I've been looking to do much the same here in the USA. A number of years ago I worked for Honeywell, for about 15 years, designing and programming temperature control devices, from individual room devices, all the way up to campus level with a hundred buildings, from using assembler to object. When I started, I knew computers, I could write good code, I knew which end of the screwdriver to hold, but I couldn't tell a controller from a thermostat, and telephones required actual dialing.
I was also an instructor and traveled making contacts from NYC to Brisbane.
I'm a current paramedic student, retired LEO, my experiences have been varied. I can see applications for a Pi across those careers as well, and finally the prices for sophisticated devices are down enough to be reasonable for an inventor to dabble.
What I haven't done is my own home via Raspberry pi, so I'm in a new application of skills I thought I would never use again.
I'm writing because it appears some of you are making yourselves crazy over what you may not know is minutae...
Indoor temperature doesn't change very much.
Valves, relays, and control points can become expensive, especially if you wear them out or they get stuck in the opposing state. How does a 10 cent relay become expensive? I saw someone both trying to control heating in a zero °F environment and conserve power as electricity is expensive. If you send out a control change to a valve or relay often, you wear it out. For example, for me to send out a change in position to a valve, the change must be significant, not every time a calculation creates a change. If my valve has a range of 0 to 100, and my calculations based on temperature change means the valve would move from 50.1 to 50.2, I'm not sending that value to my valve to command it to change. I'll wear it out internally, or worse, it'll get stuck and not move at all, and worse than that.. it'll get stuck closed and freeze the room in the campus library above the 300 year old document storage...
... or the boss's office..etc.
For relays, they'll get stuck together. If sufficient power is going through them, they'll weld shut, if its low power, being toggled numerous times a day may produce oxidation between the contacts sufficient to mean ON is not on..the relay is in the correct position, may even show that there is voltage going through it, but minimal current.
If anyone is further interested, I can go further, and also suggest some algorithms, not actual code, to explain and assist further.
TEMPERATURE = SENSOR + OFFSET , for example, for every input.
Now I realize offsets may be unnecessary if they're accounted for elsewhere, and other such things I don't know yet as I'm at the beginnings of my Pi knowledge, but from what I have read in the forums thus far, I stopped being a JAFO and joined to say the above.
I hope the above can save you some grief.
"A wise man learns from the mistakes of others."