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Question DetailsAsked on 6/5/2016

One of my air return filter box has a 16 tube and a 6 inch tube with damper. Why?

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2 Answers

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Assuming you are talking outgoing ducts (though sometimes you see dampered return ducts too, especially if running through a dehumidifier), the 16" is probably your main trunk leading to multiple rooms, and the 6 inch would usually be an additional run to a room that either needs a significantly different heating/cooling environment that the others, or maybe a late addition for a room that was not originally direct fed by ducting when the house was built, or maybe an addition. Common for basements that were originally unfiinshed but now finished, and for attic conversions to living space.

The damper would be to regulate the airflow to that specific space - usually the ducting would be oversized to allow (at wide open damper setting) more air flow than the room should receive, then the damper is used to regulate that to a reasonable flow to balance out the special room versus the rest of the house.

Other potential uses I have seen for a separate duct like this -

1) dedicated feed to a sickroom or "clean" hobby room, leading to a HEPA filter box then into the room

2) dedicated feed through a humidifier/dehumidifier or controlled climate "box" for specialty storage like art or wine cellar

3) dedicated feed through HEPA filter and carbon filter for a "safe room" or fallout shelter.

4) feed to a previously unheated/unconditioned but now finished/conditioned space like basement, garage, garage converted to living space, attic converted to living space, new addition or room, or greenhouse/atrium

5) for a tight house, occasionally a fresh makeup air duct will be configured like this - in which case it would be on the return duct part of the box, and pull air from outside, with the damper regulating the fresh/recycled air mixture.

6) a bypass duct installed to provide extra return air from an area that was stagnating - basically increasing reutrn ventilation from a room that was not getting enough air exchanges or that was consistently too cold or hot to it needed individually adjustable airflow - though usually would be a case of not enough, because too much you jusat close down an adjustable register somewhat.

You would have to trace down where the 6" duct runs to (with that size likely only to one register) - if ducting is not visible for enough of the run to figure where it is going to, try closing the damper (assuming the 16" one remains open so you are not stalling out the fan), and (with the air handler running) see which register(s) the airflow has stopped coming out at (or going back through if return duct).

Answered 4 years ago by LCD


Here is better description. Inside my filter/return is one 16" and one 6" line. I understand that the 16 is return to blower, why is there a 6 with damper behind filter? House is all original. All rooms with doors have flow thru vents for return air.

Answered 4 years ago by bigdawg


OK - could be a return from another room or area that was added to the air handler after the fact to avoid overloading the 16" duct, split return from a humidifier/dehumidifier/HEPA filter unit (sometimes only part of the return air is rounted through them to keep the unit small and changes slow), or could be makeup air duct for outside air to blend inside and outside air. With a damper on it I would guess the first or third most likely.

You will have to chase down where it comes from - might be able to do it by having on person holler or whistle into it or put portable music player up agaisnt it while the other 16" one is blocked off with a blanket or other sound-absorbing means (with unit power off of course) while they listen at the return grates around the house (or vice versa) or use a vacuum hose to listen into the 6" with, by using vanilla or perfume or lysol spray or such sprayed into return grills to track which ones teh smell tracks through from or not. For the latter, since in the return air handler, you would have to be creative and route a piece of tubing or such a little ways up into the 6" (so would only getting that air even though 16" is flowing), then run that through a vacuum to get the smell to your nose while the HVAC unit is running. A commercial contractor would probably use a smoke pencil, but a bit pricey to get one just for this purpose.

You said 6" is "behind the filter - could be they put in a HEPA filter in place of regular pleated filter so reduced airflow, or you have undersized air handler fan for the application, so they routed a bypass for some of the return air around the filter to increase airflow. Another reason for doing this would be if the air handler throughput was increased (larger/more powerful fan) to reduce icing or condensation on the AC evaporator, but the existing filter opening was too small for that airflow, so they bypassed some of the return air to reduce the "negative side" resistance.

Answered 4 years ago by LCD

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