Hot water runs hot and cold - new sensor or heat exchanger needed ?

Hi - our systemmate 2000 is delivering hot water at variable temps - so in the space of a few seconds will go from hot to cold.

Looking at the values

Store =71
T1 on = 62, T1 off = 69

PHE T2=32

DHW T3 = 49

When the hot water is running the display shows H of between 29 and 39.

The hot water pump is running okay and set at speed 3. The boiler is set at maximum and the water pressure etc all looks fine.

Any advice on next steps please? Could it be one of the sensors or the plate exchanger ?

Also maybe relevant is that I recently had the PCB replaced with a repaired model and we might have a soft water unit fitted as there is a rectangular metal unit to the right of the plate exchanger.

Thanks

Hi is the hot water pump running constantly or only when a hot tap is turned on. Regards Murray

Thanks Murray - the hot water pump only runs when the hot tap is turned on, thanks

OK that was the easy diagnosis out the window.  You might have looked at my videos on YouTube or already know how the system works and the fact that the hot water temperature is controlled by the sensors which speeds up or slows down the hot water pump to try to keep an even hot water temperature.  The temperature sensors are active devices so they have a chip in the end bit that the pcb interrogates to read the temperatures.  Like anything electronic they can become unstable and not report accurately, this will lead to fluctuations in the water temperature.  I know people have had sensors where when they read the temperature it always reads what seems to be sensible figures but replacing the sensors cured the issue.  Equally of course it could be the pcb not controlling the pump correctly.  The easiest way to check this is to replace the pcb!  There are a few other possibilities such as low water level (usually accompanied by gurgling's) or blocked heat exchanger that sort of thing.  Anyway some ideas to think about. If you want to try a replacement pcb I can help there have a look at my website for more information, click here:  Website