[Greenbuilding] Cold air return/s

[Bob Klahn]

Larry,
I agree with Michael; tightening the ducts and the house shell is 
essential, as is free air flow back to the return system.

Consider going a bit further to balance the duct delivery system; 
provide a clear path back to furnace from each room, regardless of door 
position.  Door undercuts simply do not work adequately.

Ducting each room in an existing building is probably not feasible or 
cost effective; however, you could put transfer grills in the walls, 
connecting each room to a hall or central area.  If the grills on 
opposite side of a partition are offset (one high, one low) in a cavity, 
it tends to dampen sound transfer.  This was one function of the old 
over-door, operable,  transom windows.

Alternately, you might be able to fit a jumper duct in the ceiling, from 
the ceiling of the room to the ceiling of the hall or common area.

Take care.
Bob Klahn



On 6/28/2011 2:03 PM, Michael O'Brien wrote:
> Hi, Larry--
>
> In theory the supply side and return side should have the same air flow.
>
> However, typically each room has at least one supply while the entire house has one return so the system may be out of balance right there. And, when doors are closed, individual rooms experience positive pressure while the main house goes under negative pressure. The same is true for ductwork that has air leaks in it--in an out-of-balance system, supply ducts will leak more conditioned air while the return duct will pull in more attic and garage air.
>
> An out-of-balance system may increase air leakage by increasing pressure differences across the envelope, especially if no air sealing is done as part of the upgrades you listed. Increased air leakage will contribute to the house becoming too dry in winter, by increasing the loss of humidified air and its replacement by cold dry outdoor air.
>
> So ideally the return should be able to "see" the whole supply side, with no barriers to flow. That can be aided by installing transfer ducts on rooms that need to be closed for privacy. Because warm air is buoyant a single return should be located high.
>
> All in all, probably a good idea to invest in sealing the envelope and ducts in unconditioned spaces in order to get a single return to function properly.
>
> Please let us know your solution.
>
> Best,
>
> Mike O'Brien
>
>
> On Jun 26, 2011, at 12:15 PM, Myers, Larry A. wrote:
>
>> I am in the middle of a near total renovation of a 30 year old house (1600 sf, 3 BR, 2 Bath), that wasn't great energywise to start with. I am making some modest changes to save energy -- more insulation, triple pane windows, Energy Star roofing and appliances, etc.
>>
>> I have a reasonably efficient gas furnace/AC unit. My question: Is there an optimum place to site the cold air return? And is there any advantage to having two returns, or more? If so, where?
>>
>> Thank you.
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