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Tool Talk Discussion Forum

Furnace ductwork question?

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blueridge

06-08-2014 10:35:50
12.74.49.173



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Installed Natural Gas 90+ furnace on the upper deck of my shop/pole building 32x40 ft. ran 12" round duct. from furnace down center of trusses. for 12 ft, then 10" for another 10 ft. then 8" for another 10 ft. to almost the end of building between 2 10x10 service doors, capped off the end of the run. Will it heat O.K. if i cut holes into side of ductwork with holesaw angled down toward sides of building? or should i run short ductwork off of this main run? Building has 2 in. styrofoam on sides & ends and 1/2 in. under tin on roof? Thanks! Gene

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ss55

06-12-2014 06:49:40
50.81.112.224



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
If you keep your duct area equal to the outlet size of your furnace, you won't restrict you airflow too much.



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blueridge

06-12-2014 06:41:20
12.74.50.88



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
Thanks for the info, the furnace is 80,000 BTU input. i will agree the 12 in. main ductwork is probley too small to start off with, I can tap into plentum with a 6 in. and go over to my workbench with ductwork. would that be enough? or should the 12" be bigger. i also didn't mention the long run is 13 ft. high on the truses.



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David G

06-12-2014 07:09:37
204.29.138.33



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 Re: Furnace ductwork question? in reply to blueridge, 06-12-2014 06:41:20  
That is a good sized furnace, I think you are way undersized on duct work. How did you determine that 12" was correct?



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dh in Carolina

06-09-2014 15:24:59
173.212.10.195



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
12 Inch round pipe will only handle around 650 cfm. Would have to be really small furnace.



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David G

06-09-2014 15:06:05
205.215.206.18



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
I agree, need furnace info, but the 12" sounds real small.



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jeffcat

06-08-2014 17:48:00
69.244.99.202



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
If you already installed everything you can measure the flow with an aroemeter. they look like a little wind turbine on the top of a short pole. A little pocket one is at many kite shops and looks like a plastic paddle with a little gauge. Figuring ducts, I would ask at a fabrication shop for heat ducts or a store like for heating and air conditioning and refrigeration. National refrigeration.Etc. Jeffcat

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evilboweivel

06-09-2014 09:59:03
24.56.68.110



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 Re: Furnace ductwork question? in reply to jeffcat, 06-08-2014 17:48:00  
need more info!! what size furnace?

what you can do with what you have already is to cut holes with a hole saw in the duct until the static pressure drops in the duct to about .5 to .75 at that time you will be roughly right.
HOWEVER you probably will not get enough air flow across the heat exchanger with the 12" main duct and your furnace will cycle on the high limit.

If you are in the mid south based on your handle then you need to add more ceiling insulation and propably ok on sidewall insulation
In the north way short on insulation

Good luck
Ron

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wgm

06-09-2014 03:10:00
66.11.102.200



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 Re: Furnace ductwork question? in reply to jeffcat, 06-08-2014 17:48:00  
Those things are a joke. The only accurate way to measure air flow is by doing a pitot traverse.



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wgm

06-08-2014 15:03:25
66.11.102.200



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
What size furnace?



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bhb

06-08-2014 11:32:34
173.88.160.25



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
It has been many years since I have had my HVAC company, I would have to look up the amount of BTUs that a 12 inch duct will carry. The amount of BTUs will determine how many take off runs can be used. Maybe someone else that is more current in the business can help. If you cut holes in the duct you will heat the building, but it would be best to run 6 inch round pipe to the outside walls. You would then heat the outside edges of the building and the center would be warm also. Registers would help defuse the air along the out side wall. It would also be best to put dampers in the 6 inch runs to balance the system.

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Dick2

06-08-2014 11:20:21
184.101.236.132



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 Re: Furnace ductwork question? in reply to blueridge, 06-08-2014 10:35:50  
Depends on a lot of factors. The insulation you mentioned will have a lot of heat loss, especially through the roof. A lot will depend on which climate zone that you are in.

The lateral main run in our house is a 24" diameter; the 12" that you describe seems undersized to me. Sideruns off the main should go to near the outside walls, assuming that the air return intake is towards the center of the building.

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