uneven air control

JSPAK

always wondered ...in lets say a 2 zone duct hot air system ... wouldnt it be more efficient to put zone dampers in the return side too .. thus if u got one zone set at 55 and the other set at 68 ... u wouldnt be drawing 55 degree air to be heated up to be sent to the 72 degree side

Brad White

It depends

on how open or how isolated the spaces are from one-another. Not to mention the temperature differences maintained. If one area really is 55F and the other is 72, that has to be two very isolated spaces/portions of the house! 



The actual effect you describe depends on the proportions of course. Half the air at 55F and half at 72F means that the furnace or coils will see 63.5F air. Good for the cool side, a bit of a climb for the warm side. Your active zone coil output (if hydronic), will increase somewhat to meet this due to higher delta-T on the air (and water), to a point.



I think you are thinking about it correctly but maybe over-thinking it a bit too. For example, in about 95% of my designs, we have VAV boxes on supply only. (Granted we do not have a wide range of temperatures to be maintained; the systems strive for the same temperature versus variable loads such as sun in some rooms, computers in the other.)



But the other 5% would be medical facilities and laboratories where pressure relationships rule. A sudden heat load in a lab forces in more supply air yet the lab may have to be negative pressure to its neighbors, so we provide an individual interlocked air valve on the return/exhaust side.



To do that on a house with otherwise reasonable return air paths? I would have to come up with a reason to justify doing so, adding expense and complexity. Not saying it is not advisable, I just cannot think of a rationale for doing this in a house as I presume you are discussing.