Automatic by-pass valves and variable speed pumps
 

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?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
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Automatic by-pass valves and variable speed pumps
 

"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.

Automatic by-pass valves and variable speed pumps
 

In an earlier contribution to this discussion,
dennis@home wrote:

"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.]

SNIP
Any comments?

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

The idea of what?

The idea of a variable speed pump certainly *isn't* to maintain constant
flow - but to back off as more amd more TRVs close, maintaining flow just
through the rads which are still open. This implies keeping the pressure
constant (by reducing the speed) as the flow reduces.

The idea of an automatic by-pass valve is to maintain *some* flow through
the boiler - particularly during the pump over-run phase, when the zone
valves are closed. Arguably it also needs to open a bit when *most* of the
TRVs are closed in order to maintain the minimum required flow through the
boiler. But since it is operated by (differential) pressure, if it is to
open by different amounts under different circumstances - which it needs to
do - then surely the pressure (pump head) needs to change with these
circumstances. It can't do this if it's 'clever' control system is striving
to keep the pressure constant.

Any further comments?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

Automatic by-pass valves and variable speed pumps
 

"Roger Mills" wrote in message
...
In an earlier contribution to this discussion,
dennis@home wrote:

"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.]

SNIP
Any comments?

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

The idea of what?

The idea of a variable speed pump certainly *isn't* to maintain constant
flow - but to back off as more amd more TRVs close, maintaining flow just
through the rads which are still open. This implies keeping the pressure
constant (by reducing the speed) as the flow reduces.


Ok so I should have said a minimum flow.
The pump I have came with a chart which showed over which head and flow it
would operate on the different settings.