The "Window open" function can only be switched on or off, the default value is 3 minutes, this cannot be changed. According to the information in the help, all thermostats in the affected room are switched off. I have not tested it yet.
I operate 20 valves in an old house with thick walls and 10 rooms on 3 floors and this works with only one relay. But I had to test several sockets until I found one where the relay could connect to all the valves.