Proper duct systems has a supply air and a return air run in each room (excluding RA in bathroom and kitchens) for proper balance of the system. Thirty foot long may seem long if the ductwork is not steeped up as it gets closer to the furnace. As with supply air ductwork it also must be stepped up in size as it goes out from the furnace. The size of the increase in both duct systems depends on how much air you are pushing out or returning. This is what is called a venture system as when going out on the supply side the ductwork is decreased as runs to the rooms are taken off the main extended plenum. Remember that air takes the path of less resistance and with out the venture system the far runs will be able to get the proper amount of air needed to supply the room. The duct system is engineered (not necessarily by a Engineer) to distribute the air flow evenly. I hope this helps.

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Well thats my 2 cents.
Brian Kelly
Kelly Home Inspections