The Idratek module is now connected and working. It fails safe on HW on, so in the event of the network going down, the hot water will come on.
What I am now attempting to do is to sense whether the pipework from the boiler is heating up when the HW system calls for heat. That way, if there is a problem (boiler hasn’t fired), the immersion heater will switch on instead, and generate a warning.
After some discussion on the Idratek forum, it made sense to monitor the output of the grey wire on the 3-port valve. This goes live when either the HW is off, or the HW is on but the cylinder thermostat has reached its temperature and is therefore not calling for heat. Logically, if the above is false, and the system is calling for heat to heat the hot water, then the pipework should be getting hot.
My first challenge is to sense when the grey wire is live. I have added a neon indicator (240v) to that wire and neutral, which will be on when the HW is either off or not calling for heat.