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smontanye
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Registered: 01/15/09
Posts: 131
Loc: hollandale, mn
ductwork
      #6480186 - 08/24/13 08:28 PM

last winter I installed a 120k btu forced air furnace and never did any ductwork. Just made a partial plenum with a large opening and let it blow wild. now im looking to add the ducts to better circulate the warm air. My shop is a 56x40x10h. I believe the blower is a 2000 cfm. any ideas whats needed for sizing? main trunk line would be about 40' and thinking of 3-4 lines coming off of that. I got some calls into some pros but everyone is really busy that I cant even get a call back so might try myself.

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jasfrank
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Registered: 03/22/06
Posts: 254
Loc: rural mn
Re: ductwork new [Re: smontanye]
      #6480222 - 08/24/13 09:21 PM

With just 3-4 outlets they'll need to be huge. They figure 100 cfm per 6" pipe and you need to divvy up 2000 cfm if I read you correctly. I'd measure the plenum size and if its 20x20 I'd estimate you would need 400 sq inches of pipe coming off it or slightly more. Theres about 28 sq inches in a 6" pipe and about 50 in an 8", about 80 in a 10" pipe, and about 110 in a 12". So if you wanted 4 drops I'd make them 12" pipes. Also you should size the plenum above so its full sq in to start (same as plenum outlet on furnace) or half of full size if going each way. Then drop the size each time you bring a pipe off it. Then make your intake the same size in sq inches as the plenum outlet. I hope this makes some sense to you.

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smontanye
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Registered: 01/15/09
Posts: 131
Loc: hollandale, mn
Re: ductwork new [Re: jasfrank]
      #6480714 - 08/25/13 03:22 PM

jasfrank said:


With just 3-4 outlets they'll need to be huge. They figure 100 cfm per 6" pipe and you need to divvy up 2000 cfm if I read you correctly. I'd measure the plenum size and if its 20x20 I'd estimate you would need 400 sq inches of pipe coming off it or slightly more. Theres about 28 sq inches in a 6" pipe and about 50 in an 8", about 80 in a 10" pipe, and about 110 in a 12". So if you wanted 4 drops I'd make them 12" pipes. Also you should size the plenum above so its full sq in to start (same as plenum outlet on furnace) or half of full size if going each way. Then drop the size each time you bring a pipe off it. Then make your intake the same size in sq inches as the plenum outlet. I hope this makes some sense to you.




my plenum opening is 24''x21''and will be roughly 7' tall to reach the ceiling. each of those 4 drops would be about 24' long with multiple vents coming off them. I got a simple layout I could post if I knew how to post pictures...


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thooksModerator
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Registered: 04/05/01
Posts: 50376
Loc: Marietta, GA
Re: ductwork new [Re: smontanye]
      #6484024 - 08/28/13 12:54 PM

2000 CFM of supply air at 0.08" static pressure per 100' of ductwork will require a 20x16 (plus insulation). Run this for 20' and pull (4) 9" or 10" round branches off. Transition to a 12x16 for the next 20' and pull four more 9" or 10" round branches off.

You should terminate the round branches with a 10" round necked diffuser or one with a 9x9 square neck at minimum. A 12x12 neck will help slow the air down better and reduce stratification.


That's about as simple as it gets...


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thooksModerator
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Re: ductwork new [Re: thooks]
      #6490112 - 09/03/13 08:02 AM

Is it built yet?

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smontanye
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Registered: 01/15/09
Posts: 131
Loc: hollandale, mn
Re: ductwork new [Re: thooks]
      #6490775 - 09/03/13 05:09 PM

no, I finally got some pros to look at it but now waiting on their bids before I decide if I tackle it myself. My access to ductwork is prob gonna be from menards. so would 8x24 work for main line then have 12-14 6'' take offs with diffusers work? im gonna install the coil for ac too now so I don't have to redo the plenum in spring.

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thooksModerator
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Re: ductwork new [Re: smontanye]
      #6491412 - 09/04/13 10:18 AM

8x24 (I would say 24x8) will flow about 1000 CFM, half of what you have.

A 6" round branch is good for about a max of 125 CFM. 8" round is good for about 215 CFM.



You are probably looking at $2500-3000 for a decent job with sheet metal and minimal flex. Lotta flex = less efficient, higher energy bills. Pay now or pay later, as they say.

I wouldn't have fiberglass ductboard installed. Not gonna get into a discussion about it here. I'm sure there's many that approve of it.


If this is for a shop, you could look at having a small amount of properly sized and insulated rectangular duct installed with sidewall grilles or even drum louvers put in that will throw air across the shop effectively.


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smontanye
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Registered: 01/15/09
Posts: 131
Loc: hollandale, mn
Re: ductwork new [Re: thooks]
      #6492578 - 09/05/13 08:26 AM

If the duct work is all in the conditioned space does it still need to be insulated?

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daddo
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Registered: 06/13/04
Posts: 8382
Loc: McQueeney,Texas
Re: ductwork new [Re: smontanye]
      #6493014 - 09/05/13 02:07 PM

smontanye said:


If the duct work is all in the conditioned space does it still need to be insulated?





Maybe- maybe not.

"Conditioned space" is, of course where the space is conditioned - meaning even during a time of vacancy, saving an energy setback time.
In a shop where the system may run a few minutes, hours, between off times, it could be a problem.

If the shop gets warm/hot and humid, then the a/c is turned on, the duct can very well get below the surrounding airs dew point and start sweating until the humidity is dropped in the space relative to the temperature.

During no cooling days, the paper, cardboard, sawdust and wood will gain humidity during this time and it could take a day to a few days to drop that back down. Infiltration can be a problem as well (Outside warm humid air entering the space through doors, windows, cracks, ceiling tiles, etc.).

This would be one reason to make sure the duct is the right size and the system is moving the proper air- (Filters, returns, ducting, system sizing cfms).

If it is exposed duct, you may try it first and if it becomes a sweaty problem, the ducts can be easily wrapped with insulation.

--------------------
"There are no strangers- only friends I haven't met.


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thooksModerator
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Registered: 04/05/01
Posts: 50376
Loc: Marietta, GA
Re: ductwork new [Re: smontanye]
      #6493056 - 09/05/13 02:27 PM

Yes, it needs to be insulated, for at least 2 reasons. One is what Daddo posted about, sweating. The second is for efficiency.

It's going to hurt more being un-insulated than it will insulated. I know it's not inexpensive, but nothing is if you do it right.

Unless the HVAC contractor has a special crew/people that just do insulating, you can probably find it done slightly cheaper and probably much better if you get a mechanical insulation contractor to come and do it. Yes, there are particular contractors that specialize in only insulating piping and ductwork. Those that are good are smokin' fast and do a job that is hard to duplicate unless you do it day in and day out.


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