Adding zone controls to my rheem 90+

[masstech]

I have a 2000ft/2 ranch witha Rheem 90+ (105k input/97k output) forced air system. I'd like to finish part of my basement, roughly 13x26 (338 ft/2) and add heat by tapping off of my existing supply ducting. What issues should I be concerned with in tackling this project? Should I be concerned about my system being unbalanced after the fact? What is the best way to recirculate the excess air when the smaller of the two zones is calling? My background is mainly forced hot water systems not forced air so any advice or tips would be greatly appreciated.

[mark beiser]

Is it a mod?

[masstech]

Do you mean modular? No it isn't

[mark beiser]

No, I mean is it a Modulating furnace?

[beenthere]

If it was a boiler. Would you just tap off the existing supply piping to heat the basement.
Or would you run a seperate supply line.