Mark, from the other live thread: "Zoning a system is a snap with the ECM drive blowers." Didn't want to hijack that thread, so I started this one. I've been wanting to find out what's involved, and what the potential outcome is, of zoning. I've got a split level house, with all four levels as living space. No cathedral ceilings, loft spaces, just closed stairways between all floors. Also have a split (no pun intended) CM 4 ton system, air handler in the unfinished attic. Upper 2 floors are warmer than lower 2 year round. When system is moving air, things are more even/comfortable. I've been running the fan constantly and that does help, to a degree. But when the compressor goes off in the dead of winter, the lower floors (where we watch TV, etc...) cool down very rapidly. I've looked at the Arzel equipment you've been recommending, and understand the overall concept. I have the manual J, and the calculated load looks consistent with my usage history. However, according to that report, the number and sizes of ducts specified is not what I got. For example, if a bedroom was spec'ed for two 4 inch supply ducts, I got one 6 inch instead (measurements for example purposes only). I have just one central return located in the hallway ceiling on top floor. How can I be as sure as possible that I'll get good results? What I'm struggling with is determining if my ductwork is "good enough" before incorporating zoning. And the answer to this seems obvious, but I'll ask anyway - can I expect my power usage to increase with the second zone added? Thanks!