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Question DetailsAsked on 7/4/2015

Should registers be replaced when new ducts are installed?

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2 Answers


That wouyld be extra, if you want new ones. They will work just fine, assuming they are the correct ones.


Answered 5 years ago by BayAreaAC


By the "right ones", I think BayAreaAC means obviously the right physical size for the duct exit point (which might change with new ducts), and that they are compatible with the duct "sizing". Registers, particularly some of the adjustable vane ones, consititute a substantial part of the friction in a duct system - the "friction" or more technically "head loss" of bends in ducting commonly are equivalent to a foot or two of straight duct friction losses - but a register or a grill can run from around 3 to as many as 50 feet of "head loss" - meaning in many residential duct systems the grills can add up a significant majority of the resistance to airflow in the system. I remember one major new skyscraper troubleshooting job I was involved in where the ventilation was absolutely terrible to the point the building was closed by the health department for odors and mold buildup in the new ducts - turned out over 90% of the fan energy was going into extreme turbulence at the architecturally fancy custom-made but terribly designed artistic twisted strip mesh grills, which were basically acting as chokes on the entire system.

So, if your HVAC contractor ran his Manual D calculations (which size the ducting and determine required fan output) for open-area, low resistance grills and you reuse high resistance ones, your airflow could be a small fraction of the design flow and result in serious underperformance, as well as freezing up of the A/C evaporator and significant loss of efficiency and potentially overheating of the furnace. If he ran the calcs assuming leaving the existing ones in, then assuming they are correct cross-section dimensions for the new ducts, and you don't want them replaced for appearance sake or to install low-level outlet hoods instead of straight-out airflow, they would be fine.

Answered 5 years ago by LCD

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