I've got an idea for an emergency dump on the cheap. I plan on removing the 2 registers I've got mounted in the top of the second arm of the "plenum". Fabricating a door that is slightly bigger all the way around. Attaching said door to a piano hinge that allows door to swing in, effectively sealing ductwork into crawlspace off. Will use heat activated fusible link from granger, and also an appropriate spring to put tension on the door. The links are rated around 8 lbs of holding force. I think this will work as a poor man's heat dump. Thoughts?