Questions on heating system corrections


Have a house that was converted into a 3 family about 20 yrs ago. The first floor apt heating system (gas, forced air) is running off the furnace that used to heat the whole house. (The other two apts have their own heating systems.)
My problem is that their return air for this apt is drawn from the common hallway so it is really not returned air. Nor is there a vent sending heated air into this hallway. I would like to put in a return air in each room that receives hot air and leave the hallway unheated..
If I do this I have an opportunity to shorten the trunk significantly. Is there a good reason to have an overlength trunk? The trunk originally went outside the house at one time to draw in fresh air and there will be / is about 20 excess ft of trunk.
I am thinking that for the new returned airs I can use flexible ducting?.....if I get square inches of the ducts sending heated air into the apt I can only get about 75% of that area as return. I understand that this will mean a velocity difference. Any problems associated with this?
This is an interim fix...will be replacing the furnace next year. Main thing now is to help with keeping heating costs down and not do anything detrimental to the installation of a new system.
Bill
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<Bill> wrote in message

Do the manual D calcs and get back to us.....If you don't know what that is, then hire somebody that does know.
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