Yes. As regards figuring out errors just put loads of debug nodes everywhere so you can figure out what is going on. Often nodes don’t believe how you think they will.
As regards your main point: aswell as the msg that flows around there are two types of variables you can set. The ‘flow’ variable which is only limited to the flow page it is in and ‘global’ which is everywhere.
For example you could use a change node to set flow.BoilerON to the Boolean true when the boiler is on and a second change node to set BoilerON to the Boolean false when it isn’t. Then you can just check the state of that Boolean anywhere in a flow with a switch node on that flow page (eg. Switch Node set to if flow.BoilerON boolean true output 1, if false output 2)
The default is for the status of that variable to be saved in memory. That might be okay but you might want it saved to file. In that case you need to edit the settings file for the node Red docker.
This details how to save variables to storage rather than memory
https://nodered.org/docs/user-guide/context
There is a guide here in how to manage files
While you are editing the configuration file you could also turn on password protection so not everyone on your local network can edit your node Red.
https://nodered.org/docs/user-guide/runtime/securing-node-red
Just beware when it says password in the text it means the hashed version of the password (either done easily locally in Linux or using an online tool).