Flexible ducts

soup006June 4, 2011

My house uses flexible ducts in the attic for our heat and air conditioning. After doing some reading, I have learned that the flexible ducts should be supported with straps to keep as few bends in them as possible. Is this accurate? I have some ducts that put out a good deal of air but others that do not seem to put that much air out. The ducts run to a vent in the ceilings of my house but the vents come up into the attic vertically, allowing for the flexible ducts to bend. Would this be the reason for the less air flow in some locations? One location is at the front, not the end, of the main trunk but it has reduced air. Do I need to use straps to support the ducts or an elbow by the vent so it does not allow the flexible duct to bend? Both?

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There are a ton of factors that affect the amount of air that will come out of a duct. The best thing with flex duct is keep the duct as short as possible but also with as gentle of bends as possible. I do not see any big issues with what you have pictured as long as they are sealed well.

    Bookmark   June 5, 2011 at 10:23AM
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What else can I look at, other than leaks, to see why some vents aren't blowing out as much air as others?

    Bookmark   June 5, 2011 at 7:24PM
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restrictions in the ducts
location of ducts on pleunm should be no ducts on end cap
dampers at plenum (if you have dampers) may need to be adjusted for air flow.

ducts should be strapped with duct strapping ( I use 3" wide duct strapping instead of 2") no more than a 1" sag per 4' section of duct.

ducts should be mastic sealed..you'd be suprised at the amount of leakage at plenum and supply boxes.

I like 90's at supply boxes, but this would entail taking ducts off supply boxes, screwing 90's on to supply box
and reattaching ducts..kinda a big diy..

you can support the duct at the supply box with duct strap..but you better get a couple of rolls of duct strapping!

best of luck.

    Bookmark   June 7, 2011 at 10:55PM
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you may need to have your system balanced. I had a similar issue and same kind of rigging in a second floor of my prior house.

You could have too much air flow in some rooms and not enough in others. A lot depends on duct length, register settings, and the location of the room with respect to sun orientation. Rooms that have windows are on the west and south will be hotter and need more cool air flow. Long duct runs in the attic will need more cool air flow because the air temp will rise more at register output.

When we first moved into the old house (built in 1993), the second floor was very hard to keep cool. And I had hot spots - some rooms were cooler than others.

One of the things I did was to have a tinted film put on all the windows that faced south and west as well as the large skylight in the master bath ceiling. I was amazed at how much heat was reduced with the window tint. In the skylight they used an extra strength reflective tint similar to what atronauts helmets have. I had this done before low e-glass was common in house windows and doors. And of course it cut down on solar gain in the winter so it probably cost me a little more in heating. But I didn't care about that becuase summer was brutal and the attic temps are unbelievably hot - the second floor just wasn't right in summer.

Once I was able to minimize solar gain in the hotest hot second floor rooms, then I had the hvac system on the second floor balanced and fianlly got the hot spots under control. They just partially close some registers in the cooler rooms and open hotter rooms all the way to make sure you get the right air flow to keep temps stable acorss the second floor space. It doesn't cost much to do.

Oh - you also need to make sure that you replace your air filters routinely. A clogged filter will really cut down on air flow.

Good luck

You can do a cheap version of this by getting some cheap window shades (or closing your blinds if you have them) and experimenting with cuttting down solar input in hot rooms during the day - it can make a big difference.

    Bookmark   June 8, 2011 at 2:05PM
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There are a ton of factors that affect the amount of air that will come out of a duct.

apart from leak, i think no other factor is involved here :S

Here is a link that might be useful: flexible duct

    Bookmark   June 28, 2011 at 3:02AM
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