Front defrost vent doesn't blow any hot air.

It's the vent actuator, it moves the baffle that changes the direction of the air flow.
Here is a video of a broken vacuum hose that creates the same problem.

It could also be a failed actuator.

HONDA EXPERTS accord vents dont switch anymore? ' Yahoo Answers

https://answers.yahoo.com/question/index?qid...Sep 13, 2008 - my 1994 honda accord ex has a problem. the heater vents dont switch ... defroster and feet etc.. none of those work anymore. nether does my AC, ... still no change. they will blow hot and cold air out of the front "in your face" ...

Honda Prelude - A/C delete - YouTube

? 7:41
www.youtube.com/watch?v=41k6UxSUM8Y
Jul 30, 2013 - Uploaded by Nicolas Alvarez
Honda 98 prelude 5th gen A/C delete H22A New belt size = 37 inches (same as 97 accord) ... New belt size = 37 inches (same as 97 accord) ... How to: 1990 Honda Prelude - Diagnosis Air Conditioning Inop and a Vacuum leak ... Fixing A Car Air Conditioner That Blows Hot Air - Duration: 3:42. by Scotty ...

These vehicles came with 2 types of ac controls.
The vacuum control system uses vacuum to actuate the vents and ducting from hot to cold. A lot of times the HVAC vacuum line in the engine bay (black hard plastic) is either broken or disconnected from its vacuum source.
The electronic vent control: does everything at the control knob via vent actuators and the control panel itself. With this one it could be any of the components. You would have to locate them and verify operation by moving temp/vent selector.

. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ?
Air Flow Control
Air flow control is accomplished in the following manner:

• Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
• The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
• The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
• Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
• The position of the function selector knob determines the manner in which the system will operate.
• Each position of the function selector knob is detented for positive engagement.
• Vacuum Control Motors The vacuum control motors (18A318):
  
  • are located on the A/C evaporator housing, A/C recirculation air duct and heater air plenum chamber (18471).
  • direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
  • The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).
  
  

Sounds like the switch nob itself is wore out. you may need to replace that assembly, not just the nob but that cluster that has the nobbs on it. good luck

you have a vacuum leak in the selector switch for front heat explorers had similar problems

blend door

heater box is warped doors not closing fully

check out selector switch,vacuum hoses ,check transfer door write if did not solve

check the hoses connected to the vacuum advance ball, it has probably begun to decompose...