The internal bypass is part of the problem which causes low efficiency on the Vitodens 2xx series. It has a fixed threshold of around 140 mbar. This means that in almost every use case the bypass will be opened when the heating is on. Even if there is adequate flow in the system. This bypass flow water mixes with the return water. Increasing the return flow temperature, reducing the condensation effect and hence showing higher then normal flue gas temperatures.
The boiler requires a minimum flow of around 125 l/h, yet even at the lowest speed the bypass will open when the flow dips below 680 l/h.
Like a lot of things at Viessmann, it doesn't make any sense. They should have opted for a better approach. E.g. a pressure regulated pump (like grundfos autoadapt) in combination with a smarter internal bypass that only opens when it detects that the flow is too low. Or alternatively no internal bypass and educate your installers on installing an external bypass with correct pressure threshold on a per use case basis.
