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We have a ranch with a wood fireplace in the living room. It's fantastic at heating the main living area, but the bedrooms down the hall get left in the cold.

I'm going to install insulated flexible ductwork in the attic, with a blower near the inlet (red), with outlets in each room. The initial duct will be 10", with 6" to each room.

This is roughly to scale, about 24' between the inlet and 2 aligned outlets. Fireplace is to the left: enter image description here

I can either run 10" to a 3-way manifold, and take 6" runs from there. Or I can run 10" to an 8"/6" splitter, take one 6" off there, then run the 8" to a 2-way 6" splitter, and take the remaining 6" off there. The first is preferable, as I'll get more efficient use of the duct, which comes in 25' sections.

Will I see a noticeable difference in airflow one way or the other? If it matters, the lower-right bedroom is the master, and it's fine if airflow is a little less there (other 2 are kids' rooms, which we want a bit warmer).

  • What kind of wood fireplace is this? – DMoore Feb 3 '16 at 21:58
  • @DMoore it's a regency wood burning insert – Drew Feb 3 '16 at 23:21
  • "It's by no means the primary heat for those rooms," but it's the main reason those other rooms are cold: stack effect. – Mazura Jun 3 '16 at 22:51
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In either case you have a 10" duct as the bottleneck in the trunk line. Expanding it downstream won't get you much in terms of flow. I'd go with Plan A.

Perhaps a more important question: Is the inlet attached to the fireplace somehow? You'll probably be disappointed if you just pull air from one room to another. The differential would be so small that you won't see much benefit. In fact, the air movement may make those rooms actually feel cooler.

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    Gotcha, thank you for the answer. That's what I figured, but wanted to be sure. It's not attached. My dad's done a similar setup and it's worked really well. It's by no means the primary heat for those rooms, but it gets the 80 degree air from the living room ceiling into them, so it can be moved around by ceiling fans. – Drew Feb 3 '16 at 21:14
  • Feel free to wait for more answers. I'm not an HVAC specialist. – isherwood Feb 3 '16 at 21:27
  • Possibly a lost cause (assuming it's the source of primary heat), plus one. – Mazura Jun 3 '16 at 22:44
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I'd take the 6" branches down to 4" with a very small blower or standard fan doing all of the work. Your 10" is fine as long as it's capped a couple or few feet after the last branch at the end so it can be pressurized for even distribution.

Your goal is or should be to sip the living room's heat & not gulp it to leave the living room, kitchen or bath(s) cold. Other than all that work, you may find that your blower or fan by itself mounted to the ceiling at the head of the hallway will make the difference...unless you have snorers, screamers & talkers.

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Will I see a noticeable difference in airflow one way or the other?

The fan creates the flow according to the amount of resistance in the ductwork and the fan's CFM rating. If your setups are approximately the same the flow will be the approximately same. Doing the math on 3x6 vs an 8 and a 6. I think the 3x6 would have less resistance and therefore more flow overall. Just a guess though.

However, the resistance of each 6 inch run will depend on its length. The longer runs have more resistance and therefore the farthest bedroom will get the least and the 2nd bedroom less flow than the first on the line. So, from the point where you split the flow, the longer runs will receive less flow because they have more resistance.

The other thing to consider is the higher the flow the louder the velocity noise from the moving air. I don't know what the number is for air but in a hydronic heating system they try to keep it less than 4 ft/sec and size the pipe accordingly.

Flow and pressure works similar in pneumatics, hydraulics, and electricity.

Good luck with your project!

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