I own a small 2 story home that is heated by a forced hot air fueled by natural gas (6 year old system). It is a one zoned system. I have a two story addition going in (garage w/MBR over it and a family room behind it that connects to the kitchen). I have a proposal from a heating contractor (that my GC agrees with) that I'm not sure about. He wants to add a new furnace to the system to heat the new section with 2 zones, up and down in the new section and leave the old system alone creating 3 zones. Would it not make more sense to have 2 zones with the old system doing the first floor and the new doing the second (where all the bedrooms are)? The way it is proposed would seem to force me to have my old system running most of the time as it would be heating the 2 floors of my original home. The only savings during the day would be in the MBR as it would be a zone unto itself, and the only savings at night would be in the family room as it would also be a zone but with bleed over from the original system.