Roger Mills (aka Set Square) wrote:

Yes - as long as any balancing gate valve in the HW circuit isn't turned
down too much. If you can contrive to run the HW heating and CH at
*different* times, you won;t need to do any HW/CH balancing.

The plan is to have the HW running relatively seldom, so I hope to
balance the system such that it discriminates in favour of the HW
whenever both are running, ie such that the gate valve is up a good bit.
I only ever use HW in the morning and briefly in the evening, so it
would seldom be running long enough to make the house cold

The pump overrun thing is a bit tricker than I thought. I had planned
to follow one of your recommendations and run a five-core cable
between the new wiring centre in the hotpress and the boiler
downstairs, but it turns out that there is not conduit all the way
(it goes across a floor as well as between walls) and tracking walls
to install cables is not something I want to get into. Instead I'm
going to re-use the existing three-core cable, which up until now has
supplied mains to the pump, to carry the SL and PL signals to/from
the box, and then provide the boiler with it's own permanent live
from the existing mains connection in downstairs. I guess that should
be easy, just a matter of putting new sleeves on the wires to ensure
that it is clear they are live.

That will work, but it's very bad practice! It is highly recommended to have
the *whole* system powered from a *single* FCU so that anyone working on it
can be *sure* that nothing is live. Powering it from two separate places
will not achieve that - and you may get sued for electrocuting any
unsuspecting BG (or whatever) person who comes to work on it!

I accept your point and the note of caution, but running a five core is
going to be too damn expensive to do, I'll have to take up boards and
remove my present bath to do it.

As a related point, surely a reasonably knowledgable person would use a
phase tester before risking his life on the assumption that things had
been wired the way they appear. I've seen/heard of examples of far, far
more dangerous things around houses than what I'm thinking of doing,
undoubtedly you have too. I bet you'd never stick your finger near mains
wiring without testing it first