This is NOT a good way to condition your crawlspace.
There is a lot of confusion out there about the purpose of conditioning crawlspaces. In other words, why would you want to? From my (biased) perspective, the MOST IMPORTANT reason to do this would be to get your ducts inside the conditioned space. If the ducts are not in the crawlspace, all the other reasons make it a tough sell.
So, this got me to thinking. What would the ultimate conditioned crawlspace look like? First, I would start with a single story home with really tall, super-insulated walls, say ten feet, and an insulated slab on grade floor. Then I would install all of the supply ducts right along the concrete floor. Once they were all in, I would install a removable metal grid floor so that it just clears the largest diameter duct. Granted, this would look strange, but the point is that now any losses from the ducts would certainly benefit the conditioned space.
You would still want the ducts to be properly sized, well sealed, and reasonably insulated. If the ducts leaked, the air would not be going to the register where the air was intended to reach. This is not so much an energy issue as it is a comfort issue. The BTU’s are still inside the conditioned shell, it’s just that they are not where they are supposed to be when they are supposed to be there. This may result in very small temperature variations between rooms. If you think about it, though, when is the airflow ever perfect? NEVER! Even if you balanced the system down to the exact CFM at every register. There is no single set of room-by-room airflows that is perfect. The ACTUAL room by room loads on a house vary as the sun moves through the sky, as people move around a house, as lighting and appliances are turned on, and as doors and windows are opened and closed.
So, condition that crawlspace, make sure it is clean and well insulated, let it communicate with the house, don’t freak out over sealing (or insulating) the ducts, and size those ducts right!
Happy Trails,
Russ
Duc-Bloc register blocks for duct testing
HVAC 1.0 Book – Introduction to Residential HVAC Systems
Kwik Model 3D HVAC Design Software
Russell King, M.E. 
Jun 21, 2012 @ 04:45:53
You might want to consider termites, code and code officials in future blog.
Jun 21, 2012 @ 06:23:18
Those are excellent points for a future posts. Thanks for commenting. The point of this particular post was to emphasize the *reason* for doing it, not the details of *how* to do it. It was also from the energy perspective. Many people think the reason is so the floors will not be as cold, or because there is less foundation wall to insulate, etc. As you point out, the actual practice of conditioning crawlspaces can get very complicated, especially in existing homes. How about this? When I define the crawlspace as *clean*, I mean that it is code compliant and termite- and code official-free. 🙂