In an earlier contribution to this discussion,
dbroms wrote:

System boilers do not have header tanks

mine has definately got a header tank. The valve is a honeywell 3
position diverter valve. The tank has a built in header tank that
supplies cold water to be heated up and the other header is in the
loft which tops up the sealed heated water that flows through the
coil in the tank unless i have it totally wrong.

Well, something is definitely not right!

Your primary circulation system is either vented or unvented (sealed).

If unvented, there is no header tank - by definition - but there is a
pressure vessel instead, to allow for expansion, and a filling loop to allow
the system to be initially filled with water.

If vented, there is a fill & expansion tank - but no pressure vessel or
filling loop.

The term 'system boiler' is usually associated with a sealed/pressurised
system - which has an expansion vessel - and often a pump also - inside the
boiler casing.

So a slightly more detailed description of your system would be helpful.
--
Cheers,
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