Relay and Opentherm together
The Tado extension kit can control the boiler through a relay (simple on-off switch) or through the Opentherm bus. The bus allows for better temperature stability and higher efficiency as it tells the boiler to turn the heating up or down as necessary.
If a heating system contains other elements, like a shut-off valve or external pump they would require the relay to switch them on or off. Unfortunately there's no way to use both control mechanism together: the Opentherm bus for boiler heat output and relay to turn on other elements of the system.
It would be really nice to have a hybrid mode where the relay and Opentherm bus work together, possibly with the relay switching the auxiliary devices off after a short grace period.
This seems to be rather simple thing to implement in the firmware, but there's no option in the settings menu to enable both.