Using Raspberry Pi 3 and Domoticz to Replace Euroster 2006 for Junkers Euroline Boiler

Hello everyone,

I'm planning to do a small project using a Raspberry Pi as some sort of base to create a 'smart' home. I am a beginner in the subject of home automation.
The aim is for the Raspberry Pi to replace the Junkers Euroline gas boiler controller (currently a simple Euroster 2006 controller is fitted) and to allow on/off based on temperature values and manual on/off (via a browser interface or preferably an android app).
After reviewing posts on various forums I decided to use the popular Domoticz software.
I currently own, (or are actually on the way to):
- Raspberry Pi 3 model B + case + heatsinks + fan
- A Xiaomi bluetooth temperature and humidity sensor (the version with the display), which can supposedly be coupled with Domoticz

As far as I can see, the Euroster controller works very simply - it short-circuits and opens the circuit. Therefore, the pump operation, nozzle modulation, etc. will not be controlled. So it's not the pump operation, the nozzle modulation etc. that will be controlled (this is taken care of by the boiler electronics), but the sending of the heat demand signal. Consequently, I definitely need some kind of contactor / relay that will be managed by the Raspberry Pi. Hence my question:
- What contactor / relay should I purchase to be suitable for the Raspberry Pi and to work with Domoticz?
- Are there any other components I will need (not counting the power supply for the Raspberry I have) to make such a project ?

I would suggest DS18B20 temperature sensors because RPi supports them natively on Raspbian. They are hardwired, but if this is a problem you can connect them to a cheap WEMOS on ESP8266 and ESPeasy soft.
I would suggest giving relays like those for the Arduino are available with optoisolation. You can also build your own, but you need to remember that the RPi logic is 3.3V and the maximum current per single pin is 16mA, so you can't do without additional transistor control.

@xury Thanks, just started to find out that most relays need a 5v signal. But I found something like this:
https://elty.pl/pl/p/Modul-z-4-przekaznikami-z-transoptorami/669
Do you think this will work giving additional power from Raspberry to 5v? In theory they write that it's compatible with 3.3v systems.

Will you be controlling any pumps?
The surges created when they are switched off can cause interference. This opto-isolator should help but remember from separate power supplies....

@ditomek No, the boiler sort of has a pump built in and controls it itself. I won't cut into its competence .
In the sense you mean that for the safety and stability of the Raspberry this relay should get its current from another power supply, and from the Raspberry only the signal for on/off?
I'm thinking of leaving the old controller as a fallback in case, for example, the Raspberry hangs up while I'm away from home.
Then it would look like this:

schema..png Download (3.3 kB) .

And if the raspberry hangs up with the relay in the open position, the old controller could prevent the flat from getting too cold. The other way around probably won't work (as the raspberry hangs up with the relay closed), but better a fish than nothing.

Gentlemen, does each boiler controller operate on an on/off basis and the rest of the furnace measurement/operation is sewn into the furnace electronics?

Not everyone. There are some more modern inventions out there that also control flame modulation etc.

I'm going to bump up the topic a bit. My cousin has a valliant gas furnace and a controller for the furnace. He has an outdoor temperature sensor included in the package with the controller. When he had it disconnected, the cooker worked on the on/off principle, i.e. it heats the water up to, for example, 70 degrees and switches off. Now that the sensor is connected, the cooker switches on more frequently, but apparently uses less gas and has a lower output water temperature. The question now is whether this can be controlled from the RPI/domoticz via modbus or whether this function is performed by the controller.