Mid position valve as bypass

[John.g]

Yes, that's exactly what I mean, a straight by pass between flow & return. I wonder was the system first conceived with the mid position valve only which I think is not allowed with a unvented HWC as it will power fail to hot water.

 

[SJB060685]

Yes, that's exactly what I mean, a straight by pass between flow & return. I wonder was the system first conceived with the mid position valve only which I think is not allowed with a unvented HWC as it will power fail to hot water.

If using a mid position valve on an unvented cylinder then the only acceptable way is with the use of an additional 2 port after 3 port on flow to cylinder, with its own high limit stat wired in series to motor feed. As you said this is to prevent heated water being circulated through cylinder in the event of fault and boiler left on constantly.
If you were to install new then S plan is far simpler and compliant

 

[John.g]

It wold need to be a NO valve.

 

[SJB060685]

The 2 port after mid position would need to be a spring return normally closed. Or have I misunderstood you?

 

[John.g]

No, I was thinking of a normal s plan with no mid position valve and using a 2 port spring open valve in a bypass between the flow and return.

 

[Spectric]

Hi there

I can see you could use a V4043B1257 across the feed and return as bypass as it is normally open, so if either of the other valves are powered to open then this valve would close when powered. It could potentially have made the pipework easier in some synarios.

 

[doitmyself]

Quite a lot of boilers, if not the majority, will not modulate below 5 kw, there two reasons, IMO, why a minimum flow rate should be maintained, one is, to avoid thermal stressing of the HX, if one allows say 25C delta t, then a flow rate of - 3LPM. The other reason is more practicle, if the delta t is too high then the boiler will very quickly cut out on high temperature (SP+5) especially on start up when the boiler output can be at 75% during the ignition phase.

I may not have made myself clear. I wasn't saying that a minimum flow was unnecessary, but that it did not need to be the same as that specified for full ouput. A 24kw boiler with a 20C differential has a flow of approx 17 litres/min. If the same flow is maintained when the boiler has modulated down to 6kW, the differential will be only 5C. Whether that is a "good thing", I don't know.

 

[John.g]

I'm not that familiar with domestic gas boilers but I was always under the impression that the minimum flow rate was at minimum output, for example a vokera Vision 20S, a 20kw boiler with a minimum output of 5kw specifies a minimum flow rate of 350 litres/hr or 5.83 LPM, by calculation, this results in a deltaT of 12.3C, if the boiler output increased to 20kw then the deltaT would be 49C so clearly the flow rate at full output would need to be ~ 14/15 LPM for a 20C deltaT.