One boiler overrun, two pumps

22 Jan 2017
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United Kingdom
I have a heating system with a BG300+ boiler and a pump/valve for the CH circuit and a pump/valve for the HW circuit. I noticed that both pumps come on whichever circuit is active. When I looked both pumps are connected to the single overrun signal coming from the boiler.
The problems are:
  1. the CH pump is quite hefty and it's using a lot of power when it's not needed
  2. Some heated water is getting into the CH system when only the HW system is active. I'm wondering if the CH pump is forcing water past the CH valve which isn't completely closed
  3. The CH system is always hot when required, but the HW system isn't getting hot enough especially when it's really cold outside.
Any ideas about how to stop both pumps running and do you think it might be stopping the HW from working properly in the Winter?
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Each pump (circulator) will have to be wired through a relay.
If the HW circulator utilises a bypass to which is fitted before HW MV then use brown core of heating MV to power heating circulator.
Is this really a twin pump system or is it a thermal store?
Thanks for replies. I've attached a picture of the plumbing system in case this helps.

I don't think it's a thermal store - there are a lot of radiators so I think the previous owner decided to have two pumps, one exclusively for the CH. It's a Grundfos UPS2, BTW.


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Here are some photos. It's a constrained space but I hope you can see what's going on.
220231 is quite good. The red pump is for the central heating, the blue for HW. You can see the flow for the central heating splitting into the two radiator circuits.
In 215639 you can see at the top of the picture the cold going to the boiler on the right and the hot coming back on the left.
I'm pretty sure that the diagram I also posted is correct.


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what a bloody mess, get someone who knows what they are doing and get it sorted

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