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QUESSTION by Adam Zielinski

Homeowners with forced air duct systems always want to close the registers in rooms they aren't using, and close the doors to those rooms, in the belief that doing so will save energy and money.
I always thought doing this was over-rated and unlikely to save a significant amount of energy or money.  I could see doing it for one or two rooms perhaps, but sometimes homeowners close off half of their house or more.
This creates unbalanced air flow in the duct system and likely results in over heating the furnace heat exchanger, and or short cycling the furnace.  So the furnace spends a lot of time in start up mode and less time at peak efficiency.
I have not seen any real studies done on this however.  I'd like to see some data or research on this.

Reply by dale conner
Adam, most furnaces will move the proper amount of air through the blower and heat exchanger if the furnace cabinet pressure doesnt exceed .5 IWC and the furnace capacity was chosen based on a manual J calculation. This information can be found in the furnace installation manual or IOM (installation,operation,maintenance) that comes with a new furnace. However, this does not mean we are getting adequate air delivery to all of the rooms in the house due to excessive air leakage in the ducts and/or incorrect duct lengths or diameters routed to each room.

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QUESTION by Judi Lyall

How about using a motorized damper ?

Reply by dale conner
A single motorized damper can be used to control a zoned area but you also have to install a barometric by-pass damper to prevent over pressurization www.zoningsupply.com

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See full discussion: http://homeenergypros.lbl.gov/group/hvac/forum/topic/show?id=6069565%3ATopic%3A7207