On Tue, 29 Nov 2005 13:05:38 -0000, "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.

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?


There is a control to set the operating speed/power.

In any case, though, there is not a need for a very great flow for
bypass purposes - only enough to stop the water boiling if it were to
otherwise stop flowing. Also, remember that this is more of an issue
with cast iron boilers than those with low mass heat exchangers


--

..andy