Maximin duct runs

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68bucks

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So I have a weird house, its really long and more or less divided in two. We have been planning an addition for the house to get us a first floor master bedroom/ bathroom and a great room of sorts. In the planning I want to update the home's HVAC system since its 30+ years old, original to the house. Currently I have 2 FA propane furnaces, one on each end. One end of the house is heated by a furnace that does 2 smaller bedrooms and full bath upstairs, and a large utility/ laundry/ studio space downstairs, that is where one furnace is. There is no AC but it is fuly wired to have one and we will add one in the project scope. The other furnace heats a large combined kitchen/ living space down and a good size loft master bedroom and master bath upstairs. It has central air. The two spaces only come together in the upstairs bedroom area. All together its about 2500 SF. The addition will add about 1500 SF. I am trying to decide the best plan for the HVAC system. One thought I had was a boiler and 3 air handlers. Its a good idea but really expensive and has a few logistical problems. Another thought was add a 3rd system for the addition and just update the 2 existing systems. I'm not wild about having 3 HVAC systems.

So I had a guy out to look at it and get me some numbers. He suggests updating the system in the current laundry room area and installng a larger system for the addition and the current living area. the two existng systems are 77,000 BTU inputs, he suggest a 125,000 BTU system for the new unit with multiple zones. My concern is the distance that the new system would have to push the air. It would be in the new utility area in the addition and I estimate that the longest supply run would be 80'-90' to the existing master bath. the retun would be a little shorter but not much. I could maybe abandon that return. It comes from the loft master bedroom and the air will flow down naturally anyway. Does anyone here have HVAC design experience? Are those sort of lengths for ducts common and practical? I don't like the idea of a system that has to run to the limit of its ability on a daily basis. Any thoughts?
 
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