It's not so much the size of the pump but the shape of the working curve which is wrong, i.e it is cannot deliver the correct flow at the actual head.Are you thinking the current pump is too large or too small?
Do you mean the primary or secondary heat exhanger? If it's the secondary one I might take it off your hands anyway as the condensation tray (integral to the secondary heat exhanger) in my boiler is already badly corroded and looks like it might be ready to pinhole.
Cheers,
iep
Are you sure it is a Grundfos?So, looking for manual for grundfos pump ups15-50 part number 988123, IP42.
The older pumps are nothing like as efficient as the modern ones. I suspect that the pump is even older than your boiler. I am just guessing, but is the Energysaver the original boiler or has a new one been installed? The reason I ask is that old cast iron heat exchanger boilers had a much lower resistance compared to modern boilers, so the pumps did not have to be so powerful.As you can see, the power figures bear no relation to those from the two versions that you posted. In fact, assuming a similar pump efficiency, this one is more than 2x as powerful as the current new model!
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