The cable runs through a metal tube with 230V cables.
(a lot of interference and spikes)
Alternatively I could use optocouplers or a digital potmeter I poll every minute for the temperature and a pulse that triggers every hour to keep the time.
Here in the U.S. that would be an electrical code violation.
What does that mean?
And the other end goes to (or can go to) the heating/cooling system?
POE sounds like a reasonable solution, but what's the network connection for? Just to get the time? Obviously, if you use the wires for networking you can't use them to control the heating/cooling system so you'll have to build something else to do that.Where is that pulse coming from? (I don't see how a digital potentiometer fits into the picture.)