New problem with old stuart grohe 2.4 bar SRGD50 shower pump

[dfoss]

My son reports his shower has started to fail to pump cold water if a second person goes in for a shower - but continues to pump hot water. I got him to check that the header tank was being refilled properly and stayed pretty full throughout.
Can the pump itself fail intermittently at one end only? Is there an electrically operated sensor that can fail on one side only - in this case on the cold side? or is there a filter in the pump which if clogged could perhaps trigger this on a small drop in pressure - like when re-starting after a short stop when the header might be down just a bit?
Can't find any data sheet for this pump to teach myself.
Any help greatly appreciated.
ps - his shower has been working perfectly for several years.

pps - looking at a later monsoon pump schematic there are reed switches and filters. but is it not the case that if either flow switch reeds failed the whole pump would stop?

 

[dfoss]

OK - since posting I have found and watched Seb Hibbert's youtube videos on checking out Stuart pumps. I can see that the impellers are both keyed to the shaft

So am I right in thinking that - given correct flow and pressure of water to both hot and cold sides of the pump - the pump will run both sides or not at all?

If so it seems likely that I need to go there and double check the failure mode he described - clean the filters, look at the installation to ensure it's plumbed right with the hot feed above the cold feed etc. and double check that the tank is refilling correctly.

Any other suggestions most welcome.

 

[dfoss]

Well - I checked all that out and have ascertained the plumbing is OK and well laid out. the filters are clean. The reed switches are clean and functional. Flow is good and similar to both hot and cold. Pump is good.

-but the Trevi e3115AA that it feeds does not pass much cold water. when set to hot or a mixture the pump kicks in ok so normally the thing works well. I am now of the opinion that the Trevi is at fault.

But I cannot see how to get the knobs off!!!! - so I will close this thread and ask in a new 'un about the Trevi.