What does the internal bypass valve do in a Viessmann combi?

[dr911t]

Could someone please explain what is the function of the internal bypass valve in a Viessmann combi and how its settings MIN/MAX or malfunction somehow would affect operation of DHW and heating?
Thanks in advance.

 

[ShaunCorbs]

When the boiler switches from hot water to heating the water moving has to go somewhere when the diverter switches over

 

[dr911t]

When the boiler switches from hot water to heating the water moving has to go somewhere when the diverter switches over

is the bypass value always in the same position? does it only function when the diverter is in motion between DHW and heating?

 

[ShaunCorbs]

It’s set to x pressure so when the diverter moves the system increases pressure and the bypass activates

 

[dr911t]

It’s set to x pressure so when the diverter moves the system increases pressure and the bypass activates

So it should always be set on "MIN" and boiler does the rest?

 

[ShaunCorbs]

Should be set so under normal operation it doesn’t open

 

[MaxPower]

The MIN pressure seems to be around 140 mbar. But what ist the opening pressure in the MAX position? I cannot find this information anywhere.

 

[John.g]

Vaillant have a internal bypass ( on the primary circuit) on all their system and combi boilers, it is factory set to 250mar but is adjustable from a minimum of 170mbar to a max of 350mbar, it provides the minimum flowrate required by the boiler at reduced flow demand and pump overrun after boiler shutdown, Viessmann setting may be similiar.