2000 blazer. With the fan off I still get air thru my vents. More flow as my speed increases.

 

Mine has been like that since the day I bought it new. Same with every Chevy I have owned since before buying the Blazer. The 2006 Monte Carlo never did it and neither did my dad's 1999 Silverado. Don't worry about it, they all do it. If you don't like the air coming out the vents, move to the feet or defrost.

 

If you have the manual hvac system, when you turn the right hand **** to "off", it stops all air flow from entering the vehicle.
If your's is the manual system and that does not stop the air flow, then you possibly need to replace that hvac controller as it has numerous vacuum lines and a rubber gasket that go bad at the back of the unit.

 

I've done that. No vacuum lined. No change.

 

I usually leave mine to the "windshield and feet" selection when off... it does the same thing with my 2000 civic, I'm thinking it might be a lack of thinking on the manufacturing end, when driving air might put pressure on the intake door enough to break the seal. Honestly never looked at it to know what all it looks like to begin with lol. Stereo goes louder then any noise I hear... I worry when it vibrates lol...

 

Back in the day a vehicle had a vent you could open to allow outside air to enter down by your feet - My understanding is newer style vehicles are sealed to the point that for any type of vent it has to be thu the system - and as stated by Zenith the off setting on a proper working unit should stop the flow .....

 

I remember the old floor vent with the pull ****. I'm sure you're right. There has to be a logical cause and fix.

 

Not sure what the problem is, I replaced my controller unit like in the picture and it works perfectly. Could be in the vacuum line cluster that plugs into the back of the controller. Sorry I can't be of more help, but this unit is designed to "Stop" airflow when in the off position.