Duct ineffecient design - bedroom 1 high flow, bedroom 2 low flow

[snakepit27]

See attached screenshot.

I'm doing duct work for my basement. There's two bedrooms. Notice one is a 10 to 15 ft run while the other a 22 ft run with two elbows.

I had both go about 1.5 ft from the window.

Room 2 pressure is low, while room 1 is high.

I transition from 7 inch to 6 inch pipe.

What would be a good way to optimize?

 

[Dana]

I don't see where room #1 is connected at all? Is it teed off at the partition wall between the two rooms, or is it a straight run to the trunk/plenum?

Are you possibly confusing pressure with velocity? The cross section of a 7" pipe has 36% more area than a 6" pipe, and at the same cfm would have lower velocity. Or did you measure the pressure with a manometer?

Each turn at an ell or tee adds considerable "equivalent length", which adds duct impedance due to the induced turbulence. What is the total equivalent lenght (straight pipe + e.l. for the turns) is there on each run?

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[snakepit27]

Thanks Dana. Would it be better to shorten the run in room 2 and have the vent at the 2nd elbow? That would essentially be 10 ft from the window. I'm not sure, is it bad practice to have the register far from the window in the basement. My theory is I could get more air flow if I shortened the line.

So I guess the question is, is it better to have more airflow and not have it by window or have it by window with low flow?

 

[snakepit27]

btw, that equivalent length chart is amazing!!!!!!!!! basically i'm adding an extra 60 ft in length with those 2 elbows!

 

[Dana]

The whole "throwing it at a window" made some sense when houses were basically un-insulated, with air-leaky single pane windows.

With any reasonably tight house and even a single-pane + tight storm window there's very little additional comfort value to putting the supply register next to a window.

In very high performance houses heated with the pipsqueak air handlers of a ducted mini-split they usually use the absolutely least amount of duct possible to maximize air flow, and use registers designed for directed throw to guarantee mixing within the room, eg:

<<Yes, that one tiny ceiling register heats & cools that whole room!

With a bigger deal air handler you just have to keep the duct impedances low enough to deliver the right amount of flow for temperatures to balance with other rooms on that zone. With basements that can be tough if zoned with upstairs rooms, since the heat loss characteristics of basements don't change as much with outdoor temperature as fully above grade rooms. But basement rooms have less of the lossy above-grade wall exposure, and usually smaller window sizes too. That makes it even LESS important to put the register at the window.

Hopefully you've insulated the foundation walls?

 

[snakepit27]

Yes. We are going to insulate the basement walls. This is really good info and makes sense. I like the idea of the shorter duct run. Thanks a ton.

 

[Dana]

Make sure to insulate the foundation walls in a manner that won't turn it into a mold farm. The particulars have been detailed in multiple threads on this site, but I can walk you through it in a climate-specific manner if you can share your ZIP code or county. (Utah covers quite a range of climate zones, from US zone 3B through the cold edge of 6B, and it matters!)

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