Set Square wrote:

In an earlier contribution to this discussion,
Andy Hall wrote:


You may be able to alleviate this by changing the pump to a Grundfos
Alpha.

Or turning the existing pump down, as long as it still gives satisfactory
circulation at a lower setting.


These automatically back off their power as the flow resistance
increases, which I suspect here is the reason for the noise.

Is there any danger of them backing off so much that there's insufficient
flow to cool the boiler during pump over-run conditions?

Yes, I have heard that caveat (though I haven't come across that situation
in practice, but then I've come across vanishingly few Alphas)