If there's no leak, there's no point in replacing a hose. Blowing from just the vents under the windshield is a common problem with vacuum-controlled systems. The various vacuum motors are spring-loaded to go to the defrost position when there's a failure. Your feet might freeze in winter but the windshield will be clear.
Before tearing into the dash to check for vacuum, look for a small plastic hose going from the engine to the firewall. There will usually be some kind of check valve or storage canister inline with that hose. That's a good place to check for vacuum when the engine is running.
If you hear something moving in the dash when you operate the heater controls, there could be a defective actuator or a broken linkage. The clue is some functions will work and others won't. It's when nothing changes and there's no sounds that a loss of vacuum should be suspected.
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Tuesday, August 9th, 2011 AT 8:07 PM