How to connect Sonoff-SV to replace non-smart wall thermostat to control gas heater?

[Andreas Krienke]

Hi,

living in a rented flat with a gas heater and a central, non-smart wall thermostat, which is connected to the gas heater with 2 wires, I would like to replace this central thermostat with a Sonoff-SV unit. But I am unsure how to connect it. The two wires are as far as I know just to close the circuit and give the signal to the gas heater to fire up. The wall thermostat is a very old one (https://www.heiz24.de/index.php/de/Siemens-REA11.01101-Raumregler/c-KAT43/a-Y6927).

I have read some hints to use the Sonoff-SV as a dry contact, but I really do not know how and what to adjust and how to connect?

Could someone guide me here to get this up and running?

Gas Heater is a Esprit 20 E, the wires are 24V.

Thanks a lot.

[Philip Knowles]

The thermostat will be a volt-free contact. You need to put the SV into 'isolated' mode. The Sonoff SV web page shows how to do that. Once that is done it should be fine.

Regards

Phil K

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Sent: Sunday, December 22, 2019 11:58:17 AM
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Subject: How to connect Sonoff-SV to replace non-smart wall thermostat to control gas heater?

 

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[AKS-Gmail-IMAP]

HVAC control voltage is AC and when your volt meter reports it at 24 V AC that is the RMS average. It peaks above 24 volts. The Sonoff SV takes DC voltage according to its specifications. The Sonoff will be seeing more than 24 V DC if you try something like a simple rectifier circuit to get DC. Perhaps it will fry. You need to provide a small power supply to convert the 24V AC control voltage to a DC voltage within the SV’s power input range. 

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[Philip Knowles]

Yes. It wouldn't be possible to use the thermostat supply as a power supply anyway for the SV as there wouldn't be a neutral or 0V just a signal return.

What I was describing was how to isolate the output of an SV to make it volt-free.

Bear in mind that the SV doesn't need to be in the same place as the thermostat. The thermostat could be replaced by a smart temperature transmitter that could even use the existing wiring (disconnected from the boiler) just to power it and the SV could be connected to the thermostat input at the boiler controlled by the temperature transmitter.

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From: AKS-Gmail-IMAP <akse...@gmail.com>
Sent: Tuesday, December 24, 2019 12:55:50 AM
To: Philip Knowles <knowles...@gmail.com>
Cc: TasmotaUsers <sonof...@googlegroups.com>; Andreas Krienke <sha...@gmail.com>
Subject: Re: How to connect Sonoff-SV to replace non-smart wall thermostat to control gas heater?

 

[George Ioakimedes]

That's actually not technically true. Thermostats can use what's called a floating ground reference. 1 leg of the AC is referenced as ground and the other leg gets rectified by with a half wave bridge. The earlier response was correct that this rectified voltage will be higher. If you have 24VAC the transformers typically put out closer to 27VAC. A half wave bridge will result in 27 * sqrt 2 = 38VDC and depending on how much capacitance is after the rectifier there will be ripple on this 38VDC so any chip attached to that could see even more than 38V. The next problem then is converting that high voltage down to 3.3V that the ESP uses. This can't be done with a LDO because it would have to dissipate too much power (38V - 3.3) * 0.300A (peak consumption of the ESP during WiFi) which is over 10W.

I tool a quick look at the Sonoff-SV schematic and they have a switching regulator at the input which solves the heat problem but many switchers can't handle an input voltage above 36VDC. I didn't see a part number for the switcher so I can't tell what it can handle. I saw a mention of this accepting AC input voltage but I didn't see that on the schematic I looked at.

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[Pila]

I recently Tasmotised my new gas boiler. Standard minimal wall thermostat is one wire going from the boiler to the thermostat and back. My was 230v.

I used Shelly 1. First tested for some time replacing wall thermostat with it. Then open the boiler and connected it inside. Thermostat is now connected to Shelly as a switch set to follow.

In such a minimal setup use the coldest room for reference. TRVs may create a problem if they start competing with the thermostat.