Missing ductwork on returns

I’m looking at a condo in Mexico and when removing the air filter on the return it just goes into the crawl space and there is no duct work present. Curious if someone can comment on this being normal or where the return actually comes from?

I am no physiatrist but I will try to help you.

There is such a thing as a conditioned crawl space but it should be sealed from the exterior of the foundation. A basic central duct system is one that takes return air from the same place as the supply air is discharged into ( a closed loop), some central systems do not have ducts connected as a return to the blower side of the air handler/furnace. If duct work for the return is not used its considered as a local return.

I’m no physiatrist either,
“A physiatrist is a medical doctor or doctor of osteopathic medicine who specializes in Physical Medicine and Rehabilitation (also called PM&R physicians). Physiatrists diagnose and treat both acute pain and chronic pain and specialize in a wide variety of nonsurgical treatments for the musculoskeletal system.” https://www.google.ca/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=physiatrist%20define

but what Charley says, depending on what’s in the crawlspace, eg, dry no dirt, its actually a great way to ventilate it and may also help in keeping the floor warmer than it might be, although in Mexico that may not be an issue.

This could work in a very dry/warm climate. I would suggest having the dwelling tested for radon gas before occupying it.