By the way, I looked into the postUpdate vs sendCommand differences.
What happens is this: when you use postUpdate, only the value of the item is updated, but the “command” (which it really isn’t) never reaches the binding. This results in an updated value in the UI up onto the point where the next evohome update is recieved and it is set to the actual value again.
Using sendCommand the value in the UI is updated AND the command is send to the binding, which, in turn, forwards it to your evohome. If that was successful, the next evohome update which ,of course, reflect that change.
I think this is due to openHAB and it is by design. This way you can update UI values without triggering any rules or bindings. I’ve used it to update the value of an item from within a rule where that rules was triggered by that same item. Works flawlessly.
See also: sendCommand vs postUpdate - #3 by Tango2, https://stackoverflow.com/questions/28981946/what-is-the-difference-between-postupdate-and-sendcommand-in-openhab