we recently had a new unit installed and I think it is a little oversized. It does push out more CFM‘s then the way the ductwork was designed. I have a room that I recently added that I would like to run a branch to. I can run a 6” from the plenum and another 6” from a trunk supplying the living room which has too much airflow right now. The reason why I think it has too much airflow is because the thermostat is in that room in the unit cycles on and off a lot. Plus the air is really pushing out. After making the holes in the ductwork the airflow is still sufficient.

My question is can I take these two 6 inch runs and combined them into an 8x6x6 wye so I can run an 8 inch into the new room. The new room has west facing windows so it is going to need more than a six-inch. By combining them into an 8 inch will I lose pressure since I’m going from a smaller flex to a bigger flex? Thank you

  • Check to make sure your flex is fully stretched; also, have you had anyone check the static pressure across your blower? Apr 11, 2020 at 16:28
  • I have not. How does that play into this? Apr 11, 2020 at 16:38
  • I did some reading on static Pressure so I understood a little more now but no one has measured it. I just know the airflow is much stronger than prior unit. When 90 degrees it only takes 17 minutes for unit to shut off. When around 85 degrees or so it takes 11 to 13 minutes. Other times only 10 minutes Apr 11, 2020 at 16:55

2 Answers 2


8" to twice 6" sounds reasonable.

twice 6" has a little more cross section than the 8", but it also has 50% more surface, so the smaller tubes have a higher friction. Over all I'd say that the match is close enough that it doesn't matter.

  • Thanks. I’m testing by using an 8” from plenum and plugging both 6” holes. This may also help with neutralizing the airflow with the short cycles. I made a new post about that this morning because my electricity cost has gone up Apr 12, 2020 at 12:34

The way I look at it, you'll be loosing about 6 square inches of duct flow by using the wye connecting the two 6" ducts into an 8" duct. Depending on the air flow of each of the 6" ducts, it hard to say whether the combined flow will be more or less. I would be thinking about running the two 6"ducts into you new room and locating them on separate walls since it's a western exposure. Then you'd be utilizing the maximum flow from each duct.

  • Understood. The new room has a flat roof so there is only one wall ceiling I can use. Based on an electrical line blocking some of the truss entrances the two ducts would have to be about 2 or 3 feet apart. It is also a challenge to run because that room is a flat roof. This is why I would love to just run one line but I understand what you are saying. Also, the two six inch lines are traveling almost 20 feet each to the wye. Then another 17 to 20 feet to the room. Thoughts? Thanks Apr 11, 2020 at 16:52
  • @DominickOrlando 40' of flexible duct is a long way. Like ThreePhaseEel suggested, make sure your flex it stretched to the maximum and even then, be prepared to add an 8" duct fan to the run.
    – JACK
    Apr 11, 2020 at 19:24
  • Thanks. No choice in the length as the room is furthest from the plenum and other run. I can try and run something bigger from the plenum I just don’t want to sacrifice more are elsewhere. If I have to stay with the 6” do you suggest running them separate instead of joining to an 8” wye? Apr 11, 2020 at 20:24
  • @DominickOrlando You'll have more resistance with the two 6's than the 8.
    – JACK
    Apr 11, 2020 at 20:32
  • Thanks. I will have to play with it. I can probably do a 7” from the plenum. I can connect the 6 and 7 to the 8 inch wye and see if I get pressure build up. I need at least 150 cfm in the room Apr 11, 2020 at 21:01

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