On Sat, 16 Mar 2019 14:15:31 +0000, Davidm wrote:

In step 3 para 6 it refers to "Gradually open the primary flow to the
hot water cylinder again". This implies that there should be some sort
of flow control valve fitted to the HW circuit, which my system has
never had, so when HW is called for a lot of the flow will be passing
through this loop.
Could this be why my boiler sometimes trips out with an overheat error?

No, you dont need a flow control valve. You should have a bypass valve,
or, one of the radiators without any TRV control.
Thats so if all radiators have reached their set temperature, the TRV
shuts, stopping flow through that raditor. If all rads have TRVs, and all
are shut, there is nowhere to go for the heat output from the boiler.

The boiler will only come up with an overheat fault if there isnt enough
flow to keep the heat exchanger output below the recommended temperature
(70 degrees?), or, there is a fault with the boiler, or a restriction
somewhere.