Flex Duct Mastic Procedure


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Old 06-27-15, 11:32 AM
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Flex Duct Mastic Procedure

So my AC system installed 2 years ago is leaking like a sieve around where the flex duct goes onto the plenum. Obviously, I cannot fix it now because its sweating. I have mastic but what is the procedure for it? Take off the ducts, put mastic on the takeoff and then put the ducts back and zip tie them? Or, do I leave them there and just mastic around it? Not going to help, but Im trying to get this 2 ton AC to handle 100 degree temps in a 1350 square foot area (yea, I know, they f---ed me on the sizing).
 
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Old 06-27-15, 11:52 AM
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My fingers feel sticky just thinking about trying to glue that flex into place as is. I have never had to do this, always worked on rigid ducts. Even applying the mastic and then sliding the flex over it sounds messy and it would be a one shot fix.

Just throwing out thoughts while we wait for the pros, how about a large hose clamp or two? The clamp in a smooth fashion all the way around. If you wanted to seal it perfectly, I would use some silicone with the clamp, that way if you wanted to remove it, you could.

Note, flex duct doesn't have the r-value I would like to see so anywhere you can add more or bury the duct in insulation would help.

There are two sides to satisfying your cooling requirements, increase the size of the ac unit or (as you are doing) reduce the losses (gains). Duct leakage is a double hit, the air that escapes has to be replaced by that hot outside air and the humidity it brings along.

Have you done any other air sealing and insulating?

Bud
 
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Old 06-27-15, 02:48 PM
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Yes, as much as can be done in a completed build. I am NOT replacing this unit. Its gonna have to 'do' until it dies. Instead of matching the system to the building, Im going to have to get the building closer to the system. I am going to add insulation to the ceiling as its not even code minimum now. I may try hose clamps if I can find any to go around 6, 8, and 12" flex duct. That's a good Idea. Hell, I may even be able to crank down on the big zip ties they used. Didn't try. LOL.

I was able to clamp it down more, but the leak is where Damper pivot is. They put the clamp closest to the plenum, but it probably should go closest to the room.
 
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Old 06-27-15, 04:28 PM
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In hot country when you cool the house, that air is heavier and escapes the lower portions of the house. That reduces the pressure inside and allows warm air from the attic to push down into the house as a necessary replacement. Air sealing both high and low help to reduce that leakage. Link on air sealing below. Recognize that link is from Vermont so some of their discussion may relate to cold climates, but air sealing is air sealing.

"I'm going to have to get the building closer to the system." best approach in my opinion.
Bud
http://www.efficiencyvermont.com/ste...ide_062507.pdf
 
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Old 06-27-15, 06:44 PM
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Ill look at that link. Prior to now, in the winter, I went around every single light fixture and took every grill off of every register and caulked the gap between it and the drywall. Some were better than others but they are all sealed. There is no way AIR is coming through my sealing...heat...yes...definitely yes. I have not had much luck air sealing teh floor though, and that sucks since below it is outdoors. But its a closed finished build and is what it is. When I recarpet in 10 years, Ill caulk every floor seam and base board since I cannot get to the framing anymore.
 
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Old 06-27-15, 08:19 PM
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From the attic the top of every wall, and unfortunately that includes the outside wall, is a source of leakage. Basically, every wall cavity is connected to every other cavity and to the attic. Basically, if you turned the house upside down and filled it with water it would be pouring into the attic.

In most cases it is just important to know where to air seal, if and when you are there for other reasons. Other than that, as long as you get the major leaks, plumbing, chimneys, and drop ceilings, then you have the most taken care of.

And of course all duct leakage with emphasis on leaks outside the heated envelope.

Bud
 
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Old 06-27-15, 10:17 PM
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Yeah, im hitting the big ones. Not much I can do about the top of the wall, except caulk it, and thats going to look goofy from inside. LOL. There isnt much leakage going on there unless there is a crack in teh tape job....unless I misunderstood.
 
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Old 06-28-15, 03:45 AM
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The leakage isn't from the corner at the top, it is behind the drywall, in the cavity that leads everywhere. Basically, the drywall isn't sealed against the framing and although paper thin, every stud, every top and bottom plate offers a path. If you filled a single wall cavity with water it would quickly flow to the adjacent cavities and out onto the floor. The upside down example says it also has a path to the attic. Top plates, if the drywall wasn't sealed to the studs, are usually sealed with foam or mastic from the attic side where the top plate and top edge of the drywall are visible.

Bud
 
 

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