Trying to creating a smart thermostat (TRV) based on classic mechanical thermostat

Electronics
1

Hello everyone,
currently I’m trying to make the heating in my home more smart. For this purpose I’m using HomeAssistant with ZigBee Hydrometers and smart thermostats (TRV) on my heater. It’s working, but the majority of the smart thermostats have one large problem in my opinion: They always need electricity/batteries to work. Because of this, most vendors implement a non optional functionality which are opening the thermostat valve automatically when battery is running low. So when this happening the heater is permanently heating, independent of the room temperature. The reason behind it is to have a freeze protection, but this might become a problem when beeing away for a longer while, for example 2 weeks vaccations. So when the battery is running low, smart thermostates are far away from beeing smart.

Then I was thinking about classic TRV thermostats. I think the way they work is very nifty, especially because it is working without any electrcity, so the anti freeze protection is always working. As a short explanation the classic ones work like this:
You can usually set a target level with turning the handle between freeze protection mode up to 5. Every level is representing a specific target temperature. Inside the thermostat is a material which is automatically expanding and shrinking depending on the temperature. So when it get’s warmer, the material is expanding and automatically closing the valve.

So my idea is: Why not combining the already very clever behaviour of classic thermostats with the advantages of newer smart thermostats. So now I’m searching for a way to control the level options of the classic thermostats via my central HomeAssistant device. So for example when I want to have the target temperature at 20℃ at evening I want that the handle automatically move to level 3 (which is representing 20℃).

I have some experience with ESP devices, which can be implemented within HomeAssistant and I’m also able to code. But I don’t have any idea yet how I can implement the mechanical movement of the handle in an efficent way.

Classic thermostats I’m talking about are looking like this:

thermostat-gfceaabe78_1280
thermostat-gfceaabe78_1280924×1280 311 KB

Does anyone here has some experience with similar projects and some tipps for me and my project?

Best regards and thank you

2

Do you essentially want to be able to remotely turn a thermostats control knob to a specific setpoint?

3

Exactly, that is what I’m looking for. And when the battery is getting low I would automatically let the thermostat move to “freeze protection” level, which will then still work when battery is empty fully.

4

My first thought is to use a stepper that is connected to the knob or to the knobs shaft.

5

I was going to suggest hooking up two thermostats side by side at the same time, a mechanical that would just remain set for freeze protection, and a smart that would handle the day to day operations. But if the smart thermostat switches the heat on when its battery gets low that wouldn’t work since the mechanical would never operate.

6

Mmmm would it be better to just fix it so the smart thermostat battery never gets low?

You’re going to need power to remotely control the manual thermostat anyway so I assume you will have mains power?

Build a DC supply for the smart thermostat.

7

What a relative has done on the East Coast, which sees power outages often, is he left the mechanical thermostate at it’s normal wired location and set it to 50 degF as the backup for the remote battery powered ones. His are not mechanical like yours so only using the mechanical thermostat as a backup applies to your situation.

If you were to make your own battery powered thermostates and could change the default to not turn on when power failed you’d be good to go. Otherwise, what Don said has validity. If there is a wired thermostat control at each site, they often have 24VAC there and I have rectified that to power my own thermostat with just some diodes and a couple of capacitors. If that’s not an option, maybe a small solar panel mounted on the wall or put in a window and use rechargable batteries.

8

Solar is a good idea!