First the good news. I’ve picked up a second hand 3.5kw 300L Rheem unit for a song

And it seems to work!

But …

the heat pump component appears a little temperamental in its decision to come on or not.

and the heating element is always on as a function of the conventional thermostat

I’m wondering about replacing the controller with a more modern one - ideally with some control over the parameter and the ability to monitor sensors and operation

 

If used for temperature control of you living space, you need an updated controller suitable for your unit. The conventional thermostats are on-off for preselected heat or cooling only.

But, as your intro refers to water heating, you need to sort out what you want the unit to accomplish.

 

If used for temperature control of you living space, you need an updated controller suitable for your unit. The conventional thermostats are on-off for preselected heat or cooling only.

But, as your intro refers to water heating, you need to sort out what you want the unit to accomplish.

Thanks Dave

what I want to achieve is to bias the heatpump over the element and have far more confidence about when and why it is coming on or not

the controller seems fickle

 

The PCB is "052119" and exposes a socket, which I assume could be for programming or diagnostics

Maybe my first step is to understand the protocol and pin out for this, and this may tell me if the NTCs are all sane and what the run parameters are