It was the way you had it drawn with the pumps on the return side, IIRC. The flow came from the boiler and split to the CH and DHW. The returns from these came to the twin pump and then the combined output went to the boiler.
In reality, the pump needs to go on the flow side, so the boiler flow goes to the common point of the pump. Then one pump output goes to the CH and the other to the DHW. The NR valves prevent flow from each circuit back towards the boiler. The returns from the circuits combine and go the the boiler.
I'm not so sure. The pump will let by and I was thinking of gravity circulation. With a sprung NR valve that won't happen.
However, it's academic since the boiler doesn't need it and likely shuts down in absence of electricity anyway.
.andy
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