Heater problems

The heater will only blow out of the botton floor vents. not matter what setting I put it on the hot air will only blow through the bottom. If I put the cold air on it will blow throught the defroster vents. Can anyone please tell me what the likely cause is.

Car Info 92 Chevy Blazer.

Check for loose or disconnected vacuum lines under the under the dash behind the heater controls.

Do you hear any hissing?

Check to see if the vacuum line is attached to the vacuum canister under the hood. It is probably a round black plastic ball about 4 inches in diameter. If the vacuum line is off or the ball is cracked, this is probably your problem.