Viessmann Vitodens 333 Service Instructions Manual page 96

Function description
Extensions for external connections
Internal extension H2 (accessories)
The internal extension H2 is inte-
grated into the control unit casing
instead of the internal extension H1.
The cylinder primary pump is con-
nected to relay output sK.
96
(cont.)
An external extractor fan interlock can
be connected to aBJ.