Sorry this is so long.
I had a new Lennox Heat Pump and variable speed air handler installed in 1999, I think it's 4.5 tons and it has 2 zones. Recently while researching HVAC information at work for our line office expansion I realized how undersized my duct work is. I would like to come up with the best course of action then when I call a couple HVAC contractors in I can specify exactly what I want. I have no intention of doing this retrofit myself.
Basically I had a large home addition added in 1999, I had a 2 year old 2.5 ton Trane XL1200 heat pump for the original house that did an excellant job, when I added the addition I let the HVAC man talk me into scrapping the old system and adding a larger 4.5 ton Lennox unit to serve the whole house. I initially just wanted to add a second furnace and heat pump for the addition.
He took the old air handler out that was in the center of the supply trunk run, had about 20 feet of supply on each side of the AH. He then added the new unit at the far end of the house in the garage and tied the main trunk from the new AH into the end of the old trunk. Basically this gave me a total length of about 60 feet of supply trunk with 3 very sharp 90 degree turns just off the AH. The AH is set up to run 2150 CFM, I have 19 - 6" diameter supply lines off of this trunk which I think is okay for 2150 CFM, the problem is the trunk is only 8" X 16". The 3 supply registers closest to the AH actually have negative pressure and is pulling air out of the room.
I can't make the present supply duct bigger so my plan is to add a second 8" X 20" supply duct that will run half the length of the house. I would then take the first 10 6" diameter supply ducts off of the 8" X 16" duct and connect them to the new larger duct. So the old smaller trunk would supply the end of the house furthest away from the air handler. The AH is in the garage, where it comes into the basement it makes a 90 degree turn, I would connect the new supply duct at this 90 and actually turn it into a "T". Since the new duct will be the path of least resistance, because it will be a straight shot down the new duct I want to have a trunk damper added to limit the air into the new duct. I want to also have supply dampers added for all 19 supply lines.
If I could have the system reworked I would, but I just don't have the room to enlarge the present duct, unless I remove the return. Looks like when they built the house they ran the trunks then all the other plumbers and electricians did their work. So there is plumbing, lighting, drains, vents, freon lines...... preventing me from doing so.
Will this work? Any advice would be appreciated. I paid top dollar for this system through a licensed HVAC company. I don't want to dump a lot more money into it and still have problems. Will I have a hard time getting another HVAC contractor to work on this system as screwed up as it is?