Quote Originally Posted by Chkadii View Post
At risk of derailing the thread (though it looks like OP made their decision), here's a diagram I created at work because I'm a huge dork:

Hahaha, that's brilliant. I'm a huge dork too, no judgement here. Thanks for the explanation/visualization!

So if the goal is to have the backup T-stat on on/off mode, could you get away with using an on/off T-stat rather than a proportional one? Just for the backup, not the main T-stat.