Can I put two register vents on one duct?


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Old 05-18-18, 05:50 AM
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Can I put two register vents on one duct?

I currently have two vertical ducts going from my first floor to my second floor. The first Duct (A) is rectangular with an outside diameter of 3 5/8" x 8" and the other (B) is 3 5/8" x 10". Duct A supplies heat to an upstairs bathroom (5'x8') while duct B supplies heat to an upstairs bedroom (13' x 24'). The bedroom has another supply register vent as well as return vent. Currently, all ducts, including the secondary bedroom vent, come straight from main trunk and terminate with only one register vent.

Both A & B ducts enter a small upstairs attic area before terminating into their respective rooms. From the attic, I have good access to the backs of the walls that hold the duct boots. Register vents are wall mounts located about 4 feet from each other.

I would like to remove the wall currently holding the vertical ducts. My intention is to instead run a new duct from the trunk and route it to the small attic, then out to both the bedroom and bathroom using the same register locations. The overall length of the new duct will be about 10' longer than either of the two current individual ducts, although the new duct would originate from the trunk closer to the furnace source.

Question #1. Will the amount of heat I push through the single duct be radically different from what I'd get from two separate ducts?

Question #2. What size and style duct should I used for the new duct? My assumption is that I should go as big as possible to push more volume of air, but I'd also like to make installation no harder than it has to be.

I'm sure I'll have a few follow up questions, but that's for your help whatever advice you can offer!
 
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Old 05-18-18, 07:11 AM
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Things that affect air flow include bends, duct run (length) and area.

You currently have approx 72 square inches of duct supplying the two rooms.

Assuming that the current air flow is sufficient then the only real change is the length, which would be minimal.
 
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Old 05-18-18, 09:53 AM
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Marq1,

Thanks for the reply. That's good to know. Truth is, I usually close off one or both of the bedroom vents in the winter because the room already gets enough heat coming up the stairway. Even so, I'm glad to know the change will be minimal.

I'm comfortable with plumbing, electrical, basic and finishing construction, but I'll never had to mess with duct work. Moving on to my #2 question, should I just use something similar to the 3 5/8" x 10" duct or is their an easier option? Maybe something bigger? Maybe something cylindrical?

As it turns out, I'm redoing the kitchen under the attic space and I have mostly good access to the basement joists where the duct goes through and to the vertical wall going up. I'll be ripping out the kitchen floor and, if necessary, I can cut through the subfloor to access the joists where the new duct will lay but there might be a few short runs where I'll need to push the duct over a wall where I don't have exposed access. I think I can lay and fish the rectangular duct into place, but I'd appreciate any tips to make it easier or to avoid pitfalls.

Also, unless there's a reason not to, I'm planning to feed the bedroom vent first, then veer off to the bathroom (4' away). Even if I use the rectangular duct to get me to the attic, might I be better off using flexible venting to get to the boots behind the registers?

Thanks again for any help you can offer.
 
 

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