# Splitting Round Ducts

### Splitting Round Ducts

I have existing ductwork in attic for ac/heat pump and want to tie a coal furnace into it from the basement through one floor (raised ranch). The existing ac/heat pump trunk is 10"x16"x16' long with qty of 7 -6" flex duct coming off the trunk feeding 3 bedrooms, kitchen,living room and sunrooom 1650ish total sq ft. The ac/heatpump air handler has a blower running at 870 cfm. I used an online ductulator and determined i am getting 783 FPM which i believe is good.

The ductulator also told me if i run a 14" round duct from the furnace to the attic to tap into the main trunk i will get 814 FPM assuming the blower on the new furnace runs the same speed as the ac blower in the attic (I do not plan on using the attic blower. I only used this as a frame of reference/baseline for FPM.) The magical ductulator also told me i can split the duct coming from the furnace into 2 round ducts instead one 14" duct.

This is all a long-winded way of asking the following question: The cross sectional area (CSA) of two 7" round ducts does not equal one 14" round duct, so do i need to have the two ducts CSA equal the one 14" duct CSA?
14" round= 7x7x3.14=154 csa
7" round=3.5x3.5x3.14=39 x 2 ducts =78 csa
10" round=5x5x3.14=79 x 2 duct = 158 csa

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Thanks,

Mark

heatwithcoal
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### Re: Splitting Round Ducts

a 10 " round AND a 12 " round would handle the flow of a 14" round duct
300 and 480 cfm respectively
Rigar
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### Re: Splitting Round Ducts

Ps...
The volume increases dramatically as diameter does

(ex. two 4" ducts do not equal one 8" duct)
Rigar
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Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
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### Re: Splitting Round Ducts

I have another question regarding length:
If I have the existing 10x16x16 trunk in attic (listed above) is it realistic for me to tap into this 16' trunk with a 30' run (20' horizontal and 10' vertical) from a furnace in the basement? This 30' will be tapping in at the middle of the trunk, so 8' along the 16' length. I recall reading that a trunk should not exceed 24' unless it is reduced.

Mark

heatwithcoal
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Posts: 209
Joined: Wed Sep 12, 2007 9:48 pm
Location: Western Massachusetts
Stove/Furnace Make: Leisure Line
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### Re: Splitting Round Ducts

Ok....couple questions....so we are on the same page and i dont misguide you.

Is the ductwork in the attic space th ONLY ductwork the furnace will supply?
Is the furnace a "true" furnace?....
Is there to be a cold air return back to tbe furnace from each room.
What size fan is on the furnace? (blower cfm)
...cfm ratings arent created equaly...
the ductwork coming from the furnace..... before it supplies the ductwork in th attic.... will there be take offs?..... To supply heat to other rooms

.. it is not so much the length of the trunk that determines when it is reduced.... as much as it is the number and size of take offs ( supply branches) ..
for example.. if I had 24 feet of trunk.... and the first 8 feet were supplying several areas.... I would reduce the supply trunk....
... all I can say is distribution is critical.... and you may need to consider the blower size on the furnace

... 1 last question... the registers for the ductwork in the attic..... do they come out on the floor or up high in the top of wall
Rigar
Member

Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
Stove/Furnace Make: Keystoker
Stove/Furnace Model: A 150 warm air furnace

### Re: Splitting Round Ducts

only ductwork furnace will supply? yes
looking at keystoker a125 (a150)
I plan on either having one cold air return on opposite end of house or none at all at this point. Basement is semi insulated and I have aprox 1'x3' opening above my basement door and grille on door . This door is in middle of house (raised ranch) so i suppose i could easily run cold air return from the space above the door back to furnace. approx 24'. (house is 48'x28' with 12' x 24' addition.)

I was told it was 1400cfm. I am still waiting to hear if it multi-speed. (i.e. low-med-high terminals)

I want to have a take off from the plenum 8" maybe? to heat basement which is wide open 1350 sq ft.
Registers in attic are on the ceiling in the middle of house. Apparently the HVAC installers were pinching pennies. Although it seems to keep the house cool in summer and the heat pump keeps the house warm down to 25 degrees.
Electric baseboard house, 2x6 construction, well insulated, very tight house. I will have an exterior cold air supply feeding the furnace.
existing trunk is 10x16x16'

Did I miss anything?
Thanks for any guidance

Mark

heatwithcoal
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Posts: 209
Joined: Wed Sep 12, 2007 9:48 pm
Location: Western Massachusetts
Stove/Furnace Make: Leisure Line
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### Re: Splitting Round Ducts

The keystoker A150 utilizes a convection blower rated at 2060 cfm.
I believe the inlet and outlet (throat size ) for cold air and warm air is 24 by 25
you could easily dedicate more than one 8 " for the basement( start with 2 at least- and damper them)

I think you will be very impressed with the Keystoker A150...
( we bought 1 new this year)
Rigar
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Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
Stove/Furnace Make: Keystoker
Stove/Furnace Model: A 150 warm air furnace

### Re: Splitting Round Ducts

I am assuming you meant outside air for combustion?

RETURN AIR to the plenun for heat will be critical in this case...
that much air in that size trunk could create high velocities...
but that would be easily controlled in the basement (with 2 or maybe even 3 )- 8" take offs with dampers
Rigar
Member

Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
Stove/Furnace Make: Keystoker
Stove/Furnace Model: A 150 warm air furnace

### Re: Splitting Round Ducts

Mark...

for what it is worth... I originally wanted the Coal trol with my furnace..
Keystoker would not warranty the convection blower if I got the coal trol as they claimed it tax is the motor too much

Neil from coal trol says he has done it in the a 150... with no problems
I strongly suggest you consider cold air returns directly to the furnace.
without them I think you would produce negative pressure in the basement where the furnace is located... which would result in draft problems.
it is a very strong convection blower...
for around under 200 dollars.... you could get a variable speed motor if you wanted it.... but I think you would be happy with the 1 that they stocked with the a 150
Rigar
Member

Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
Stove/Furnace Make: Keystoker
Stove/Furnace Model: A 150 warm air furnace

### Re: Splitting Round Ducts

Yes the outside air is for the combustion.
I am not really worried about warranty as this is a used furnace.
The gentleman I am planning on buying it off is telling me it is a a125 which is the predecessor to the a150. I spoke with keystoker and they told me this is the same furnace, although this used furnace has a 1400 cfm blower not the larger one you have. Apparently it is not the original blower. I guess my main questions are:
Can I run successfully run a 30' duct to the middle of my 10x16x16' trunk and have favorable results with the available space without a lot of noise?
I have about a 10"x 14" area to run a duct through my closet from basement to attic, is this enough? Also, can I do it with simple round duct? (probably not round, huh?)
I can run a cold air return as it sounds like this is a good idea.
Maybe I am thinking about it too much? I suppose dampers can fine tune the flow?

Ultimately, I love my two stokers but I am getting a little tired of feeding them and emptying ashes, plus I want to take advantage of my new ductwork. This is my rationale behind moving to a furnace. Well that and i like to tinker.

Mark

heatwithcoal
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Posts: 209
Joined: Wed Sep 12, 2007 9:48 pm
Location: Western Massachusetts
Stove/Furnace Make: Leisure Line
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### Re: Splitting Round Ducts

I think youll be fine with 14 " round (you can only do what you can)
...unless there is another chase somewhere

i would still leave a couple of dampered 8 " takeoffs in the basement to help you balance your system

...you could always upgrade the blower if need be...but i really think it will be fine

be critical and seal evey seam and joint on any duct you install...it makes a noticable difference.
...lastly...return air is very important....and i doubt it would be noisy with that size blower.

For a few extra bucks i would install isolation boot material between the furnace bonnet and plenum...
Sold by the foot usually.
Great for noise reduction and to service if you ever need to

...hope this helps
Rigar
Member

Posts: 419
Joined: Tue Dec 04, 2012 7:30 am
Location: central new york (syracuse area)
Stove/Furnace Make: Keystoker
Stove/Furnace Model: A 150 warm air furnace