How to wire up for hot water overrun ?

[Gudge]

I'm confused about how hot water overrun works. I have a system as per fig.4 of the installation manual. As I understand it, hot water over run leaves the HW BDR91 on for a while after the Boiler BDR91 is switched off, in order to keep the pump circulating hot water through the cylinder after the boiler switches off..... is this correct ?

In which case, in my case, I cant see how it will work, since the pump and boiler demand are linked together in the junction box (same as is the case in figures 9 & 10 of the installation manual), so even when the boiler BDR switches off and the HW BDR stays on for 'Boiler overrun' there will still be a demand to both the pump and the boiler (due to the link).....as far as I can see.

I could remove the link, but then when there is no HW demand, but there is room heating demand, the boiler BDR will switch on and the boiler will start but the pump will not be activated ?

I know I must be missing the point some how, so could somebody please explain how it works and how it should be wired ?

Many thanks.

[The EVOHOME Shop]

What boiler have you got?

AFAIK, the pump over-run is only really there for boilers where the pump is wired and controlled by the boiler (usually boilers with a low water content heat exchanger), as they normally have a pump over-run at the end of heating demand. I think the idea of the evohome over-run facility on the hot water side is to prevent heat blocking of the boilers heat exchanger, but TBH most systems when you install evohome with have an Automatic Bypass Valve or Bypass Circuit fitted so not sure how relevant the over-run facility really is.

[Gudge]

What boiler have you got?

AFAIK, the pump over-run is only really there for boilers where the pump is wired and controlled by the boiler (usually boilers with a low water content heat exchanger), as they normally have a pump over-run at the end of heating demand. I think the idea of the evohome over-run facility on the hot water side is to prevent heat blocking of the boilers heat exchanger, but TBH most systems when you install evohome with have an Automatic Bypass Valve or Bypass Circuit fitted so not sure how relevant the over-run facility really is.

Hi and thanks for the reply, my boiler is a Worcester Greenstar Danesmoor Utility 12/18 Regular Boiler Oil ErP. So it sounds like I really don't need to worry about the HW overrun then, as I have ABV, but it is still bugging me, as I cant see it can ever work with the fugure 4 / normal wiring arrangement, so seems like a pointless option. (I'm sure its not really pointless, but I just don't get it)