Set Square wrote:

When installed properly, a Baxi Solo PF *definitely* needs to control the
pump in order to provide pump over-run. Imagine the scenario where the
boiler is going flat out, and is suddenly stopped by the room stat. The
metal parts of the boiler are *very* hot, and the water inside the boiler
will get dangerously hot if it doesn't continue to circulate for a while to
carry the residual heat away. If the pump is simply wired in parallel with
the boiler, it will stop when the boiler stops - and the boiler will
overheat. It may not be a problem with your existing installation because
the pump is always on. But when you put a 'proper' control system on it, it
will be!

This sounds a bit scary, I better get a plumber to take a look at it.
Right now the boiler has been running for about an hour. I turned it off
at the timeswitch, and confirmed that the pump definitely goes off. Is
this like, a red alert situation or (merely) something that I should try
to get sorted at some point in the near future ?

When the timeswitch turns the boiler off, it appears to be completely
electrically isolated, so I don't see how it would have a separate mains
supply that would allow it to control the pump in that state. Best thing
to do is get someone in to have a look at the way this has all been wired.

One or two people have mentioned by-pass circuits. You may or may not need
one. A by-pass provides somewhere for the water the go - principally during
the pump over-run period, when the zone valves are closed. If you go for an
S-Plan system with 2 x 2-port valves, you will definitely need a by-pass. If
you go for a Y-Plan system, with a single 3-port valve, you may not -
because there is always at least one port open. Provided you follow the
recommendation of having one radiator with two lockshield valves and no
TRV - so that it can't be turned off - you're probably ok with a Y-Plan. If
you *do* need a by-pass, the easiest way would be to install an automatic
by-pass valve between the flow and return, just before the zone valve(s).

Yup, the bathroom radiator is setup with two lockshield valves, so I
suspect I'll go with Y-plan.