Baxi Duo-tec 2 Combi 28 GA OpenTherm Compatibility

4 May 2023
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United Kingdom

I've been having a look around here at various forum posts which have been useful, but I can't find the exact information I need. So I decided it was time to sign up and get involved. I'm hoping someone might be able to help.

I have a Baxi Duo-tec 2 Combi 28 GA boiler and a Tado v3 wired thermostat. This is all currently working fine using 230v on/off relay but I was looking at changing over to OpenTherm hoping to improve efficiency. Not entirely sure how much OpenTherm will help, but everything I read about it makes it sound positive.

I know the changes I need to make. I currently have 2 wires going from the Tado thermostat to the boiler terminal block. These need to be moved to the boiler M2 low voltage block and then also moved to the low voltage/OpenTherm terminals on the back of the Tado. The Tado thermostat then needs to be put into the engineer menu so it can be setup to use OpenTherm.

The problem is that I am getting conflciting information from Tado and Baxi. I have spoken to two people at Tado who have said that the boiler is compatible with OpenTherm and they see no issues with the proposed setup. However I messaged Baxi, who I would think would know best, and they have stated "The Baxi Duotec 2 Combi 28 GA is not compatible with OpenTherm connectivity, only low voltage or 230v switching". Does anyone have any experience with this? I would presume Baxi have it right, but jusut wanted to check.

I know the Tado configuration lets you pick OpenTherm or other Bus protocols and does have an option for BS-Bus which is shows as being for Baxi boilers. But is this anything like OpenTherm or is this just on/off switching but on low voltage as the Baxi message suggest?

I'm just looking for the most efficient way to connect everything up. If it's any help I have multiple Tado radiator thermostats and zones. So I was hoping that by using OpenTherm it would be able to better modulate based on load as quite often only a few radiators are on to heat the kids bedrooms.

Thanks for any assistance anyone can provide.

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I will leave the experts to give a definitive answer, but I can find no mention of Opentherm in the installation manual.
The "Low voltage terminal block" only seems to be used for connecting the optional outdoor and wired sensors.
Thanks for the quick reply. I couldn't see anything in the manual I have with the boiler or anything online either. So that coupled with the Baxi engineers response pretty much nails on it's not supported I guess. Tado must just be making things up unfortunately.
It appears Tado don’t support OT, as per @RandomGrinch the M2 block is for low voltage, not OT


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Baxi will be right .
also worth noting open therm on baxi boiler is temperamental and only usually works properly with baxi products
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