Thanks for the info.

The upstairs master bedroom never seems to get as warm as the main part of the house (it is the last duct on the end of the main run) - do you think it would be worthwhile to install one of those "booster" like things to its duct? Or maybe get one of those fan-like booster that sits on top of the floor register?

When you say "damper down" - do you mean on the new ducts I am adding?

well it depends, on what your existing blower capacity is, assuming that the blower capacity was accurately sized, adding two new 6" ducts won't affect the system too much. However, if the system happens tobe undersized and you add two new registers you may be taking too much capacity from the rest of the system. I mean in that 5oo plus square feet are there any vaulted ceilings? Do you live in snow country? and how well insulated is the rest of the home? Its just a consideration, but myself I would investigate the blower capacity and size of the unit before I made any changes to the duct system.

Thanks for the input. I will definitely look into the blower capacity. I live in the suburbs of Philly - we get snow, but certainly is not snow country. There are no vaulted ceilings - it is acutally more like a basement - max ceiling height would be just under 8ft.

I just saw a response from 'theeagle' where he suggested putting the heat registers up by the ceiling and doing the returns by the floor. It sounded logical, but want to see if anyone else had some thoughts. Thanks!