Heater problems

[esr43]

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.

[BlazerLT]

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

Do you hear any hissing?

[Rick Norwood]

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.