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Old 09-27-2010, 01:35 PM   #1
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Sealing air return duct


Hi guys, new member here. I've done my share of projects around the house, but never done any HVAC work before, so please bear with me.
I'm trying to seal the return air duct on my furnace (non-condensing, down draft). The return duct is made of galvanized sheet metal (rectangular - 24" x 10" x approx. 15ft length) and it's anchored to the garage ceiling with straps). The duct transitions to a joist cavity in the ceiling (that seal is also very poor). To properly seal the duct it, I might to have to remove it from the ceiling, although I'd very much like to avoid that - the thing probably weighs 150lbs+.

My questions to you are:
- How much can I twist/bend this duct without breaking it? The backup plan would be to seal it "in place" - remove some of the anchors (while supporting the duct), twist the duct a bit (to distance it from the wall/ceiling) and apply mastic or tape.
- What should I be using to make a good transition to the joist space? Right now there's just a hole in the top of the duct and another hole in the drywall. I'm thinking something like a collar would help? (then seal it with foam or tape/mastic).

Thanks in advance!
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Old 09-27-2010, 09:57 PM   #2
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Sealing air return duct


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Old 09-29-2010, 09:35 AM   #3
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Sealing air return duct


There are a variety of tapes and sealing compounds available. Go to Lowe's (or your local HVAC supply if they'll let you) get a roll of UL approved tape (181 A/B) a gallon of duct seal and a brush. Go to town, but be careful removing hanging straps.
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Old 09-29-2010, 05:11 PM   #4
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Sealing air return duct


Thanks! How do I make the proper transition from the galvanized duct to the joist space? Whoever built the system left a 1/2 inch gap between the hole in the ceiling (leading to the joist cavity) and the duct. Is there a specific connector i should be using?
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Old 10-04-2010, 11:42 PM   #5
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Sealing air return duct


I wouldn't even bother with this. I believe the more air that goes in the furnace, the better. If it's like most systems, there probably isn't enough return air to begin with. The installer left that half inch gap for even more airflow. Someone correct me if this is wrong, but I think that's how it goes. Always remember that you need as much air coming in to your furnace, as you need coming out.
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Old 10-05-2010, 06:14 AM   #6
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Sealing air return duct


What climate is this in, where is the furnace located, is the garage heated, and where is that wall cavity in relation to hot/cold sides of the system?
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Old 10-06-2010, 12:26 AM   #7
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Sealing air return duct


The furnace is in the garage, so if there's a gap in the return duct, air from the garage would be sucked into the house.
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Old 10-06-2010, 04:18 AM   #8
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Sealing air return duct


Quote:
Originally Posted by jasonj View Post
I wouldn't even bother with this. I believe the more air that goes in the furnace, the better. If it's like most systems, there probably isn't enough return air to begin with. The installer left that half inch gap for even more airflow. Someone correct me if this is wrong, but I think that's how it goes. Always remember that you need as much air coming in to your furnace, as you need coming out.
Not suppose to be any air transfer from a garage to a return duct. The code for several years now, requires the duct to be enclosed if its in a garage to prevent garage air from being drawn into the duct system.
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Old 10-07-2010, 07:46 PM   #9
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Sealing air return duct


Right I never realized it was in the garage! You wouldn't want to suck up car fumes that's for sure! Thanks.
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