Do I need a new pump?

[Dave876]

Not sure how long this has been happening but some symptoms have been there a while - only noticed now as the house is cold in this weather plus I had a stuck TRV.
House was extended - new radiator on the longest run has never gotten properly hot - it's usually cold. 11 rads total, worcester bosch greenstar CDi30 boiler, grundfos alpha 1 pump.
Everything is running but none of the rads get particularly hot as the boiler is short cycling. Tried balancing the rads but even with all trvs and lockshields open still short cycling. pump is running full speed. All rads do get hot at least depending on what's open or closed. If I shut all other rads down then the last radiator does get hot but limited heat due to short cycling boiler.
No matter what's happening - feed from the boiler gets hot as expected (and the boiler goes up to expected temp) but the return is never hot enough to be even close to the same temperature.
Boiler was serviced 2 years ago, system drained, flushed and refilled, bypass valve replaced. System is s-plan (not zoned) and both valves work fine (one replaced not so long ago). Magnaclean was checked a 2 months ago.
I don't think that the pump is up to the job hence can't push the water round fast enough for the boiler. Or have I missed something?
I think (from an online chat with Grundfos a while ago!) the next one up is UPS2 25-80?
Any help or advice appreciated!

 

[Knappers]

Not the size of pump causing your issue, you have a restriction somewhere. Air, sludge, valve etc.
I would check pump head, two ports, any valves and then start flushing through flows and returns at rads before considering a power-flush if needed.

 

[Dave876]

Not the size of pump causing your issue, you have a restriction somewhere. Air, sludge, valve etc.
I would check pump head, two ports, any valves and then start flushing through flows and returns at rads before considering a power-flush if needed.

The rad that doesn't get warm easily - I had that off for decorating 2 months ago and verified that water came through the valves and it seemed a reasonable pressure without the pump running. The runs are quite convoluted given it's an old house and certainly not optimal. There's actually 2 drops from upstairs, and effectively 3 seperate and distinct loops (not zoned), It was flushed 2 years ago. Power flush was when the boiler was fitted (12 years?). No cold spots in the radiators when they are warm. 2 port valves all work fine (plenty of heat in the pipework after the heating valve).

When you say check the pump head - what do you mean please?

 

[Knappers]

Take head off (4 hex bolts) look through veins of impeller to check for deposits/blockages.
Flushing through at rads is for multiple reasons, check for deposits or blockages and restrictions but also flush through any airlocks.
Best to do with all rads isolated except one your working on.

 

[John.g]

If its this model then you can derive a lot of info from its power output in watts W once the mode/setting is known which should be displayed via the LEDs, post a close up photo of the pump display (obviously, running).
There should be a label somewhere as well showing if its a 4M,5m or 6M (40, 50,60) pump.

 

[Dave876]

By way of an update - I replaced the pump with the UPS2 25-80 and everything is working fine. Still need to balance the rads though.

Something that came to light - when there was no call for heat and both valves closed the old pump continued to run for a short time (as may be expected) but the bypass never opened (the pipe work never got hot at least). bypass was (and still is) set to 0.5 bar. Note the bypass is located very close to the pump so not a long route at all.

Knappers - I missed your reply sorry - yes I could have taken the head off then and had a look. Might do that anyway just to see.

John - couldn't see your images until I actually registered! But the old pump was an alpha1 15-50/60. I tried various settings (proportional, constant etc) on the speed but with no change. Essentially it was on speed III and reading 18W.

Right now - all rads get hot. House is up to temp for the first time in god knows how long - I suspect this wasn't a new issue at all. Also running much quieter in fact. The bypass is now open however with the pump running on "2". Need to check if all rads work with turning the pump down to 1 (once balanced). But the boiler isn't short cycling and runs at a constant 70 when needed - everything makes me think the short cycling was down to the lack of flow through the boiler. It's certainly managing to modulate itself down as needed.
Thinking about everything further - maybe just turning down the bypass to improve the flow through the boiler may have worked to stop the short cycling but I'm not convinced it would have helped get the water all the way round the system when it failed to do that with various loops effectively shutdown anyway. I didn't set it to 0.5bar - done by the last plumber - I'm only just learning about the bypass now although the general purpose of it has always been pretty clear.

 

[John.g]

It couldnt have been pumping anything on speed III at 18W.
A 4M pump will run at 18W at zero flow, a 5M at 26W & a 6M at 33W so pump either knackered electronically or impeller/ports full of sludge or never running in speed III !!. As post #4 suggests, be interesting to remove the head and inspect the impeller vanes and pump ports, stick a tie wrap through the vanes to prove clear.

 

[Dave876]

Ah it's all sealed and can't get in to see the actual impeller. it's not the one you posted - and won't let me post a link here!
I can look into the ports - some deposits but not terrible I think. Tie wrap poked in moves the impeller easily enough. Prising off another bit I can get to the spindle and it turns freely enough there as well. Pump is only 2 years old so disappointing but the new one is in, quiet, working and more oomph.
Depending on the rest of Christmas - I could wire it up and see what it does outside the system just out of interest.