OT 3.0 Smart Power

This Forum is about the Opentherm gateway (OTG) from Schelte

Moderator: hvxl

OT 3.0 Smart Power

Postby rustyx » Sat Nov 09, 2019 11:52 pm

First of all, fantastic work with OTGW, Schelte, the level of detail and the quality of work is mind-blowing :shock:

Secondly, a small suggestion for the website: the matrix page is a bit wide. You can wrap the text part in a DIV like
Code: Select all
<div class="content">

And add a style:
Code: Select all
div.content { width: calc(100vw - 178pt); }


And finally, to my question - would it be possible to share the details about the OT Smart Power protocol? I noticed that the OTGW attempts to enable high power, but how does it do that, exactly? I guess the current sensing values would all be different, so not sure how to handle it in hardware, plug what message needs to be sent? I'm asking because I'm investigating the possibility of building a self-powered OT master.
Or, if someone can share the OT 3.0 spec with me, will be much appreciated!
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Re: OT 3.0 Smart Power

Postby Templar » Sun Nov 10, 2019 6:47 pm

As far as I know, the OT specs version 3.0 or higher is not published. You could start with reading this old topic: domoticaforum.eu/viewtopic.php?f=75& ... ent#p63761
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Re: OT 3.0 Smart Power

Postby hvxl » Sat Nov 23, 2019 6:07 pm

Thanks for the suggestion on the matrix page. I'm not sure where the 178pt number comes from. It seems to me that 165pt should suffice.


Based on reverse engineering and some bits of information I found online, I figured out the following approximation of Smart Power:
  • Low power is the normal situation, as described in the OT 2.2 documentation available elsewhere on the forum
  • For Medium power, the output from the OTGW to the thermostat is inverted. So when the line is idle, the line current is 17-23 mA, instead of 5-9 mA.
  • For High power, also the input from the thermostat is inverted. This means that the voltage on the idle line is 15-18 V instead of 8 V or less
The thermostat indicates that it wants to use one of the raised power modes by inverting its output for more than 5 ms. At that time the OTGW inverts its output. If the thermostat keeps its output inverted for more than 20 ms in total, then the OTGW interprets that as High power mode. If the thermostat returns its output to normal sooner than those 20 ms, the OTGW will operate in Medium power mode.

To exit High power mode, the thermostat just returns its line back to low for more than 5 ms. For Medium power it must make the line high between 5 and 20 ms.

Of course, a thermostat should not try to use Smart Power until it has received confirmation from the slave that it supports Smart Power (MsgID 2 HB0).

Note: The timings are just observed values which may not match the official Smart Power specification. But this works with a Remeha iSense (which was graciously lent to me in order to investigate smart power).
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