VW Jetta Heater Problem Solved. 84F now to 150F - Narrow Down the Causes 

It's not always the damper doors. The foam you may see coming from the vents could be the baffle seal. Regardless CLR is some pretty powerful stuff. Flushing your radiator more often prevents these issues or just use quality coolant. You get what you pay for. OAT - Organic Acid Technology types are best for aluminum engines.