How do I size a CH pump?

[stewamax]

I have just replaced my central heating boiler*, and sized it using Stelrad’s excellent STARS heat-loss app.

Since the radiators now seem to take longer to warm up (perhaps because the boiler temperature needs to stay within the condensing temperature window), the pump** now seems under-powered.
But how do I size a pump? It is tempting just to buy the next pump upwards in Grundfos’s range - a 180 25-80, but having done my sums for boiler sizing, I feel I should finish the job properly. And I want to avoid having a too-powerful pump that spends all its time (even on its lowest setting) over-pumping into the header tank. I am also uncertain whether to elect for conventional technology (e.g. the 180 25-80) or clever ones (e.g. Gundfos Alpha 2)
I study the various pressure and flow-rate curves for pumps servicing different heads of water, but DAK where do I go from there?
Grundfos have a sizing app which I am about to try, but what do others (e.g. CH installers) do?

Usual disclaimers - I don't work for Grundfos!


* - by a 35KW Viessmann Vitodens 100W open-vent (feeding 22 radiators and two DHW cylinders)
** - the ubiquitous Grundfos UPS2 50/60

[bungeejumper]

Not really an answer, but our monster (an open-vented gas Rayburn boiler supplying 17 radiators) is running well on a Grundfos 15/60 Alpha 2L - the upgrade from 15/50 to 15/60 was quite a significant advance in power, but it was needed because we have a helluva lot of drop pipes which needed a helluva lot of pumping. I'd be a bit surprised if you need the 80, but I'm sure others can advise better than I can.

The Alpha 2L has about seven different programmes, of which three (IIRC) are 'intelligent' (i.e. it senses the resistance in the system and adjusts the pumping power accordingly). And the other four are simply straight pumping speed choices. AIUI, the fancy power-managing electronics don't save you much money - maybe £10 a year - but they keep the Eurocrats happy. I've never noticed ours over-pumping into the loft tank, or not excessively so. Anyway, it's been in for four years and it's very quiet and has given no problems. Does that help?

Oh yes, the Alpha 2 fits in the same standard-sized hole where the old 15/50 did. No extra pipe-shortening required.

BJ

[Sobraon]

I would phone up Grundfos and ask them for advice - I say this because of my experience after replacing a shower in the last year. I was getting the run around between the supplier and the manufacturer because the shower would not 'trigger' the flow switches on the shower pump.

Phoned Grundfos, straight through to the technical people and problem resolved. Chap I spoke to was obviously a pump 'nerd' and we had a pleasant and informative (for me) conversation.

(Usual disclaimer I don’t work for Grundfos).