When the air conditioner is working cold air only comes out of heating vents

No hot air or no airflow at all ?

In a car the heat and filter both go through the same filter. In homes there are normally separate filters for the air and heat.--------------------------------------------------------------------------------------
VAG DASH CAN V5.17

As per your problem mentioned.There are few solved help links to troubleshoot these problems.Go through the list and Click the link below:----
Air conditioner blows air but not cold air http://repairhelpcenter.blogspot.in/2012/07/air-conditioner-blows-air-but-not-cold.html

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Car overheats and Air blows hot at idle? http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html---------- Hot air on the driver's side vents only? http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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2005 Chrysler PT Cruiser: A/C Blows Warm? http://whoisbyme.blogspot.in/2012/06/2005-chrysler-pt-cruiser-ac-blows-warm.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold? http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold? http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html

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These will help.
Thanks.

Jeep Grand Cherokee is notorious for broken blend-air doors. This is the door inside the A/C-heater housing that switches the air from hot to cold. You must remove the dash to repair it.

It works off of vacume. The butterfly in your ducks arent opening, to allow the cold air to pass. good luck .

if line out is not geting hot then the core must be clogged

Sounds more like a control valve is not working correctly.. probably stuck in one position because you should always have heat from your heater core...like when you chose fresh air or recirculated air...it switches a valve to allow the change...well from cold to heat there's a valve also that switches the vent inside dashboard

You have a blendoor motor that controls the hot/cold door inside the hvac box. i have seen the motors stop workink. also the actual door will break wher the motor meets the door. either way dash will most likely have to come out..good luck...hope this helps

none of the above, the vent door is stuck inthe open position most probable cause is vacum leak look closer to your fire wall and you will see a very thin vacum hose going into the vehice which seems to be unpluged!!

The vacuum line on the drivers side of the intake manifold just below the throttle body fell off.