"Malcolm Reeves" wrote in message
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"FrancisJK" wrote in message
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Has anyone any opinions (silly me, of course you have) about alpha
pumps?

With an alpha, as TRVs close the flow falls but the head does not rise
as in normal pumps. With a normal pump you need a bypass. So normal
pump+bypass gives constant-ish flow rate and return temperature rises
as TRVs close. Alpha gives reducing flow and a lower return
temperature.

Hence Alpha's are a good idea if you have a condensing boiler since
those are more efficient with lower return temperature. But normal +
bypass is probably better for a standard boiler since they do NOT want
a lower return temperature due to the risk of condensing causing
boiler corrosion.

People use these having TVRs on all rads. Boilers require a minimum flow
through the rads, using TRVs on all will reduce this flow. A flow switch
that cuts out the burner is required when the flow gets below the boiler
makers stated flow.

Using a by-pass on a "modern" condensing boiler defeats much of its purpose.
Older condensing boilers had the same design specs as regular boilers: the
Ideal Minimiser stated 80C flow and 11C delta T on flow and return. The
makers did say that you can stretch it to 15C.