I'm building a 15 x 26 addition off of current family room to use as a music room. The family room has 2 vents(registers) where the family room meets the new room, and because of convenience they extended the 2 into the new room. That's fine cause they were lined up perfect.

The issue is they were told to replace what they stole from one room, and now want to split that duct run from the one register to two. Basically they want go up to feed original duct, then keep going and feed new room.

My concern is the family room will not be getting all the conditioned air that it originally did, and will be too warm in the summer and too cold in the winter because of the reduced flow. they robbed Peter to pay Paul so to speak. Am I crazy for asking for replacement runs with registers to original family room?

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    Edited since with the previous title, someone searching for your question would have no idea what you're asking and ignore your post.
    – BMitch
    Commented Jan 8, 2015 at 14:23
  • It depends how big the runs are and the size of the vents. In most residential settings you get one vent per run but could get two with a larger run (or smaller vents - which will effect output).
    – DMoore
    Commented Jan 8, 2015 at 14:45

1 Answer 1


Typically Ducts are sized for the space they are conditioning. Splitting a duct will reduce the efficiency of conditioning that space. You will need to know if the HVAC system has enough CFM Flow for the addition to begin with. Ideally it would be best to run a line direct from the main box coming from the HVAC unit. You could also increase the size of the line to the main unit and then branch from the larger line to the addition.

Will it cause a Problem in the HVAC System, not really, especially if it is not zoned. If you have a Zone system then your bypass damper may need some adjusting as the back pressure of that run is now lower.


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