ive been making short trip in this cold weather and when the heat gets warm for a few seconds it will smell like you just turned your home furnace on for the first time this year "hot burning smell" then it will go away then a few minutes later it will come back!!
sometime you can even smell it outside truck, i thought i could get a wiff with the hood up on drivers side!

any ideas what might be wrong if something is ????????

 

When not on recirculate, the thing draws outside air from the grille at the base of the windshield.

Could that be packed with dust, or ash?

Otherwise, there could be gunk inside the HVAC housing. It might be worthwhile to take the fan out (only takes 3 screws down by passenger foot well), and clean up the fan and/or reach up in there with a rag and wipe out as much as you can.

If that doesn't work, there is a product called DB Frigi-Clean. You put a little schrader fitting in the housing between fan and AC evaporator, the can attaches to that, and you spray it in while fan is on high. It's supposed to foam up and clean stuff out...

 

alright i try checking out the fan and recirculate, but it seems to be a hot smell, almost like a short or something, not mold, or something
thanks

 

Is the seal at the back of the hood in good condition? It could be sucking in fumes from the engine compartment.

The other thing you might look at is the resister for fan speed control. It is mounted near the fan motor.