Thread: Plenum length

Is there any length to how long a plemun run can be for the air return. I did an inspection yesterday and the run was about 30 feet (or more) back to the unit. I did ntoe notice and problems with the unit and was wondering if this would affect the unit over item.

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Not that I am aware of, the size of the return would be based on several factors, including that length - especially if flexible duct was used (because of friction losses in the flexible duct).

It would be one part of the 'engineered system' (the duct work and equipment specified).

Bill,

No size limitations for return plenums that I am aware of.

You do though have to watch out for pentrations through fire separations, and leakage. You want the air to be returned from the space you are supplying air to.

Sound control is another consideration when planning a return plenum.

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.

Thanks for the responses. I had just never seen one run that long before.

William, as Brian mentioned, the system should be engineered (planned) for the amount of air flow, type of duct and length of run. One point I wanted to make is that IF there is only one return grill, the size of the duct will remain constant all the way to the unit. IF there are multiple grills, then the ducts will be increased at each wye to allow for more volume. The purpose is to insure each grill is pulling air in rather than everything drawing from one grill if the same size duct is used throughout, dampers could also be used to control airflow, especially if it is a 2-story house with heat and air that needs seasonal adjustment.
Lots of variables can come into play on ductwork design.

Jim,

Good points. Just for clarification, the second story was added to this home. They moved both air handlers to a space in the attic off the second floor. Them main house unit run was OK. The one for the bedroom started in the master bedroom and ran all the way around the attic to get to the air handler. I guess this is the only way they could get the duct to be the same size for the whole run.

Brian,

"Remember that air takes the path of less resistance "

Air, like water and electricity, takes *all* paths.

The path of least resistance gets the most air/water/electricity through it while the path of greatest resistance gets the least - BUT ... it (they) will go any and every where it can go.

30 feet is along way, what will happen is this, the return air will pick up heat from the attic. we compensate forthis by using oversized return boxes and undersized flex. no need for an engineer just a good hvac guy