The heat only come out the vents. I turn knob to defrost or floor and it does nothing it just stays on the vents

SOURCE: heater control 98 chevy s10 blazer

These trucks have a problem where moisture builds up in the heater box then freezes. When this freezes and you change your heater settings it breaks the shaft for the blend door. If this is what happened you should be able to look under the dash above and to the right of the gas pedal and you will see two vaccuum accuators one of them you should be able to wiggle around the linkage if it is broke. To fix this you need to remove the entire dash to get the heater box out which involves discharging the AC and draining the coolant because the heater core and evaporator come out. Please rate.

SOURCE: Heater/AC only works thru defroster vents on '98 Contour

you prob have a vac leak on the back of the intake. good luck

SOURCE: cannot get heat out of floor vents -only defrost and dash heat

i had a simmiler problem on my 95 chevy pick up. on mine it would not chg. from floor to defrost, no matter where i put the selector. with mine it was the controls. if you go under solutions for k1500chevy gmc trucks it should tell you what your problem is, or give you an idea whats going on with yours.

SOURCE: heat only comes out defrost port

What you need to do is check for a vacuum line that has deteriorated.When you lose vacuum the system will revert to defrost mode.You might have to remove the engine cover and check to see if you can find where you are losing vacuum.These lines are plastic so they get brittle and break.You can repair them by getting some rubber line and cut out the bad part of the plastic line and splice in the rubber line.Hope this helps.Good luck.

SOURCE: heater/AC vent option knob produces knocking sound

If I am reading correctly you are hearing the vent doors moving to redirect the airflow to the vents you choose. This is normal and can be more noticeable if the blower fan is on, putting air pressure against the doors.