Domestic Hot Water Pump Issue

[MrBeauregarde]

Hi All,

I have a system boiler (Glow-worm Ultrapower Sxi) where the boiler is over-heating resulting in the system switching off and fault codes F5 and F25. Before the system switches off there are rather uncomfortable grinding/banging noises focused on the heating water expansion vessel. I have had a plumber out and he said the issue is that the DHW pump is not working properly (it clicks/rapid on-off) and therefore due to poor circulation on the DHW, the heating loop and therefore boiler is over-heating. Have run through the diagnostic menu and the banging noise is happening as the flow temperature (d.40) goes over 85 degrees. He believes the DHW pump is OK and recommended replacing the PCB. I wanted other's thoughts on this as I don't want to replace the PCB if the pump is the issue or potentially something else.

Here are some other tests I have run which hopefully can assist with the responses:

1/ I have run a multimeter across the DHW pump when in operation and it shows the voltage rapidly goes from 0 to ~140V and then back again. This rapid cycling is done in 0.5 second cycles.
2/ I have checked the resistance across the DHW pump live and neutral terminals and seems fine with around 220 ohms of resistance.
3/ When I loosen the bleed screw without isolating the DHW pump, water pour/sprays out which could be a sign of knackered bearings? but have read elsewhere this is not certain for certain types of pumps (this is a WILO pump).
4/ Have isolated the DHW pump, removed bleed screw and rotor shaft is turning freely.
5/ When the system is in operation, the DHW pump is not showing as switched ON in the diagnostic menu (d.13) i.e. is 0. This is despite the Hot Water Activation (d.25) being ON.

Thanks

 

[picasso]

Could be the diverter valve is stuck in heating, the pulsing on and off is a pump unblocking feature.

 

[MrBeauregarde]

Hi, Thanks for response. No I don't think this is the issue as the CH Flow pipe remains cool to touch, DHW tank does heat up (the over-heating errors occur towards end of DHW heating cycle) and diverter valve position diagnostic (d.35) does show DHW.

But interesting that you note that the pulsating control signal could be message from the PCB to try unblock the pump and therefore problem is the pump rather than the PCB.

 

[picasso]

I would take the plate heat exchanger off and check its not blocked before buying any parts.

 

[MrBeauregarde]

Can I check what would be driving the pulsing on/off control signal from the PCB to the DHW pump. There must be a thermistor or other sensor giving this feedback to the PCB.

The CH pump operates smoothly without this problem. You mentioned it is an unblocking signal but what drives it? Are you sure it not just a faulty PCB?

 

[picasso]

The central heating pump does the hot water through a plate heat exchanger, I would have thought if the hot water pump wasn't working you would not get any hot water at all, it sounds like the boiler is not modulating down so going to overheat, it could be a pcb or hot water pump but I would check the plate first, if the boiler recognises the temp is rising fast it may be programmed to pulse the dhw pump but I don't know the boiler that well.