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A little bit off topic:
Feel free to update the table of the messages supported by my boiler with my capture. It supports more things than currently shown in the table.

Following are supported:
0, 1, 3, 6, 9, 14, 15, 17, 18, 19, 25, 26, 27, 33, 48, 49, 56, 57, 100, 125, 127

Some notes I took:
15 had value 0x190A (boiler of 25kW, minimum modulation means 10% of maximum rated power)
125 had value 0x041A
127 had value 0x254F

Following messages were not supported (when requested with PM, I got UNKNOWN response).
5, 8, 115, 28, 21, 22, 23, 116, 117, 118, 119, 120, 121, 122, 123, 51, 52, 53, 54, 55, 58, 10, 11, 12, 13

I cannot test messages with WRITE. I think it would be great to let us write to the boiler manually with a command like PM but with WRITE!
Also, I would love being able to generate myself some commands and have access to the bits of the data in the sent payload.
Maybe we could think of a command with raw access? Give a complete hex command to the gateway (including whether we want to send a READ or WRITE), and it would send it as is without intervening.
Maybe many people use OTGW with Thermostat, but as in my case, the thermostat is me, I would love having even more control.

For example, I would love seeing support for the MsgID 0 master status bit summer/winter, and DHW blocking! This would allow me to completely disable hot water if required, and I could also activate summer mode once in the spring.

I moved your message to its own topic. I hope you don't mind.

The equipment matrix is automatically generated from the log files that people upload. I don't manually populate the table. That would be way too much work. The capability log feature in the latest OTmonitor will collect a log and use the PM command to check for support of all known readable message IDs. So if you use that feature to create a log and upload it, the matrix will immediately be updated. The opentherm specs suggests that it should be possible to check support for write-only messages by sending an Invalid-Data message. The boiler is supposed to respond with Data-Invalid if it supports the ID and Unknown-DataID if it doesn't. But I don't trust all boilers to behave that way.

I can add commands for summer/winter and DHW blocking. But for complete (raw) control over what the OTGW sends to the boiler, you would use the interface firmware. Unfortunately that is currently still only available for the PIC16F88. I haven't yet finished a version for the PIC16F1847.

Wow! I didn't know about the interface firmware!
That's great!
I would be very interested to have a try if it becomes available for the PIC16F1847.

What happens with this firmware if I don't send anything as a master to the boiler?
In my current understanding, boiler only answers after master has talked, in a kind of "conversation".
If I don't say anything, will slave (boiler) also stop to talk to me?

I will upload traffic capture to automatically update the table. Thanks!

With the interface firmware, you are completely in control. If you don't tell the OTGW to send something, it doesn't send anything. If the OTGW doesn't send any requests to the boiler, the boiler won't send a response. You just have to be careful not to stop the conversation for too long. Because the idle line state is low. The opentherm specs define that after 5 to 15 seconds of low-voltage state with no valid communication, the boiler may interpret that as a software short-circuit condition and start heating. You should plan to send a message at least once every second.

I have just published the interface firmware for the PIC16F1847.