At least 2 wires are required for data transfer.
Never use power line wires for other purposes, or you risk that your devices will burn as soon as somebody will establish proper power distribution.
Or treat it as data-over-powerline so the Arduino doesn't know or care if there is mains voltage on the wire.
Since these things do not work too great,
Please provide a complete circuit diagram, your description is only confusing.
First thing you need to do is document that with numbers - changes in outdoor temperature, changes in room temperature, response to thermostat settings etc - over two or three weeks. Keep in mind that underfloor heating will be slow to respond and that has nothing to do with the type of thermostat.
I suspect it will not be as easy to get an improvement as you may think.
If the present system is old you may be able to buy off-the-shelf improved control units. Don't be fooled into thinking you can do it cheaper if you put even a minimum hourly value on your time.
PPS. Make sure you can put the system back the way it is now if your project does not work.
PPPS. Check with your home and contents insurer and your landlord if DIY heating controls are acceptable. If they aren't you will have to pay a professional to install an acceptable system before you can sell the apartment.
It's an ambitious project
such as we have no idea about the current you need for the motors and actuators or the signals involved so it's hard to suggest any sort of switching.
My dad's house has underfloor heating but yours seem extra complicated with zones and such. At the very least you can improve your system by putting in better zone sensors... I'm not a fan of bimetallic sensors.
I suggest you build a test system where you can try out the scenarios and test run everything so you wont have frozen pipes one morning.
- I need to switch AC in the low milliamp range, but reasonably often say, 2-4 times per hour, 24/7. Any suggestions on an SSR that works with these currents? 3.3V preferred, but I could low-level switch around that, I will send 5V there anyway and regulate down for more stable supply
This is my apartment. There is no landlord. Assume that insurance is fine.
There are a couple factors involved here... I am guessing again but you must be talking about the switching current to actually initiate the "gate"?
Please keep us updated on the progress, perhaps we can better help with individual challenges you face.