Thermostat for two separate systems

Hello, hopefully someone can help me out with this confusing world of smart thermostats. We have two separate systems. We have four total thermostats in our house. Three of them are solely used for boiler heat and only have two wires running to them. The fourth thermostat has four wires (no C) running from the air conditioner/air handler unit to it, as well as two wires from the boiler.

I currently have a mixture of WiFi and Zwave devices in home assistant so I would prefer to stick with one of those. I would like to replace the thermostat that controls the air conditioner and boiler zone first as it is still a mechanical dial whereas the other three boiler thermostats are digital, albeit not smart. I could probably run a new 5 wire from the air handler to the combo thermostat to include a c wire on that one but the other three would be nearly impossible to get a c wire to from the boiler. What would be my best option for replacing the combo thermostat and what would be a good replacement for the three two wires? I’m assuming those would have to be battery powered. I wouldn’t mind not running the new wire to the other if it can be battery powered as well. I’m looking for something that would integrate easily into home assistant.

Smart thermostats not require presence in controlling room. Maybe place thermostats at boiler/AC and not need run wire.
Remote temp sensor in room to monitor.

There are prebuilt wifi units that allow remote sensors.

I would likely get zwave switch and zwave temperature sensor and use HA to control the OFF/ON. This may ultimately be cheapest and easiest. Just make sure you have monitor for “Fail turn ON” and “ON time greater than xx”. Maybe even have (2) safety for ON and OFF.

I had exactly the same situation. One heat/cool thermostat for a boiler and air conditioner (separate systems.) Two other heat zone thermostats.

Unfortunately, I started updating that before I had HA. I ended up having to run a “C” wire to each stat. Like you, the heat/cool stat was easy, the other two, not so much. But well worth it.

Now, all my thermostats “phone home” to Honeywell, as does HA. I can control them through HA, although I can’t change the daily schedule from there. I can do everything through the Honeywell app or web page, even if HA is down or not communicating for some reason.

Remember, HA is still in development - not yet release one.

My only problem with my setup is the dependency on Honeywell. It’s a personal control thing. Been burned by too many vendors before. But Honeywell isn’t likely to go away in my lifetime, so I’m sure it’s fine.

That said, running the whole thing through HA, no remote middle-man, could work too. The technology needed for that might get complex. For example, controlling multiple zones from one thermostat, with remote temperature sensors, gets you into commercial-grade (read: expensive) products. Setting up daily and weekly schedules for heating gets pretty deep into HA automations, which takes a bit of skill and experience.

I paid under $300 total for my three Honeywells, and they’re even cheaper today. They worked, right out of the box, once I strung that C wire. All the complex scheduling logic is all GUI-fied for me. I have a backup way to access my stats if HA fails. I can’t really complain (although I still do…)

Which Honeywell model are you using/recommend?

Does it allow you to program the fan to turn on to cycle air manually without turning on the ac?

Trane XR524 is good zwave unit.
It require c wire but work without issue

I’m using the Honeywell RTH6580WF, mostly because it was the most inexpensive “smart” WiFi thermostat that Lowe’s sold at the time. I’m sure there are others, but this one does everything I need. My only beef is that I can only access it through Honeywell’s servers, but frankly that’s a personal thing. I’m probably better off having their system as a backup if HA fails. I do wish they’d give me direct access to the detailed operation data they store about my thermostats, but again, that’s just me.

I am able to run the fan without cooling, via a button on the stat, the Honeywell app, the Honeywell web site, or HA. I assume I could automate that in HA, but I haven’t tried.

I was just playing with that to confirm, and I can even turn the fan from “Auto” to “Always On” when the thermostat mode is “Off.” I did have to configure it correctly so it doesn’t turn on the fan when it calls for heat, and the options aren’t obvious at first.