Fault Diagnostics Help Required Please

Hi,

I have a problem with my Systemate III hot water supply and suspect either the DHW pump or DHW sensor and would appreciate any advice in determining the most likely cause of the problem.

The Boiler and central heating are working correctly.

When a hot water tap or shower is turned on, there is a hot supply for a short time then the water runs cold. The plate heat exchanger also goes cold to the touch.

To test the DHW pump, I have disconnected the power supply to it from the PCB and connected a 240 volt mains supply directly to the pump. The pump runs, the heat exchanger quickly gets hot and so does the domestic hot water. The pump also gets hot as a result of the hot water being pumped through it. I assume this eliminates the pump from the fault list.

With the pump still disconnected from the PCB, my multimeter readings on the pcb leads to the pump are zero volts when no hot tap is on and rise slowly from 50 volts to just over 200 volts if a tap is turned on (I assume this is due to the fact that with the pump disconnected from the PCB and not actually running, the system thinks it needs to increase its speed and therefore raises the voltage), however, I am not convinced this is a reliable reading as when the pump is connected normally to the PCB, it never gets even gets warm to the touch which suggests that it isn't running.

Does this suggest a DHW sensor fault or something else?

I would appreciate any advice you can offer.

Panfam