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Is there a way I can monitor messages on the OT bus using the OTGW without having the OT Bus pass through the OTGW. I don't want the OTGW to add any dependency to the normal operation of the OT Bus.

I set the OTGW to Monitor Mode. I connected the OT bus to the OTGW on the Thermostat side only. I was able to see several T messages but no B messages. When I connected the OT bus wires just to the Boiler side of the OTGW, the OTGW starts to act as a standalone device because of no Thermostat detects and issues several commands to the boiler.

Also when I had the OT Bus simply connected to the Thermostat side, I could read various T messages in the Log, but about every second an Error 03 was also printed, is that because the Boiler side has been left disconnected?

I am trying to simply sniff the communication between the boiler and the thermostat. How do I do that?

To be able to communicate in two directions over only two wires, the OT protocol uses voltage levels in one direction (Master->Slave) and current levels in the opposite direction (Slave->Master). Voltage levels can be monitored by attaching the measuring device to the bus. But for current levels, the measuring device has to be inserted into the bus.

The problem with inserting a passive component (like a resistor) into the bus to measure the current is that it will alter the voltage levels and may thus affect the Master->Slave communication. You don't have that issue if you insert an active component like the OTGW in the bus. It regenerates proper OpenTherm signal levels.

I'm not sure what dependencies you are afraid of. In monitor mode, the OTGW passes the signals directly from one side to the other. It doesn't introduce any store and forward delay.

The OTGW needs its own power source, and if that PSU fails then I will have no heating, that's my fear. I am currently monitoring eBUS on my Vaillant boiler, that is also a 2 wire bus but if my eBUS monitor fails the boiler still continues to work. I wanted something similar for OT. Is that possible?

I didn't say that all communication on 2 wires will use current levels, just that OpenTherm does. The eBUS used by Vaillant uses RS232-like communication, with just voltage levels. So that can be sniffed by just attaching to the bus. As I have explained, with OpenTherm that's not possible.

But back in 2001, elektuur published a story about an Opentherm-monitor that doesn't require a power supply (it's available on the Hvac, Heating Opentherm Forum). It has the disadvantages I mentioned (affecting the voltage levels), but if your levels are not too close to the limits, it may work for you.