Worcester Greenstar 24Ri - pump shuts down prematurely

Hi all

I have a WB Greenstar 24Ri running a pressurised CH & HW (tank) system, with a hive thermostat.

Boiler is maybe 5/6 years old and previously has run without issue. A couple of months ago during the summer I noticed the water that is scheduled to come on twice a day was not as hot as it used to be.

The sequence of events appears to be:

1. Hive starts boiler demand (either CH or HW)
   
2. Pump begins (runs quietly, no screeching/bangs)
3. Boiler begins its normal ignition sequence
4. Runs for anywhere between 5 and 15 minutes, then for no reason it seems the pump stops, and boiler cuts out 5 seconds later. The HW & CH are NOT at temperature. The boiler's central blue light is solid (no flashes).
   
5. Boiler & pump will not restart for perhaps 45 minutes - then back to step 1 and repeat.

The same can be replicated using the CH (as it has now got a bit colder).

Observations:

• The hot water tank does eventually get hot and get to temperature (55C) after 2 of the above sequences.
• On CH, all the radiators get hot - leads me to believe the pump is fine. Also, hive thermostat seems fine too.
• I've bled all the raiators so no air in the system, there's a auto bleed valve adjacent to the water tank in the airing cupboard too
• Pressure is at 1.5bar
• Manually starting the boiler with the hive boost control, then stopping the boost after 5 minutes (before it gets to step 4 where it cuts the pump) will allow the pump to do its 3 minute overrun then shut down normally
• Three way valve appears to be moving as expected
• Pump is set to position 2. Reacts to positions 1 and 3 when running. Neither of these other positions makes a difference to the duration of time the system runs before cutting out.

Where might the problem lie? Something is causing the pump to stop which then causes the boiler to enter what I would guess is its standard overheat & cool down mode. Control board issue?

Any suggestions welcome!

Cheers

 

Boiler was serviced (or attempted to be serviced last week) but it went into its trip mode and the heating engineer gave up and had to leave. He did clean the filter though. His suggestion was the pump was at fault, but given it works again (once the boiler appears to have cooled down and reset itself) and all radiators & HW tank gets hot, I can't see why it would be where the problem is. I am open to the idea though...

Thermostat is set to 2/3 of the way round position. Approx 60/70C according to the diagram in the manual.

 

Thanks Mike, switch is set to Norm (see pic)

@The Teach the hive was installed c 3 years ago by BG - it appears to be behaving itself with respect to responding to the app, manual controls and thermostat

 

To close this thread off I measured the voltage coming out of the boiler when the pump had shut off and it was measuring 240V, suggesting the pump actually was at fault. Replacing the existing BG branded grundfos multihead with a Grundfos UPS3 this weekend has cured it. I am still confused as to why the pump would run for 10-20 minutes before packing up - I would have thought if it wasn't going to work it'd never work...!