I have a cheap STEGE non programable WiFi (connection between base and unit) thermostat controlling my home heating. There are two units, one inside the heater, and the other with display and knob for setting desired temperature.I was curious if i could control this thermostat remotely with arduino. As the matter of fact, i do not even need to set temperature, just to trigger start/stop of my heater( since I am using underfloor heating, i just always turn it on/off)
On the back of the conrrollable unir there are 3 contacts, NO, NC and COM but i guess those are used for direct communication with base
???
I know i coukd alaways replace current thermostat and put arduino to be thermostat controling heater with relay, bur I wanted to change as little as possibke 
are you sure it's WiFi based ? I would suspect it's some sort of radio signal / protocol but may be not WiFi.
if you have input for a physical button, may be your arduino could actually drive that part
Thanks for a quick response, on the label it says it is wifi, but on the circuit board inside i saw e13-rf-tuya zigbee so i guess it is using zigbee protocol
I only have rotating knob on which you set desired temp, and if that temp is higher than current one, it sends signal to the base(i guess some relay there) and the heater is on
may be the WiFi is for connecting with your phone to the device?
It seems that's what the Stege SG 300 offers for example
which model do you have? do you have any doc?
Unfortunately, my model is even more simple 
My model is Stege WT 100 rf
Only manual that i could find was this one on Romanian 
I'm not clear which of the units you are referring to but the description of the contacts sounds like the output to control the heater.
As for WiFi did you connect it to your WiFi when you set it up? I suspect not.
Can you clarify the above points?
I suspect you'd be better off building your own controller rather than messing with an existing one. Do some searching on the forum, you should find examples.
The base and controller communicate via some sort of WiFi which is not visible by any kind of available to me scanner, there is a procedure to pair them. You set the wanted temp on controller via fhe knob, and after few seconds, heating is on. I am sharing the images of both tha base and controller. The image with the wires is the base which is inside the heater and connected to it. The other one is the back of the controller, inside the plastic these three pins say also no, nc and com
I know i probably could create thermostat on my own, but i do not feel so confident with AC with so many Amps
Thanks for the photos. The wires to COM and NO are what turns to heater on and off. NO means Normally Open, COM means Common. The way the wires are connected is dangerous because there is considerable bare copper showing, they should be re-teminated with no bare wire visible, the insulation should stop right next to the terminal.
They do not communicate by WiFi of any sort. WiFi is what you use to connect mobile devices to the Internet, not pair 2 devices like you have. They use some kind of radio link and probably some proprietary signalling between them.
As the communication is probably proprietary you are going to struggle. Possibly someone else here might have done something similar and have better advice, but I can't help much.
Your reluctance to build your own is sensible given your level of knowledge. The thing that's going to kill you is the voltage, not the current.
My final advice is to repeat that you must connect all the wires properly, or get someone else to do so if you are not confident, and to work on a different project not connected to the mains.
Maybe someone else can be more helpful.
As usual I'm late to the party 
I have a habit of reading this forum on a tablet & hate typing on it, then when I come back to my PC can't remember where I saw something I might comment on...
I recently acquired a cheap Chinese WiFi thermostat which seems very similar. In fact the base is identical, so I thought I'd chip in.
I agree that the OP should take more care with his wiring but it's not as serious as it looks. The base unit actually has a cover plate which fastens to the top of this thing but it isn't shown.
I'll explain my system. It may be different but suspect all of these things are from the same factory.
I have a controller & a base unit. The base unit is connected to my boiler. It's just a relay controlled by (proprietary/unknown protocol) RF. In my case at 433MHz, probably 868MHz for the OP.
The controller has a Tuya WR3 controller. This is rather like a ESP8266 but it also has bluetooth. The bluetooth is used to connect to a Tuya app in order to set WiFi credentials & connect to my network. The controller does the smarts. It has a temperature sensor, sets scheduling etc. Essentially it is the thermostat. It transmits ON/OFF signals to the base unit at 433MHz. There is definitely NO WiFi connection between controller & base.
It MIGHT be possible to hack the comms between controller & base if you have a storage scope & a lot of time on your hands! It MIGHT be that something like RCSwitch understands the protocol. You need time on your hands for this too
The OP has suggested that he doesn't even need the thermostat function. In which case I suggest ditching the base unit & replacing it with a WiFi operated relay (eg Sonoff?)
I will repeat your advice. If the OP isn't 'comfortable' working with mains electricity he needs to find someone who is.
Ian
Thanks for all thw good advices. In the end, i 3d pri nted the case, put arduino inside with the relay module. Programmed arduino, and a android app to be able to work as scheduler or as an on/off switch. At first i was a bit too careful, and thought i need a 16A relay, but looking deeper into the heater, figured out there are additional high current relays, and that the one I am controlling is just used as an input signal for those relays. So regular 10A relay module was sufficient. Right now i want to extend it a bit, and controll phases, or in other words to control if my heater is using 3, 6 or 9kW.i might in the end add thermostat just to control how many heaters will be working
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