Automatic by-pass valves and variable speed pumps

[Roger Mills]

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?

[dennis]

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

[Roger Mills]

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?

[dennis]

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.