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Builder ran a 6" duct on my garage ceiling (will be spray foamed) to a vent that will heat the bedroom over my garage. My ??? is if i re-root that 6' duct and add 1 or 2 90 deg elbows will it effect heat flow to the above bedroom?
 

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Curmudgeon
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Builder ran a 6" duct on my garage ceiling (will be spray foamed) to a vent that will heat the bedroom over my garage. My ??? is if i re-root that 6' duct and add 1 or 2 90 deg elbows will it effect heat flow to the above bedroom?
Yes.
 

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Curmudgeon
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On 4" vents, every standard 90º elbow
adds the equivalent of 15' of straight pipe.
 

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NICKTECH
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the 6" elbow, set up in a 90 deg config will provide the eqivilent friction loss of 6 ft of duct. if the equivilent length is below 100 ft the loss in air volume will be negligeable. how ever exposed in an unconditioned space the heat energy will be drawn from the duct wall reducing the amount of energy delivered to that room. adding an elbow or so and some duct isnt the problem i think, its keeping the heat in the duct. insulation...insulation...insulation!
 

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I need a new chart. Mine says 10'
So adding 2 ells. One several foot from the other will add 20 of equivalent length.
If 2 90° ells are installed together in an S configuration it increases to 30' of equivalent length.

Considering that the lowest take off EL is 15', and the average right angle end boot is 35'(50' without any pipe yet). Adding 2 ells will significantly lower air flow.
 

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NICKTECH
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it'll be lower relative to the desired cfm output, the question was heat loss, not air flow, even though of course loss of air flow reduces btu output. the drop in flow wont change the temperature of the air, but an uninsulated area will. i think thats what was asked, right?
 

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it'll be lower relative to the desired cfm output, the question was heat loss, not air flow, even though of course loss of air flow reduces btu output. the drop in flow wont change the temperature of the air, but an uninsulated area will. i think thats what was asked, right?
His question deserves the full answer.

The longer the air is in the duct, because of lower air volume, the more BTUs that air will loose. So it will probably change the air temp at the grille.

So it will be a 2 fold effect.
 

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All theoretically valid, except in the real world I highly doubt the builder optimized and balanced the ducting layout so perfectly that some sort of hvac nirvhana is going to be ruined by adding a single 6" ell.

Add a single ell, you'll get a tiny bit less airflow, which means a tiny bit less heat delivered, and the smidgen of extra back pressure will cause a minute increase in air flow at other registers, but, frankly, on your average hvac system the difference will be un-noticeable.

Add 4 ells, or if that branch is already iffy, and maybe you've got another story.
 
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