VW Golf - Aircon/Heater Issues

2013 Golf Mk7 Diesel, have had issues with no heat being output.

Had the heater core replaced which worked, when set to "Hi" it blasts hot air.

However when set to anything other than "Hi" it's cold air.

Also notice that when the speed setting is set to the lower end, almost no air is output.

Noticed after initial diagnosis from the VW dealer, a thumping sound which seemed to be the HVAC flaps. The dealership removed dash to diagnose.

I've tried resetting/calibrating the flaps, but no difference.

Any idea what is causing this?

PS I took the car to an independent to change the heater core, as they charged $800 vs $1.9K at VW.

If the heater core is producing hot air (which according to you it is), then the other two factors to consider is the fan motor and/or the flaps, and my guess would be the flaps or the motors/solenoids controlling them. Have you tested all associated fuses ? Sometime they can look OK but don't pass any current.

It is also possible the Indy didn't re-assemble everything correctly when they changed the heater core.