Over the last couple of weeks I have been working on a solution to the timer for our heating as the timer with the heating system not working. The thermostat however does work when set to always on. The heating system in question is the Johnson & Starley J25-32 MK3 Modairflow.
I set about investigating how the thermostat works and found that all it has a direct 12V connection to the heating and the thermistor in the thermostat varies this voltage, with the heating coming on when the voltage drops below 4V. Knowing that it was a pretty simple control method I decided to look how I could control the voltage with my Raspberry Pi.
From looking over the forums for the Pi I came across a post about someone wanting to control a 12V train track. The process they used was a MOSFET with the control pin connected to a PWM pin on the Pi. I used the same process and connected it to the thermostat input to the heating. I also extended the original thermostat cable down to the Pi and used a relay to be able to switch between the original thermostat and my Pi control.