Mid position valve as bypass

[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.