Auto Air Conditioning & Heater Repair: hot outside air flowing thru the heater vents..., vaccuum pump, psi max

Question
Hi David, i have a 2000 express 3/4 ton cargo van with 5.7 engine.When I'm driving and the a/c,heater selector is in the off position i still have hot outside air flowing through the heater vents.When i move the selector switch across the different output selections they seem to be working.
Thank you for your input - Matt

Answer
Matt,
From your description, it sounds as if the door actuator for your vent/recirc door is weakening, or the vaccuum that is trying to keep it closed is too weak to hold it closed.
With everything off, your vent/recirc door goes to the default recirc position where it closes and keeps the outside air from entering the cab. This is done by a system of vaccuum lines that get it's source from your brake booster diaghram. When you say that all the other positions work correctly, that immediately rules that your source vaccuum is good. Too bad that is usually the easiest fix. :)  
From your source it goes through a vaccuum harness connector which recieves electrical signals from your control head and switches the valve that gets a vac pulled on or gets vented. Again, you say that it works elsewhere, so I shy away from this being the fault. Same with your control head. Now the line going from your vaccuum harness connector connects to an actuator that reacts to the vaccuum or vent. If this is weakening or the line itself has developed a small leak, it would act as you have described.
In some instances this can be a reasonably indepth repair involving removal of parts of the dash. I am uncertain on your van, being that I have never had the opportunity to work on one. But I will say that unless you have the appropriate tools in your garage, to include a vaccuum pump (capable of about 10 psi max) I would take it to a reputable shop to have it checked and replaced.
Thank you for the question, I hope I was of some assistance. If I can do anything further during this process, or in the future, please do not hesitate to ask. If you found this response to be of some assistance, please remember to rate this answer. Thanks again and good luck in everything.