Quote Originally Posted by jtrammel View Post
If the duct was sized for 1200 cfm and now you have 800cfm the velocity is lower so the cooled air stays in the ducts longer and gains more heat from the attic, this could be the reason for the 14f drop vs 18f drop.

Very good point. Plus you still have the larger surface areas of the slightly oversized ducts. On the flip side, if well sealed and instulated, the reduced static pressure can spot you as much even 0.5 SEER over a restrictive system with a VS blower.

My ductwork for example is fairly oversized now that I went from 3.5 to 2 tons and the blower isn't far from minimum RPM on low speed fan (50% of heating airflow). But if my attic wasn't conditioned, I suspect I'd see a large amount of heat gain and loss from the ductwork.

This brings up a good point. On attic installations, it even more improtant to minimize total length of ductwork, the number of supplies and returns, and using proper sizing to meet those objectives. Even better reason for attic installation in vented unconditioned attics to be banned. Just spend the money, give up 3' of closet space and stick ductwork in the floor and walls. But I suspect I'm preaching to the choir here.