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I have a Bosch 35 HRC gas-fired central heating unit connected to a Tado (opentherm) modulating thermostat, and I'm considering a future replacement with a heat-pump. One of the considerations for the type and size of the required heat pump (or alternatively "hybrid" gas/heat-pump options) , is the degree of insulation and radiator / floor heating capacity: to which extent is a low-temperature (say, ~45 degC) central heating system sufficient for my house on cold winter days ? In my case, I can't be quite sure. A common advice is then to try out different water temps, gradually colder, and test in practice whether it's comfortable enough on cold days.

I use Home Assistant (and Zigbee devices) for various home monitoring purposes (including the heating of the different rooms), so I would like to add the central heating water temperature to my Home Assistant dashboard. Although most (Zigbee) temperature sensors are not really suitable for measuring the outer surface of the tubes, I thought there must be an affordable solution for that. And while I'm at out, to get a more complete understanding of how the heating unit is functioning, I thought I might as well monitor both incoming and outcoming temperature, but then, really I would also like to see the heating power, heating status, etc. etc.

So, as an alternative to buying analog sensors, I thought maybe I should monitor the heating unit digitally, for example through the Opentherm connection.

I want to keep it cheap and simple though, because it's only for a temporary 'research' study.
Googling for solutions, I came across the OTGW.

Question: can anybody advice if this is a good solution for my purpose ? Or give more general advice to my question ?
I have a ethernet port in the proximity of the heating unit, so I could use the ethernet variant.

You may be rather unlucky with your boiler. Looking at the details in the equipment matrix, this boiler doesn't seem to provide much more information than the bare necessities. According to the log that someone was kind enough to upload, it does not support the return water temperature (ID 28) and reports no information about the current modulation level (ID 17). It also does not support reading or changing the max CH water setpoint via opentherm (ID 57).

Fortunately the boiler water temperature (ID 25) information is mandatory. So you would be able to obtain that. But the possibilities of using an OTGW may otherwise be extremely limited. Of course this is based on a single log file that was uploaded for the HRC 35/2. You may also want to look at the details for the HRC 30. They appear to be the same device, except for their heating capacity.

The HRC 30 logs show that there may be different controller boards for the boiler. Some logs report a member code of 18 (ID3:LB), which suggests it was made by Theben AG. another says 131, meaning Bosch Thermotechniek B.V. The latter does report return water temperature and modulation level, according to the log. Contradictory to what the remote parameter transfer flag (ID6:HB1) indicates, it does report the max CH water setpoint. If the remote parameter transfer flag is incorrect, I also dont trust the remote parameter read/write flag (ID6:LB1). So you may or may not be able to change the max CH water setpoint via opentherm.

I have no idea how to tell which controller board you have without an OTGW. Perhaps there is some indication on the board. If you see anything that says Theben you probably have the most useless of the two boards.

Disclaimer: This information is all based on logs provided by others. I have no way to make sure the data is correct or applicable to your equipment.

Before the glorious discovery of the OTGW, I had a homebrew flow and return temperature sensor using an ESP32 and two temperature probes just taped to the pipes near the boiler. In my case, I used MQTT as the reporting scheme.
When I got the OTGW, I compared the values of the boiler reported values and the ones from probes and they were just a fixed offset off from the boiler readings, so adequate for the job.

I see a very similar dual probe system on YT from Tomas McGuinness if you want to replicate :
https://youtu.be/hC5STZaQ3CE?si=IXsajTF-f-aMWkhN

Thank you for your responses !
I tried finding a manufacturer indication on the board, but I hesitated dissasembling the casing, as I wasn't sure how to do that properly.
For what it's worth attached are photo of the specification sticker inside the boiler, and a photo of the semi-dissassembled panel.

I took the "risk" and order the OTGW anyway, so in a few days I can post some details (and upload the log files).

If it doesn't work, I'll be glad to follow Kermit's hints how to homebrew some temperature monitoring !

Albert

received and installated the OTGW successfully !
uploaded the capability log file.
Return temperature etc. are unavailable, but a few useful parameters are : boiler water temp, and boiler water control setpoint.