Replacing a Grundfos Selectric CH pump

adamjth · 5 Apr 2020

Hi,

Moved into new house to find that I have no CH or hot water. Under more detailed inspection I find the CH pump is red hot to touch and when I opened it I was unable to turn the spindle. I have researched how to replace it and having done a few plumbing repairs in the past I am going to replace the unit. Some questions

The unit has the markings 15-50 130 p/N 59525469 on the outside. and PC:0402 which I thinks means its 16 years old and not made anymore. How, or where, do I locate the right replacement part that will mean I can replace exactly with out having to change the pipework? If I get another pump that 130 mm 15 bore and 50 does the guarantee it will fit i.e https://product-selection.grundfos.com/ ALPHA1 15-50/60 130?

Either side of the pump there is an isolation stopcock so if I lock these off I shouldn't have to drain the whole CH system?

Any other considerations I assume the ALPHA1 15-50/60 130 will work with my existing Honeywell st6400c

thanks

Adam

Gasguru · 5 Apr 2020

Yes the pump will fit onto the existing pump isolation valves but don't expect them to fully shut off ie. expect some water loss whilst the pump is out.
Always check and if necessary clean and bale out the feed and expansion vessel first (wet vac if you've got one).
I'm guessing your existing pump is a Grundfos....I'm not sure with the Alpha1 but often it's possible to just swap over the pump head and leave the rear casing in place (assuming it's not corroded inside).

adamjth · 6 Apr 2020

thanks Gasguru. Rear casing is in good shape and hadn't realised that you could replace the pumphead. Thats a win/win then if I can get a replacement pumphead that fits as it will be cheaper and less hassle with pipework

Gasguru · 6 Apr 2020

You can only assess the inside surfaces of the rear casing (volute) once the motor/impeller has been removed.
And unless you've a decent 4mm hex key you may struggle to remove the bolts.
So plan to replace the lot

D_Hailsham · 6 Apr 2020

Grundfos do not list a head-only version of the Alpha 1. I suspect this is because it would not fit the rear casing of the old UPS pumps.

You can get a head only version of the UPS2 which will fit your existing US reaar casing. There is a new one for sale on Ebay for £40.

adamjth · 19 Apr 2020

Thanks for help here. Last weekend I successfully replaced the showerpump with as new UPS3. One more question the old unit had the switch set to the third position III. and I don't know what setting that translates to on the new pump as per page 10 https://www.anchorpumps.com/mwdownloads/download/link/id/2978/ of this guide there are 7 x modes.

I have left on default which I believe is :-
"The pump runs at a constant speed and consequently on a constant curve. At speed III, the pump is set to run on the maximum curve under all operating conditions. Quick venting of the pump can be obtained by setting the pump to speed III for a short period."

it works fine so I assume its OK. BTW its in indirect CH system with 4 x rads, 1 x immersion, header tank and coldwater tank and Worcester boiler in a small house

D_Hailsham · 20 Apr 2020

adamjth said:
It works fine so I assume its OK. BTW its in indirect CH system with 4 x rads, 1 x immersion, header tank and coldwater tank and Worcester boiler in a small house

A system with only 4 rads shouldnt need the pump on speed 3. (I had 13 rads at my previous house and the pump ws on speed 1.) Balancing the system correctly made all the difference.

A significant factor is the resistance of the boilers heat exchanger as modern boilers have a much hogher resistance than the old cast iron types. The resistance is lowered considerably by running the pump at a lower speed, which reduces the flow rate. (Resistance proportional to flow squared). You say you have a Worcester boiler. Which model?

adamjth · 20 Apr 2020

Thanks for your reply it’s a green star Ri

Gasguru · 20 Apr 2020

Set it to constant speed 2 but you might get away with speed 1.
With all the radiators open you're aiming for a 10 to 20 degree temperature drop across the boiler flow and return pipework.
If the drop is too much then increase the pump speed, too low then reduce the pump speed.
Highly unlikely you need speed 3 unless you've got microbore/blocked pipework.

adamjth · 30 Apr 2020

Thanks Gasguru. So I have set it on speed 2 and all is functioning well. It has also cured something else I had noticed:- the two rads on 1st floor had been getting hot when the HW period started as set on the Honeywell. Downstairs the rads were cold and this despite the CH setting being set to OFF. Oh well this hasn't occured since I made this change. I am used to direct combi fed systems so this is all new to me but I believe this may be due to diverter valve? Either a fault or reverse circulation probs but I haven't seen the fault since the change on the grundfos to setting 2...hmmm

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Replacing a Grundfos Selectric CH pump

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