I recently purchased my house which was a new construction. Several of the vents are getting poor airflow but the installation company simply said it was normal but I suspect otherwise. I recently realized there is a branch duct coming out of the end of the trunk line and I've read this can reduce airflow.

Does anyone have any suggestions or guidance on how to fix this? I was considering extending the trunk line another 3-4 feet and moving the end duct to the side like the others.

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2 Answers 2


The fact that one branch duct is at the end doesn't mean much. You could install a damper there to balance flow a bit.

Of bigger concern is that they used flex duct and bent it severely. If the one in the photo is any indication, they didn't do a great job. You shouldn't see such sharp, tight bends. It also doesn't appear that flex duct was necessary there, but was used to save time (and maybe money).

  • Thanks for your thoughts. I ended up taking the end duct off and found they cut half a circle which acts as a damper (I think). I also blocked it off temporarily with foil tape and the vent going up blows a lot more air out.
    – Matt Young
    Commented Feb 9, 2016 at 0:51

The comment below is partially correct and bocking off more of that end run will certainly help but it's not the ultimate answer my friend. Take the insulated flex duct off that elb run cut into the side and install a proper top take off 1" larger on the side angled up. This looks like a 5". Install a 6" with one proper 90 degree elbows and reduce down to the flex size after over the duct. Connect the reducer and elbow in the flex first then put the elbow on the tto, by going an inch larger will compensate for the two 90's since each adds 10 of length to the run, in this case it's adding about 35 feet since the flex is kinked. It should be simple, then take off the foil tape and put a proper take off on the end or just a collar wth a damper the size of the pipe so it can be adjusted. Then also remove the register in the bottom of the duct and make a patch large enough that a side take off can be cut into it and run a round pipe down to 6" above the floor with a damper and a 4"x!0" boot to fit a register, you want the heat discharging at the floor in your baement whenever possible and by dropping it down it will add some resistance to that run to help add more air flow to the run not getting much. I hope this helps you out. Guys who install systems this way should be meat cutters for we refer to them as Butchers. They get paid by the number of jobs they finish at the end of the week and don't care if they work properly or not! All installation companies will say that's normal because it costs money to send someone back to fix the mistakes. You could have a balancing company come in to set it up but it means paying them to fix the butchers screw ups at about $600.00 or more ;)

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