"Unique" CH/HW design with dual pumps

[Johntheo5]

The vent and cold feed points of connection should have as little pressure differential as possible this is why they are normally installed on the boiler flow closely together with the vent before the cold feed, because the vent rises up (up to) 450mm above the F&E water level then some differential can be tolerated without pump over, if both are shifted to boiler flow with pump remaining on the boiler return then the pump outlet pressure will be EDITED, 4M, pump inlet will be 4-3, 1M, (with pump set to give a 3M head), if the pump is relocated to the boiler flow side after the (relocated) vent and cold feed then the pump outlet should be 4M+3M, 7M, pump inlet, 4M,.
Still think the combined vent and cold feed is by far the easiest and possibly best solution.

 

[Stuart981]

The only time I have seen two pumps is where there was a tee off the primary circuit that fed distant radiators and towel rails.