If you run the blower but disconnect the compressor, is the vent output hot? Possible that the monovalve is not interrupting flow of coolant through the heater core...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280