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I have an unusual house, it's a very long structure that is somewhat divided into two sections. Each end has its own FA propane furnace. One end also has central air and the other is all wired for it just never had a condenser installed. Everything works fine now and is a total of about 2500 sf.

We are planning an addition for the house to give us a downstairs master and a great room of sorts and a new mechanical room. All told its a bit over 1500 sf. The hvac equipment is original to the house, about 30 years old. In the addition project I plan to update the systems. I have thought of several options, replace the old furnaces and add a third system for the addition. Not wild about having 3 hvac systems. Switch to a boiler and replace the two existing units with air handlers. That's nice but pretty expensive. I had a guy out to look at the plans and he suggested keeping one system the same basically but install a new larger system for the main part of the house and the addition. I'm OK with the idea but I question the length of the duct runs. The maximum supply run would be in the range of 80'-90' and up 1 floor. The longest return would be close to that also. He said we would zone it which makes sense but are duct runs that long common and practical? I don't want a system that runs on the ragged edge of its capacity on a regular basis. I understand airflow and blower curves but I was wondering what is sort of the rule of thumb on such things. I have also been considering geothermal. Any thoughts?
 
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