I recently purchased an older home, built in 1914. It is a story and a half with an addition added on to it. The house had a gravity fed furnace, and in 1991 they installed a gas furnace, leaving all the old ducting, an added some ducts for the addition. They didn't add any duct work to one of the bedrooms in the addition.

We want to upgrade the furnace to a more efficient one, and at least add a duct in the bedroom they forgot to. I live in a small town, so my choice of contractors is pretty limited. I want to get an idea of what should be done. I have already contacted 2 contractors, and there prices were pretty similar, and they sized the equipment the same. But it seems no one here will do a manual J to make sure I am getting the right size. The current system is 3.5 ton 100,000 BTU NG furnace, no a/c. They both propose a 3 ton 60,000 or so BTU furnace and matched a/c. The living space to be heated and cooled is about 1112 sq/ft with a total cubic feet of 9438. the closest big town to me is Omaha NE, and that is 190 miles south east of me.

The addition is 554 sq/ft and 4416 cubic feet. The addition will have closed cell spray foam insulation in 2x4 walls with 10 inches of cellulose in the attic.

The main part is 558 sq/ft and 5022 cubic feet, with plaster and lathe walls, and a block outside wall, no insulation in between the plaster and concrete block.

I am using base board heat in the half story, and there is no ducting running up there. Both contractors thought the original furnace was sized for the whole house, even though there were no ducts in the second story. They both thought that I could do with a 2.5 ton system if I had insulation in the main part of the house.

This is what I would like to do, replace all the ducting with the appropriate sized ducts for the rooms and house. I have done minor duct work before, but never re-ducted the whole house, or had to figure out what size to use. The two contractors I spoke with would just tie in to the current ducts, and not change them. I would also like to setup 2 zones, one for the main part, and one for the addition. I want to run one large duct to the addition, cause it has to go through a concrete wall, and branch off from there to the 2 bedrooms, and 1 vent for the hall/bathroom. I can then place the damper for the zone system in that main duct. One bedroom is 192 sq/ft, the other is 168 sq/ft, and the hall/bathroom is 192 sq/ft. The 192 sq/ft bedroom does not currently have any ducting, the other ducts that are there are 8 inches.

The 4 rooms in the main part I need to heat and cool are 13x12, 13x15, 9x 12, and 11x9. I have 9 foot ceilings in the main part of the house.

I know there are probably variables I am missing, but are the contractors on with their 3 ton and 60,000 BTU estimates? What size of duct should I be using, or what are the formulas for figuring duct size, I doubt I need the 10-12 inch ducts that are there now.

What are some good retro fit zoning companies, I have looked at the suncourt stuff, and that don't seem bad. I want to do it this way, that way I can add it later, and I can get a control that works with thermostats of my choice, not some propriety system, that will cost a fortune to replace or repair.

I am sure I will come up with more questions, but thanks for your help now.

Michael