I have expanded my Idratek network sufficiently that I can now move to controlling the heating and hot water system. I will be trying this over the next few days, using a QRH Din module with a changeover relay for the HW and NC relay for the heating.

I had a boiler failure a few months ago (only needed resetting, but meant no hot water in the morning!) and needed to use the immersion heater. Therefore, incorporated into this new setup is my immersion heater on a relay contact. I've written this up here.

Here's my plan - will it work?

My heating system is Y-plan (3-port valve). When the boiler/pump are on, the orange wire from the valve is live (240v). Therefore if Cortex expects the water to be heating up (eg 5.30am) then there will be 240v at this terminal.

If I run a neon indicator from this terminal (and neutral) and then use a BPW85C opto transistor (Farnell ref: 1045524) (as in a previous post regarding electricity meters) to sense whether the neon is on, and feed this into the input of the QRH, will this work and be sensible?

I have put a pipe thermostat on the flow pipe to the HW cylinder, and this also goes into one of the inputs. Therefore, if it's between 5.40am and 6.00am and the opto transistor detects light, then the pipe thermostat should be hot. If it is not, then Cortex should switch on the immersion heater and generate a warning for checking later.

Logically this will work, but of course there could be other failures. If the motorised valve fails, there might be no current on the orange wire, and therefore still no hot water! So I wonder if I should be using two neons and opto transistors, on both the grey and orange wires. Here's why:

Grey would be live if it is not calling for heat or if the temperature had already been reached. Therefore:
If HW on (Cortex) and Grey live, then temp reached and no trigger (END)
If not above, then If HW on (Cortex) and Orange live, check for pipe hot. If not hot, turn on immersion heater and warn.

Any thoughts?