Splitting forced-air ducts

[Stereo]

I am reconfiguring some rooms in my house, not adding any square footage. My forced air furnace was installed in 1995. I’m quite sure they didn’t use any calcs back then to figure out the proper duct runs, even though I gave them all the information but they would have needed for the calculations. There is no return in one of the bedrooms and all the supply ducts, no matter how long the runs, are 6 inch, so I really don’t think I have to worry about messing up the balance of the system, though correct me if I’m wrong. I know you need a lot more than just square footage to properly size a system, but I’m hoping the information I’ll be giving you here will be sufficient for my purposes.

I have two 6 inch duct runs that I want to split to service the new spaces. I talked to one HVAC designer who said the cost to design the changes was too high relative to the benefit. He suggested just splitting the 6 inch lines into two 3 inch lines. I then had a local HVAC installer come look at my situation and he suggested a four-five split coming off the sixes.

My first question is a general one. If the duct is split, shouldn’t the two smaller ducts add up to the larger source-duct?

Here are some of the specifics:

The first 6 inch line serviced a single floor register on the second floor. I’ve had it disconnected now as part of the remodel and haven’t noticed much, if any, of a temperature change in the area that it serves which is an open area at the top of the stairs. I’d like to split that duct so that I can heat a new 35 sq ft bathroom with a wall register which would be located immediately adjacent to the 6 inch line. If you don’t have an issue with me splitting the line, what duct sizes would you recommend the two smaller duct lines be after the split?

The second 6 inch line used to T into two other 6” lines that served a room that would get terribly overheated because the line is so short coming off the furnace, plus a 3/4 bathroom that was comfortably warm. I’d like to serve a similar configuration, but I know a 6 x 6 x 6 T is not the appropriate way to split the line. Again my question is, what size should the duct be past the split, one to heat the bathroom, and one to heat the bedroom which can be on the cool side?

Thank you for your input.

 

[WorthFlorida]

A=π x r2 (square). It's the cross area of the duct. If you have round ducts, a 6" duct has 28.27 sq inches. A 4 inch duct has 12.57 sq inches. A 3" has 7 sq inches

Do the same for square ducts A= width x height.

Splitting the duct you'll find it will be too cold in the winter and too warm with the AC on. If you do go the split route, you'll need to increase the blower speed to push again this restriction to get some air flow.

 

[Sponsor]

 

[Stereo]

A=π x r2 (square). It's the cross area of the duct. If you have round ducts, a 6" duct has 28.27 sq inches. A 4 inch duct has 12.57 sq inches. A 3" has 7 sq inches

Do the same for square ducts A= width x height.

Splitting the duct you'll find it will be too cold in the winter and too warm with the AC on. If you do go the split route, you'll need to increase the blower speed to push again this restriction to get some air flow.

Lazy me. I made an assumption about the ratio rather than doing the math. The square foot calculation would suggest that I should split the 6" into two 4" maximum, based only on area, but you are suggesting that the increased resistance will cause issues. As I described, one of the original 6-inch lines had been branched off without reduction so that the single line was feeding two registers and doing a good job of it. Might you be suggesting that I should keep the ducts 6" after the split to avoid the resistance issue? Or if I really shouldn't split the lines, what size lines off the plenum do I need for those small bathrooms? If resistance is an issue, does that mean that any smaller duct, even if it comes directly off the plenum, is going to cause problems?

 

[WorthFlorida]

You can do two 6” lines and if there is a problem, with a damper you can cut adjust the air flow for a good balance.

 

[Stereo]

Thanks. As I'm mulling over your input, I realize I would appreciate a better understanding of the resistance issue. I certainly understand that if I tried to push the same amount of air in a 6"duct through a 4" duct, there would definitely be an increase in resistance. However, if I split a duct, then the air pressure in each of the two new branches will be lower. To efficiently move the air in the new branches, it would seem that the ducts after the split should be smaller in order for there to be enough pressure to move the air efficiently through the ducts. What am I missing?

 

[Dana]

There is no real penalty for oversizing the ducts then tweaking the balance with balancing vanes near the register end of the runs. A lower duct velocity doesn't mean it's delivering less air- in fact it's the opposite: The lower static pressure drop along the duct run means there is more air available to the register. The only place on the system you want more velocity is at the register, which can be induced by a tapered reducer or a register grille designed for better "throw".

There IS a real penalty for not having returns. For a doored off room with no return, create a jump-duct return path. A ventilation grill at the top of the wall on one side, on the bottom of the wall on the other of a partition wall to a common area can be a cheap & effective fix.

An Energy Star duct system would have no more than 3 pascals (0.012" w.c.) room to room pressure difference at all air handler speeds, room doors open or closed. With no return path the room gets less air, but becomes pressurized relative to the outdoors, while other rooms with the returns become negatively pressurized, which means the system is using "The Great Outdoors" as part of the return path, with understandable effects on efficiency and comfort. The air-handler driven infiltration is a common cause of extra-dry indoor air in winter.

 

[Fitter30]

In the one bedroom with no return is there a gape at the bottom of the door? Does the room have a problem with the temp? Going back to the trunk line and not teeing off a existing line is the proper way to run the duct and run another 6" just damper it down.

 

[Dana]

In the one bedroom with no return is there a gape at the bottom of the door?

Doors cuts usually need a gap big enough for a small cat to crawl through to provide an adequate return path at Energy Star pressure levels, which creates light /sound /privacy issues. Jump ducts of some sort are usually a more satisfactory solution.

 

[Stereo]

Thank you for all your input. I do have extreme space and access constraints because I'm working on a portion of the house that is almost 100 years old. Additional service runs are not an option.

I am isolating two rooms and a small 3/4 bathroom to create an apartment so I absolutely have to get return air in there. I will put in a jump duct between the two main rooms but I have to figure out how to get a return back to the furnace. All the perimeter walls are over a very low crawlspace with only 2x6 joists, or over beams, so I can't get a return into a wall. I have zero room for a chase. The only option seems to be to put in a floor return, though this, too, is not ideal as it will be in a walkway lest it be covered by furniture. Also, I would guess in a cold climate it would be preferable to put the return high on the wall. Are there any workarounds?

How large of a return duct and grill should I install? Each of the main rooms is currently serviced by a dedicated 6" duct, one per room.

 

[Dana]

There are a number of ways to skin this cat, some easier to pull off well than others.

The return path has to be at least as large as the supply duct or register in cross sectional area.

If there is a hallway to use as the return "duct" to a common-area return, partition wall stud bays are usually big enough, using a ~12" x 6" (or larger) ventilation grille on each side of the wall, one on the bottom, the other at the top. There isn't any advantage to going larger than that, but going much smaller could create enough restriction to inpart a pressure drop.

The free-air cross section of a 2x4 16" o.c. stud bay is only about 51 square inches (a 6" round duct is about 28 square inches), so even with the grille area blockage fraction a 12" x 6" is going to be enough without being overkill.

Be sure to seal up air dams in the stud bays to ensure the stud bay isn't creating leakage paths out the top or under the bottom. It could be a cut 2x4, but be sure to caulk the seams with polyurethane caulk- those edges are much bigger leaks than you might think. If building out a new partition wall (or gutting one side of the partition wall) use hard retangular duct, which is inherently more air tight.

Transfer grilles above door framing can also work, but pass more sound and light than studwall bays.

If there is no options but to go under the floor into crawl space or above the ceiling into the attic for the return, 8" (i.d.) insulated flex ducts & boots can work, but it's critical to get the air sealing details where it passes through the ceiling or floor right.

 

[Stereo]

Thank you for the details, Dana. Since there will only be one occupant in this space, noise transfer will not be an issue so I'm going to use a simple transfer grill high on the shared wall to allow air flow between the two rooms. My particular challenge is finding a place to receive the return air that goes back to the furnace. I'm going to start a different thread as we're getting off-topic from the original question about splitting supply air ducts, though all input thus far has been hugely helpful.