Will splitting a duct to extend HVAC to an addition cause problems

ductshvac

I'm building a 15 x 26 addition off of current family room to use as a music room. The family room has 2 vents(registers) where the family room meets the new room, and because of convenience they extended the 2 into the new room. That's fine cause they were lined up perfect.

The issue is they were told to replace what they stole from one room, and now want to split that duct run from the one register to two. Basically they want go up to feed original duct, then keep going and feed new room.

My concern is the family room will not be getting all the conditioned air that it originally did, and will be too warm in the summer and too cold in the winter because of the reduced flow. they robbed Peter to pay Paul so to speak. Am I crazy for asking for replacement runs with registers to original family room?

Best Answer

Typically Ducts are sized for the space they are conditioning. Splitting a duct will reduce the efficiency of conditioning that space. You will need to know if the HVAC system has enough CFM Flow for the addition to begin with. Ideally it would be best to run a line direct from the main box coming from the HVAC unit. You could also increase the size of the line to the main unit and then branch from the larger line to the addition.

Will it cause a Problem in the HVAC System, not really, especially if it is not zoned. If you have a Zone system then your bypass damper may need some adjusting as the back pressure of that run is now lower.

Scott<-

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Ceiling – reroute this air duct

I don't see a problem with that, other than the existing duct doesn't look wide enough to fit both. You'll have to get/build a reducer that goes from something slightly wider (that can support both circular ducts coming out of it) to the existing square duct size. Most metal shops can build this sort of thing for you - I had a couple 45-degree elbows built to re-route some ducts in my basement, it cost me around $50 I think.

 | |   | |
 | |   | |    <--  existing two round ducts (one moved)
 | |   | |
+---------+
|        /
|       /    <-- new reducer
+------+
|      |
|      |     <-- existing square ducts
|      |

The main thing is to keep the number of elbows to a minimum. In fact, in your case it may work better as it looks like you'll reduce the number of elbows by one.

Another option would be to extend the main duct, and split the two round ducts off at the end. Not sure how long those are now, but the box stores carry a few regular square duct sizes you can buy and assemble/install yourself.

Should I use a quick or gradual 90 in the flexible A/C ductwork

Sharp corners in the duct work are not preferred. They create eddy currents in the air flow that results in greater pressure drop than a straight run of pipe. In some situations the eddy currents can also create increased air turbulence noise as well.

Instead of using elbows for ducts you should consider using the newer style of non-metallic duct material. This has many advantages including:

It is much easier to work with.
There is far less thermal drop across the duct material than metal duct.
It is lightweight.
You can easily snake it in any direction during installation.
The insulation material is built right into the duct itself as opposed to having to be installed separately.
Duct tape sticks to it like a champ.
Tools needed for installation are very simple ones that most DIYers will already have.

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Best Answer

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Ceiling – reroute this air duct

Should I use a quick or gradual 90 in the flexible A/C ductwork

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