Heater Idle Problem

[Neal Zung]

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1997 Audi A6Q
The heater blows cold air when the engine is idling. If I depress the
pedal and rev the engine, the heater works fine. And when I am driving
it also works fine. The thermostat was changed, but this did not fix the
problem.
I am stumped and would appreciate any help.

Thanks.