That's what I do at home. I have little Xiaomi LYWSD002MMC and LYWSD003MMC devices (the latter with custom firmware - pvvx) and my radiators are switched on or off (via their pilot wire) by an ESP device, using a few rules to lower the temperature when nobody's there. It works much better than the radiator's own thermostat.

I control my devices (I have a lot more than just for the radiators) through a kind of interface that I wrote myself (PHP and only very few, reliable dependencies) because I hate maintaining Home Assistant.

I have one over complicated in floor heater that requires 2 signals to control, one for opening radiator valve and second to enable fan to blow air through it. there is only few thermostats dedicated for such setup that are extremely expensive and I decided to make my own that is also better. i have 2 temperature sensors connected, one to monitor room temperature and the second touching radiator inside, then I'm able to open valve when room temperature is below threshold and wait until radiator is hot enough and we are trying to heat for few minutes (without fan this will also work but less efficiently, but will be silent) then when temperature is reached I close valve and keep fans on until radiator cools down.

I also have temperature reported to home assistant where I have pyscript automation that controls AC based on multiple temperature sensors, open window sensors, humidity, presence, time, etc to most efficiently cool down my apartment

if you need to get signal on a wire, you can output it from the same device, from other esphome device or through home assistant using integration.

Actually I did exactly that :) I use ESPHome to control a gas boiler which heats my home. It's not without problems and required a lot of tinkering, but after the initial phase it required very little maintenance throughout the winter. This year I plan on adding an ability to control AC. https://news.ycombinator.com/item?id=40017176

I use this one and it works great: https://github.com/kbx81/ClimateSprinklerController

I have 7 Mitsubishi heat pump head units being controlled using ESPHome running on D1 mini clones (ESP8266). The D1 mini clones are powered by and interface with the head units with Mitsubishi's CN105, which is just 5 volt UART.

The total cost was maybe $30 of parts on AliExpress.

I use 433Mhz Acurite temperature sensors with a software defined radio (rtl433) running on my Home Assistant box to have remote temperature sensing. The 433MHz sensors are cheap, have good range, and have excellent battery life.

Are you doing temperature control by yourself or are you feeding them to the Mitsubishi units as a remote temperature reference?

I know the Mitsubishi "wired controllers" (basically the official thermostats) can provide remote temp to the unit and the unit has DIP switches to select between thermostat-reported temp and internal return air sensor temp.

I'm not sure if CN105 has a way to provide this temp ref - if so, you could try it. Just make sure to set your wired controller (if any) as a "secondary" controller (otherwise it will also send its temp every second and overwrite the one you sent) and then set the proper DIP switch.

If your HVAC supports OpenTherm, check out https://github.com/Alexwijn/SAT

You can have your thermostat in home assistant and make your HVAC run as efficiently as possible at the same time.

See https://news.ycombinator.com/item?id=40052824