Smart Room Thermostat replacing single "SIEMENS REA 22" controlling gas heater

Hi all,
I am still in beginner phase of home automation and just have OH2 running on my synology diskstation with 3 TP Link wallplugs controlled.
Now I wanted to go next step to heating automation. For that, I have here in rented flat a single thermostat SIEMENS REA 22, which is controlling the gas heater and thereby the information about heating based on its internal sensor (I guess). I want to replace that in order to use smart thermostats on the different heaters as well, but I am unsure with what device to replace the room thermostat first, in order to get the gas heater heating request still to work correctly.

If possible, I would like to have a solution which is not “communicating” home via internet and works just in my local environment. So solutions like Tado (which was used by a neighbour already successfully) is then not an option.

Could you guide me please to some possible devices, solutions which could be used for that?

What type of heating is installed? Water floor heating or radiators?

Hi, there is the central gas “heater” (Gas-Therme Esprit 20 E), which is heating the water also for each heater in the room. And as I said the one only room thermostat “controlling”, when the gas heater should run to heat the water circle. I have seen that it is only connected with two wires into the wall, but I have no clue where they are going and if this is directly connected to the gas heater. :frowning:

Do the heaters have thermostats? In this case home automation thermostat would be an option.

Anyhow, I assume you’re located in Germany. Since your heating is operated with gas and you rent the apartment I recommend to discuss with a lawyer which liability you are facing if you manipulate a gas heating.

Close, it’s Austria :wink: But anyway, as my neighbour did it with Tado system and replaced the central thermostat (same heating system in both flats), I would assume this is not impossible, without modifying the gas heater itself at all. (this would be my last option, as Tado needs cloud connection which I would like to avoid if possible)
The heaters in each room have normal “old” thermastat, so I guess any new smart thermostat there would fit. My main “understanding” problem is, that each heater has no “control” to communicate with the central gas heater, as the central wall thermostat is the only “thing” sending the request of heating to the gas heater.

You can replace that with a sonoff TH flashed with tasmota. See:

For each room control then it gets a bit more complicated but I use this:

You need a temperature sensor in each room that reports to OH.
Rules for opening the valve and turning on the boiler by sending a command to the main boiler sonoff

Hi, sorry for the late reply, did not found time to answer before.
I have checked your reply regarding the sonoff solution which sounds quite good. However, in the example given the thermostat is powered with 230V, while mine is connected with two (blue) wires to the gas heater with 24V only. So question is, can I use the sonoff basic still here? Or is it only intended to work on 230V? I have one Sonoff basic r3 module, but its mentioning also only AC 100-240V printed on it?

While checking the room thermostat, I figured outs its an ever older model REA11.01 (example here:
So my question would if someone can give me a hint how to proceed here? Also, there are a lot of “smart room thermostats” on amazon, but I did not found one which can control the 24V cables to the gas heater.

What does that 24V cable do?
does it provides power to the thermostat?
Does it need to be switched to activate the boiler?

Actually, there are two cables, but as I do not really know how the wall thermostat is working (“closing the power cycle”) when heating needed, I am unsure about that. I have only found that both cables are also available directly at the gas heater, with additional high voltage cables (230V) for the gas heater itself. :frowning:

Good, One is 24v supplying power to the thermostat
The other is the “call for heat” wire. When the circuit is closed then the boiler fires.

You can chage the 24V wire to 240V to power a Sonoff and wire the call for heat to the RELAY
BUT BE CAREFUL the sonoff in it’s normal configuraation will forward the 240 to the call for heat wire into you boiler circiut board and likely fry it.
You will need to modify the sonoff as a dry relay
Like this:

oh, I thought I could bring that to life without “manipulating” the sonoff :frowning: Don’t get me wrong, as I want to play around with the things, but I was just thinking (dreaming) of something like that:
This seems to work on low voltage (5-24V) and may be the solution. Can this maybe integrated into OH via WIFI? Or am I completely thinking in a “impossible” direction?

Or is maybe a shelly: a better solution?

You could use the Wifi IoT SPDT Relay Board V2 - ElectroDragon
It has dry relays so no need to “manipulate”
You can flash Tasmota and add temperature sensors
I even added an LED display at one point

I don’t know if the shelly can be configured as a dry contact of if you can add a temperature sensor to it

My current thermostat seems to be powered by batteries, not from the 2-wires. So this means, that I anyway need some “power-suply” for an additional relais, either the sonoff, the shelly or the eMylo, right?
Assumption: Cannot get power from the 24V wires as these are only “signal” lines? Is this correct?

I think the batteries are there only for back up to keep the memory in case of power cut
You need to get your multimeter out and start testing these wires

In a “normal” config you will get a constant 24V from the boiler to power the thermostat and a return pait to call for heat

Thanks for pointing out, testing brings up that you are right :wink: I removed the front part of the thermostat which contains the display and the batteries.
I have measured 23,7V (as DC) and 50V as AC. Is this correct? Does this mean I could use these wires for power-supply of a “relais”? Because then I am confused how to “connect” these wires? As my assumption is right now, that the thermostat is “somehow” closing just the power circle for a heating request? Or how may the heating request be send with these two wires?
(Again: Sorry for my beginners questions, but I really want to understand how this is working).

I am just bringing another device in: May this sonoff SV be the right solution: Also this does not ship with a case.

You said you had 4 wires…

This could be good for you. I didn’t point it out because it doesn’t come is a case

Sorry for the late reply.
Yes, there are only “2” wires coming out of the wall, so sorry if I explained that wrong. They are connected into the old thermostat on “L” and “L1”. If you compare the pictures from this link ( you see on the last one the brown “socket”, there the two wires are connected.

I have bought now a Sonoff-SV and flashed that with latest Tasmota firmware. Also figured out how to connect to OH2 via MQTT (restarting OH2 was the key to get the binding working).
So my next question would be how to connect the Sonoff-SV on the 2 wires?