I am not an engineer. But I think most engineers just use experience and a bit of logic and guesswork. Do you run your current pump at Speed 3? Below is the only chart I can find for a pump with the same name as yours. If you look at the dark black lines, the top one is for Speed 3. Your current boiler needs a flow of at least 30 litres per minute (but probably substantially more). But taking that best scenario, at 30l/min the pump is producing just under 4m of head. If we say that 1.5m of head is used up passing the water through your boiler. That means the remaining 2.5m of head is successfully circulating the water through your radiators. Bearing in mind that in the quote a separate pump has been specified purely to send the water through the new boiler, I would say that a basic 15-60 (6m head) pump costing £150 is all that is needed for the radiators etc. But your installer is recommending a £700, 10m head pump to achieve the same thing. That is why I was a bit surprised at that choice. Perhaps the explanation is that your installer determined your current pump isn't producing sufficient flow?
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