Q&A

 Master

Thanks for providing excellent info, this makes it much easier to give advise!
If the engine is achieving normal operating temperature in a normal amount of time, then the thermostat is probably OK, but if the engine temp is low, replace the thermostat.
If the engine temp is ok, then the temperature regulation flap that directs airflow through the heater core is malfunctioning, either from the control system or the flap itself. There should be a cable that attaches to the control lever and goes down to the lever that moves the flap. Try moving the lever and finding where the cable is attached to the heater box. Here is a picture:
http://bit.ly/69MWbS
#19 is the lever, it attaches to the temp regulation flap in the housing. Check this and you may find the cable disconnected or the lever broken.
Good Luck

 Journeyman

it is possible the heater core has sediment that prevents its entire area from heating up. a flush usually only clears the top section helping only slighty. try low fan speed with high temp setting, bet it blows warm. try reverse flow at the heater core hoses using a garden hose, good luck.

I have a 2004 Passat. Since the heater core is very high in the cowl it can get air bound. The first time I thought the core was blocked but it was air bound. I remove the hoses and use a garden hose to push the air from the core and its fine.