
will cause corrosion.

If your pump has several speeds, you could try slowing it down a bit - to
see whether that cures the pump-over whilst still heating the radiators
adequately. Otherwise, some plumbing mods will be needed. The fill and vent
pipe connections into the main flow pipe should both be on the boiler side
of the pump rather than after the pump, and they should be no more than 100
mm apart - so that they both experience essentially the same dynamic
pressure - making it impossible for water to circulate between them.
--
Cheers,
Set Square
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