To some extent it is a trial and error thing, but a simple way is to
close all the TRVs and then adjust the bypass valve so that it opens.

Yes, but how do I know when it's opened? There's no indicator on the Pegler
one.


You shoulld have some for of room stat anyway and in a space where
there is not a TRV on the radiator or that it is open fully. On a
simple boiler, this is used to interlock it off when the house is up
to temperature. You need to arrange things so that the rooms with
TRVs are warm and valves are closing before the thermostat operates.

I thought the boiler would just switch off when the flow/return temps were
very close, but I guess this would stop it cycling every time the water in
the pipes cooled a bit. I've got the cable in place for a room stat soI
think I will fit one as well.