"Roger Mills" wrote in message
...
I know that something similar was discussed recently, but I'm not sure
that there were any concrete conclusions.

I'm currently trying to fine tune my system, and am having great
difficulty in achieving a satisfactory set of settings. [See my other post
on non-contact flow meters.]

I have an S-Plan system with a Grundfos Alpha+ pump and a Peglar USV
16/22-PE automatic by-pass valve. If I set the by-pass valve too tight, it
doesn't open enough during the pump over-run phase, with the result that
the boiler overheats and trips. If I slacken it off so that it *does* open
for pump over-run, my suspicion is that it's actually open most of the
time - which is undesirable.

It seems to me, in retrospect, that the combination of a variable speed
pump and automatic by-pass valve is probably never going to work as
desired. If the pump manages to achieve its objective (which it won't
quite, but might get close) of producing a constant pressure regardless of
flow, that pressure (depending on by-pass valve setting) will either be
sufficient to open the valve all the time, or not at all. Surely you
*need* the pressure to increase as the flow reduces in order to open the
valve at the right time. This would seem to imply that you need to run the
pump at constant speed rather than at constant pressure.

Any comments?

I thought the idea was to maintain constant flow not pressure.