Hot air comming through defroster vent everytime the fan is turned on in any mode

No hot air or no airflow at all ?

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.

2004 Chevy Tahoe. Hot air out of vents on driver's side, cool ...

answers.edmunds.com > Edmunds Answers

Jun 25, 2009 - Hot air from vents-driver side and cool air from vents on passenger side. ... The driver's side vents were blowing out hot air, and everything else wasblowing cold. ... you'll literally have to restart your tahoe/yukon at every traffic light to keep cold air coming ... I was also getting air only out of the defrost vents.

Hot air out front drivers side vents 05 escalade - YouTube

? 3:14
https://www.youtube.com/watch?v=5lnd8UNcx6M
Aug 3, 2013 - Uploaded by South FL Mud Club
Suddenly hot air started blowing out the air vents only on the drivers... ... is a very helpful video ,I wondering if this part number apply for my 04 chevy tahoe? .... I have cold air out of thepassenger side, but warmer air out of the driver's side. .... 2003 Tahoe No Rear Heat Fix, Suburban, Yukon XL Repair!

Heating and AC Temperature and Air Delivery Mode Door ...

repairpal.com > Problems > Chevrolet > Tahoe

Driver/passenger air vents blow hot then cold when using A/C. Back to cold when you restart the vehicle. Upvote (35) ... 2003 Chevrolet Tahoe ... I have a 02 Tahoe ,and the heat system is working only on the ceiling side of the car when turn.

2004 Chevrolet Tahoe Blows Hot Air - Sparky's Answers

If the air stream is not coming out the correct vent, then the switch/mechanism that diverts the air flow is at fault.

Do you have heat? Does the fan turn on? are you getting air from the defroster vents close to the windshield? If you have heat make sure the defrost control is working. I'd open the outside air vent and turn the blower on high speed. If there is no heat, the heater control valve is not working. Double check the radiator fluid tank, make sure you have enough fluid (antifreeze) Low fluid levels can reduce the hot water flow to the heater coil inside the car. If there
is no air flow through the defroster vents you may have a problem with the fan control, or the fan motor. Good luck.

sure.

what is your goal hot air, you never said,

got full fan speed? on high>? sure.
got full engine coolant heat 180F or more holding??? if not all is lost.
if car has a heater core water valve is it open>?
are the 2 heater core water tubes both fully hot,(hand test)
yes> good.
now you need air flow through the core (heater)
that means the damper most be open,, is it?

in that order. (heat)
the dampers on hondas can be complex. (10x on electric damper motors , aka, damper actuators)
see part 5 and 6 here
http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=ACCORD&catcgry2=2002&catcgry3=2DR+EX&catcgry4=KA4AT&catcgry5=HEATER+UNIT

the fSM covers the complex tests of dampers, and not having
a real FSM makes this near impossible.
why , because your dampers are no like anyothers on earth.
other cars use different types. and are all unique .
unlike CABLES
unlike vacuum servos.
see>?

here are the basics for heat.
http://www.fixkick.com/AirCond/heater.html

defrost is the defalt mode look for a damaged vacuum line going ti the control head

ck front vacc lines defrost vent is default sounds like no vacc on door

there is nothing that is going to break by having the AC on and then turning up the temperature all the way except a little worse fuel mileage. Keep in mind that your AC comes on during the defrost mode to help dry the air inside your car so your windows dont fog up. If there is no air movement then it sounds like the blower fan decided it was time to quit. It just happened now instead of the middle of winter or the day you needed the defroster--(defogger)

Try replacing the cabin air filter.