I bought a Honeywell L641 pipe thermostat from eBay for almost nothing – and although it came with almost nothing, it does its job. It was supposed to have a cable clamp, two springs to attach it to a pipe, and a protective cover to stop anyone altering the dial. Anyway, a cable tie clamped it to the pipe, and it’s only operating on 5v!
This device is tied onto the flow pipe to the hot water cylinder, just after the 3-port valve (a Y-plan system). I’ve set it to about 40 degrees.
Now Cortex knows whether or not the boiler is heating the hot water tank.
The next step is to wire the relays in the QRH module to control the hot water and heating, then for Cortex to monitor that the pipe thermostat has sensed hot water, and to operate the immersion heater relay if the pipe does not get hot.