Thanks! SAT (Smart Autotune Thermostat) is a free integration you can add to Home Assistant, which can work with your existing thermostat but the intention is that it replaces it altogether (SAT becomes your virtual thermostat). There are many more features but it's best to head to https://github.com/Alexwijn/SAT for details.

OpenTherm is a standard communications protocol which SAT uses, however, you need this hardware:

https://www.nodo-shop.nl/en/featured/211-opentherm-gateway.h... - board

https://www.nodo-shop.nl/en/our-products/213-wemos-d1-mini.h... - WEMOS D1 MINI

https://www.nodo-shop.nl/en/solder-service/202-soldering-ser... - soldering service

It costs €29.50 + €6.25 + €7.50 soldering service + delivery , so that it comes as a ready kit for you.

You also need an indoor temperature sensor. You can get a zigbee device, such as - https://www.zigbee2mqtt.io/devices/WSDCGQ12LM.html

For outdoor temperature, Home Assistant will cover that out of box.

The installation is usually very simple, you connect two cables to the boiler and its OpenTherm module in the boiler and then it uses MQTT to connect to Home Assistant via WiFi. If you are not comfortable touching the boiler, ask your plumber to help you.

By the way, that hardware is much cheaper than Hive which does not support OpenTherm at all. Google Nest does but is so extremely limited, and from my experience, it was cycling too often and did not autotune which is the main reason why you want SAT.

If you check the manufacturer of your boiler and its model, you should be able to find out if your boiler supports OpenTherm.

This is new build~ (2017) and the heat recovery system was installed when it was built but we are not the first owners of the house and personally, I would probably not try to retrofit such system.