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DIY work on some terrible flex duct thru foundation

sippyCUP | Posted in Mechanicals on

I’ll preface this by saying yes, I know the best option is to get with a reputable HVAC contractor. But I’d rather not since the scope of the “change” (improvement) will be very small relative to the overall system. I am an engineer and I plan on learning how to run my own Manual D calc next week to verify my path forward (for funsies).

I own a 1500 sqft house in climate zone 4 that has a crawlspace. The house has package unit outside, and flexible duct (all 6″) to all the registers through the crawl space. An 350 sqft addition with full bath was built in the last 10 years (before I bought it) and to extend the existing HVAC to the addition, two approx 6″ holes were made in the existing house foundation CMU, and the flex duct run through that (I plan on asking my civil engineer buddy about that but probably okay).

The issue is I get next to no airflow from the two registers in the addition. The root causes are the way the flex is run – pinched up in the joist bays and strangled by electrical and piping, and the flex is of course not pulled tight. Each run is about 30-35′.

The rest of the ductwork in the original house crawlspace is pretty okay. I will take a closer look but nothing egregious stands out.

My plan is to remove both offending flex ducts at the boot, and backtrack them all the way to the supply plenum (within the original house foundation). I’ll buy two 6″ sheet metal splice collars, and place those in the CMU holes. Then I’ll properly run flex from the plenum to the collars (run tight and supported well), then go on the addition side and run flex from the other side of the collars to the registers. I’m also going to buy some 90 degree sheetmetal elbows for the register boots to reduce bending the flex duct. And I’ll hit everything with mastic at the joints.

Curious how other might approach this? I haven’t run duct before but this seems like a good way to get my feet wet. The only possible issue I can think of would be the sheetmetal collars getting moist from the CMU block, then rusting. The ones I’m looking at are galvanized though so shouldn’t be an issue… maybe I’ll need to avoid the HD brand and go with the most reputable manufacturer I can find.

Thank you very much,

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  1. Expert Member
    NICK KEENAN | | #1

    If the ducts to the addition are significantly longer than the ducts to the rest of the house just their length may be a problem even if they are better installed. The cheapest solution may be to install an inline booster fan.

  2. Expert Member
    BILL WICHERS | | #2

    I’d try to replace as much of that flex duct with rigid duct as you can. Flex duct has a LOT more resistance to air flow compared with rigid duct even when it’s properly installed. You also want to keep the duct runs as straight as possible so try to minimize any bends or transitions. Every bend or transition is equivalent to several feet of straight duct just like how you work out pressure drop for water or gas pipe.

    Ductwork is usually flash galvanized, not hot dipped. This just means it’s not as durable as you might think. Try to keep it from contacting masonry if you can. If you have no option, get some 20 mil tape and wrap the part of the duct that goes through the masonry to provide some protection there.

    BTW, don’t forget your duct mastic for the joints!


  3. sippyCUP | | #3

    Thank you both for your replies, some good thoughts!


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