It is controlled by two temperature sensors one in the roof switches on when the inside roof temperature moves above 74F/23C, the other in the lounge switches on when the temperature in the lounge drops below 74F/23C. With both on, the fan runs.
Where did you get the switches? I will be doing something similar but can't seem to locate the simple snap action temperature switches suitable for this. You can get these complex and expensive user adjustable switches for the DIYer but I don't trust them. I'd rather just order the simplest switch available that operates at one temperature that you specify when ordering. Less to go wrong.