Boiler will not switch to heating or DHW mode on Vitodens 100 B1KE-199

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Symptoms:

Vitodens 100 B1KE-199 reportedly will not switch to heating or DHW mode, creating a "no heat" or "no hot water" service call.

The boiler's internal diverting valve becomes 'stuck' in either the space heating or domestic hot water production position, and will not physically switch positions. The boiler display does show the correct current call for heat (space heating or domestic hot water production), but is unable to move the valve to this position.

Cause:

The check valve (located near the plate heat exchanger) has created a vacuum condition in the boiler's internal piping, preventing the internal diverting valve from moving.

Resolution:

Remove and replace the check valve that is installed in the DHW flat plate heat exchanger. For details on how to do this, please see the following document and video: