Good point. I didn't pick up that the temperature was different out the two sides of the center vent. That definately implicates the duovalve.

Nothing will get damaged in blocking the hose to/from the heater core - that's exactly the function of the duovalve! That said, you will lose some functionality by blocking the heater hose. When cooling demands are light (ie at night) the system is probably mixing in some heat to avoid overcooling the cabin of the car. You may find the car becomes uncomfortably cold, forcing you to switch off the aircon.

Also, the windshield defroster/defogger uses full heat to maximize removal of moisture from the glass. You'll lose much of the effectiveness if you block off the heater hose.

- JimY