Grundfos UPS 15-50 pump intermittently trips MCB/RCD – repair or replacement?

[sergii]

Hi,
I have an open-vented central heating system with a Grundfos Selectric UPS 15-50 circulation pump (photo attached).
The electrical supply intermittently trips when the pump is connected and running. Sometimes it runs normally for a while, but then the MCB trips and occasionally the RCD also trips.
I have checked by disconnecting other components and the problem appears to stop when the pump is disconnected, so I suspect the pump may be the cause.
The pump is not seized and appears to run. Could this be an internal electrical fault (motor/winding/insulation), and is this type of fault usually repairable or is replacement the better option?
If replacement is recommended, what modern equivalent would be suitable?
Thanks for any advice.

 

[jj4091]

If it's old just replace it.

 

[mikehammer69]

It’s a common enough issue with those pumps
Replacement usually needed

It'll probably show a low reading if you you IR test it

 

[fixitflav]

If replacement is recommended, what modern equivalent would be suitable?

I had a Grundfos Selectric UPS 15-50 which failed a 2-3 years ago and I replaced it a Grundfos UPS2. It's been fine. There's now a UPS3, which I wouldn't go for, over-complicated and possibly unreliable. For years I swore by Grundfos, but I believe there are now plenty of good makes around. I'm a DIY-er but I've heard on here about, Wilo, DAB Evosta, among others. Maybe somebody can add to the list.

 

[sergii]

Thank you for the helpful replies. I would appreciate any further suggestions for suitable replacements, especially modern, efficient models. Has anyone had experience with the newer Grundfos pumps? For example, would a Grundfos Alpha2 15-60 130 be a suitable replacement for the old UPS 15-50 in an open-vented system like mine or would that be unnecessarily powerful? Also interested to hear recommendations for reliable alternatives such as Wilo, DAB Evosta, etc.

 

[Gasguru]

Wilo Yonos Pico 25/1-5-130 is best value quality pump...no point in paying more to cover Grundfos huge marketing budget.
5 year warranty (T&Cs)

 

[gas112]

Archergas® ST15® circulating pump 15-60/130 | Wolseley

Looking for a Archergas® Heating Pump ST15? Order from Wolseley today - free delivery or click and collect available nationwide.

www.wolseley.co.uk

 

[Razor900]

Wilo Yonos Pico 25/1-5-130 is best value quality pump...no point in paying more to cover Grundfos huge marketing budget.
5 year warranty (T&Cs)

I fit loads of these as replacements. Very good pumps

 

[sergii]

Thanks for the suggestions.

One thing I am still not clear about: my existing Grundfos Selectric UPS 15-50 is a DN15 pump, and my heating system pipework is 15 mm. The Wilo Yonos Pico 25/1-5-130 appears to be DN25 (larger nominal bore), although it seems to have the same 130 mm length and the same external union connection.

Is there any issue replacing a DN15 pump with a DN25 pump in a 15 mm domestic central heating system? Would the larger internal bore have any negative effect, or is it simply unnecessary because the rest of the system is still 15 mm pipework?

 

[Johntheo5]

Thanks for the suggestions.

One thing I am still not clear about: my existing Grundfos Selectric UPS 15-50 is a DN15 pump, and my heating system pipework is 15 mm. The Wilo Yonos Pico 25/1-5-130 appears to be DN25 (larger nominal bore), although it seems to have the same 130 mm length and the same external union connection.

Is there any issue replacing a DN15 pump with a DN25 pump in a 15 mm domestic central heating system? Would the larger internal bore have any negative effect, or is it simply unnecessary because the rest of the system is still 15 mm pipework?

No issues in replacing with either as long as the external dimensions are the same.

Before purchasing the 5M Yonos Pico, have a look at the Selectric's speed setting, if its on speed 3, a 6M Yonos Pico might be more suitable, how many rads have you got?.

 

[fixitflav]

Thanks for the suggestions.

Before lashing out on a new pump, it might be worth taking the cover off the terminal box to make sure there's no damp. Perhaps get a hair drier on it.

 

[Johntheo5]

For anyone with a OV system, if installing a new circ pump, its well worth taking a photo or noting the original pump details and mode/settings as some systems are quite sensitive and can suffer from pump over/air ingress. I have seen more than one reference to the UPS3 which is supplied in constant curve mode at speed3 causing havoc with these systems, sometimes, like below, where the installer simply doesn't change the settings.

S

Thread 'new boiler cold radiators where does installers liability end'

24 Aug 2025

Hello.
I had an old gravity fed, conventional, central heating boiler which had started to seep water from the heat exchanger, otherwise the system worked perfectly (no leaks, no air locks) In June I had a company come to quote, checked the system as it stood, said it would be no problem and quoted £4300 which seemed reasonable (cf BG`s £5400) Initial experience was fine, they were punctual, engineers polite and professional. The only problem is they`ve never been able to get the new system to work. They`ve had various theories as to why this is, 1st one of the new motorised valves...

• stevemarl
• Replies: 46
• Forum: Plumbing and Central Heating

 

[fixitflav]

For anyone with a OV system, if installing a new circ pump, its well worth taking a photo or noting the original pump details and mode/settings as some systems are quite sensitive and can suffer from pump over/air ingress. I have seen more than one reference to the UPS3 which is supplied in constant curve mode at speed3 causing havoc with these systems, sometimes, like below, where the installer simply doesn't change the settings.

S

Thread 'new boiler cold radiators where does installers liability end'

24 Aug 2025

Hello.
I had an old gravity fed, conventional, central heating boiler which had started to seep water from the heat exchanger, otherwise the system worked perfectly (no leaks, no air locks) In June I had a company come to quote, checked the system as it stood, said it would be no problem and quoted £4300 which seemed reasonable (cf BG`s £5400) Initial experience was fine, they were punctual, engineers polite and professional. The only problem is they`ve never been able to get the new system to work. They`ve had various theories as to why this is, 1st one of the new motorised valves...

• stevemarl
• Replies: 46
• Forum: Plumbing and Central Heating

Good points. I don't remember seeing that thread at the time or I'd have stuck my oar in! Amazing the installers didn't try turning the pump speed down. My rule is use lowest speed at which the system works. Confirms my suspicion of the USP3.

 

[Johntheo5]

The UPS3 is a giant of a pump, 60W vs ~ 35/45W for other 6M pumps, also, its constant curves are, in effect, constant pressure curves so will still maintain a 6.2M head at over 19LPM but changing to CC1 will give a more reasonable setting at 4.2M or CP2 will maintain 3.0M, so not really the pumps fault.

 

[fixitflav]

The UPS3 is a giant of a pump, 60W vs ~ 35/45W for other 6M pumps, also, its constant curves are, in effect, constant pressure curves so will still maintain a 6.2M head at over 19LPM but changing to CC1 will give a more reasonable setting at 4.2M or CP2 will maintain 3.0M, so not really the pumps fault.

I believe the UPS3 has high efficiency, perhaps at the expense of lower reliability. I would prefer a cheaper pump and to spend a few ££ more a year on electric, as most of the extra electric power ends up in the water anyway.