Big Room With Two 14"s; Add a Return & Spilt One 14" into Two Smaller


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Old 02-07-15, 10:25 PM
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Big Room With Two 14"s; Add a Return & Spilt One 14" into Two Smaller

We have a two story house running two spilt unit heat pumps with ceiling vents. The upstairs and downstairs each have a 20x 40 return in the upstairs hallways. There's an additional 20 x 30 return for the upstairs master bedroom/bath area. I'm remodeling an upstairs rec room that's about 3500 cubic feet. It has two 14" supply ducts, equally placed in the rectangular room.

In essence we are dividing the room into two spaces; home theater and kids play/reading area. Not a firm division, just a curtain that will sometimes be deployed to define the theater area. The 8' ceiling is down for major wiring and lighting mods. So now is the time to dial-in the HVAC ducting.

My goals are two-fold, but probably have a bearing on one another. I want to split one of the 14" ducts into two smaller ducts and leave the other duct as-is. The smaller splits are intended to distribute flow on the theater side in a way that no particular seat will get "blasted," and the existing vent will be moved from it's current obstructive position, where it might also cause screen movement. I envision a "Y" of the 14" running about 6' in opposite directions. What diameter ducts should I step down to? I don't want to dramatically change total CFM; I want the other existing 14" duct to still deliver the same approximate volume the combined flow of the two smaller "new" vents. I'm guessing two 8" ducts might open up too much flow, and anything smaller might be too restrictive. What size duct would provide the closest match? Would I need dampening in order to achieve balance?

Part two: this room has sufficient airflow for cooling and heating if the door is open (often it is). But close the door, and the pressure differential is enormous. A hurricane develops in the 2" door/flooring gap! Airflow becomes compromised. I guess it's really begging for it's own return within the room. Should I consider adding a return, which would be complex due to two intersecting roofs and a labyrinth of ducts leading to the air handlers (it's really crowded!)? Not only a lot of work, but I'd be concerned of throwing-off balance elsewhere in the system. Could I try something like a pass-through vent above the door, or maybe a term I'm making up right now (I think): a passive return. What I mean by that would be a return vent in the room ducted above the ceiling to a matched return vent in the hallway on the other side of the door. It would look more professional than an above-door vent, but it would function essentially the same. Just more volume capacity. I'm thinking 20 x 20, maybe smaller.

Sorry for the lengthy description, but I hope to provide enough information for someone knowledgeable to form a mental picture of my situation. Thanks for any suggestions you have to offer.
 
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Old 02-08-15, 08:19 AM
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Instead of using a curtain, to divide the room, what about a partition that stands on the floor & doesn't reach the ceiling? That way the air flow won't be disturbed. Redesigning the duct work requires an engineer, at least the way you describe it.
 
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Old 02-08-15, 11:03 AM
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To be clear, the curtain isn't really something I'm concerned with. It will often be stowed, and will likely be cracked open for walk-thru when in use. I'm more concerned with the room vs. rest of house balance when the door's closed. And I really am targeting the same total CFM on the half of the room I propose to split the 14" into two smaller ducts. I only want two to guide air away from seating positions and the centerline of the projector, as well as the screen (to keep it motionless.)

I'm wondering about your "engineer" comment. If I can accomplish the duct spilt with a similar total CFM for the entire room, there should be no impact on the rest of the house - it's really not a redesign in the big sense. But I claim total ignorance on adding a return to the room. That's why I'm asking, especially in the context system-wide impact. I'd place money on a return solving the pressure differential in that room, just uncertain if that would have a negative impact elsewhere.

Thanks for the thoughts.
 
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Old 02-08-15, 05:09 PM
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The position of supply & return ducts are designed by engineers. Apparently, there was a mistake made, at the beginning since there is a "hurricane" when you close the door. That should never have been. Adding a return or a pass through vent makes perfect sense to me. However, I never designed those systems hence the engineer comment. At least someone who does that for a living should look at it.
 
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Old 02-08-15, 06:36 PM
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When you say 14" ducts, do you mean 14" round? If not, we need both dimensions.
For heating purposes, I strongly suggest near floor level returns. With both ceiling mounted supply & return, the warm air tends to hug the ceiling. With low returns, you pull the cool air off the floor & help to pull the heated air down.
 
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Old 02-09-15, 08:29 AM
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Bad Info On Duct Size - Sorry!

The vent boxes are 14" x 14". I thought the ducts were 14" too, but now I see they are 10" each. Flex by the way. So now I'm thinking maybe two 8" ducts to give a similar (CFM 160 + 160 = 320 compared to 290 for the 10").

Any suggestions on sizing the return?
 
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Old 02-09-15, 09:56 AM
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Somebody decided they needed my good ductulator, with the scales for flex & duct board as well as metal, more than I did but your numbers sound about right.

For return, I always go at least 20-30% more than the supply.
 
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Old 02-19-15, 11:28 PM
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Rethinking Return

After looking at the path for adding a return, it's complex. Cutting into roof sheathing to connect with the portion of the house where the air handlers are located. In itself not a show-stopper, but on the other side lies a labyrinth of flex duct. I won't say there's no room, but it looks like a lot of work.

So now I'm thinking about adding pressure relief via two vent registers connected by flex duct sufficient to match the CFM of the room's supply. Obviously one in the room itself, and the other in the hall outside the room's door. Which coincidentally is just a few feet from the home's two main returns. One disadvantage I see could be sound pass-through. The room will have a theater projection set-up, so I believe sound will be nicely ducted into the hall along with air. But reality dictates that the rest of the home is doomed anyhow in the event of a full-on dolby 7.1 action movie ... just the bass alone will not be constrained with or without any additional "holes" in the room. Putting that consideration aside, why not tackle the flow balance issue in that manner? If so, would it be prudent to place a filter on the inlet (positive pressure) side?
 
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Old 02-20-15, 05:31 AM
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160 cfm is good for metal return duct. Flex would be less (im guessing around 140 or less depending on the length).
 
 

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