[buccal]

Cool project.

Speaking of newish natural gas (CH4) heaters, they all should have modulating thermostat capability with OpenTherm/eBus or other protocol. Combined with a thermostat with outdoor temperature sensor system efficiency is increased a few percent and that should help offset thermostat and installation costs. In the end you have more efficient modern heating system.

Same should apply for heat pump systems.

[steelegbr]

OpenTherm is a cool idea but even new installations aren't always wired for it. When installing a new smart thermostat I found the installation has been wired as S Plan with the few cables running between the boiler location and valves location already consumed. Makes the job much bigger if you're not prepared for it.

[buccal]

There are many wireless OpenTherm thermostats. Wired is better but is not required for modern thermostats.