Sorry for asking again, but can anyone guide me with help on next steps regarding connecting the sonoff-sv on these 2-wires only? Is it intended to just “shorten” the two wires for a heating request to the gas boiler? Or is there something else? Also, will this be possible with the sonoff-sv or do I need to manipulate it as “dry contact” as well (like mentioned above)?
That’s it. The sonoff-sv is dry contact
saying that, means powering the SV is independent from the circuit it controls?
So, how should I connect it then best? The 2 wires into the ingoing connections and the shorten the 2 outgoing connections on the board? I have also seen a different approach where someone removed the resistors, that’s why I am unsure how to do it.
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Dear Shapoc,
i know it is an old topic but maybe…
I have the same problem now what you had in the past. I have a boiler with 2 wires 24V control old thermostat. I’d like to do a smart heating system with Sonoff SV. Please give me advice, how can I connect the 5-24V relay to my boiler? I planned I connect the 2 wires from the heater to the relay input and I do short circuit with a short cable at the output. It is the correct way? How did You do?
I bought this SONOFF relay:
Thanks in advance!
Regards,
Bubba
Hi Bubba, unfortunately I haven’t finished my project last winter, so I am not able to give you a 100% correct answer. But your approach is exactly what I had understood and I would go same way you described.
If you are faster than me and successful, would be great to here if it works the way described. thx.
Regards
Hi Shapoc, thanks for your answer! I asked some specialist and I am afraid the solution what we thought not fully exact. 
It is best for the boiler if the relay just shuts off the sonoff, but the boiler’s current and sonoff’s current are completely separated. Our relay maybe OK but we need a little modification. We need “dry contact” solution. Figure 3 on the picture.
Easiest way, to buy this switch: https://www.itead.cc/smart-home/sonoff-re5v1c.html
Here no need modification. It works immediately but unfortunately a 5v supply is required.
Regards
Hi Bubba, thanks for pointing this out. That’s even the exact needed device you mentioned! I will try to order one as well. As it is also flashable with tasmota, this should be good to give it a try.