Need help designing furnace ductwork


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Old 08-10-16, 08:03 AM
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Need help designing furnace ductwork

I am looking to install a gas furnace in my single level 1400 sq ft house. I need about 9-10 registers total. I am planning on running the supply trunk/branches through the crawl space and the return in the attic. I was thinking a trunk about 25 feet long with branches no longer than 15 feet to the registers. What diameter trunk and branches should I use? I know there are more variables involved than I am accounting for but any advice is appreciated.
 
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Old 08-10-16, 08:51 AM
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Check with your local codes. Installation of a HVAC system may require a permit and the work may need to be done by a licensed contractor.

The size of ducting and trunk lines you will need will depend on the size furnace you intend to install. The size cannot be figured simply on square footage. Your local climate, how well the house is insulated and the number of doors and windows affects the size of furnace needed.
 
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Old 08-11-16, 07:04 PM
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According to the sq ft of my house and the region I live in an 80k btu furnace is necessary. The furnace I am considering has a blower speed of 1600 cfm. I used a ductulator and came up with a 16" round metal duct but am not extremely confident in my answer. I am thinking a 16" round metal supply trunk about 25' long with 6" flex duct to the registers.
 
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Old 08-11-16, 10:04 PM
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Washington is a pretty big state, EXACTLY where is this house located? If it is in western Washington then I question the necessity of an 80k BTU/hr. furnace.

My house, in Bothell, is about 1550 square feet, all one level, and I have no problem heating it with the first stage only of a 70k BTU/hr. input furnace. That translates to about 40k BTU/hr input or about 32k BTU/hr output.
 
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Old 08-11-16, 10:21 PM
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I live in Oak Harbor, on Whidbey Island. I came up with 80k btu by using an online calculator that accounts for sq footage and different regions of the US. It came up with 67k but my house has some older windows and poor insulation so I figured an 80k might be better.
 
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Old 08-11-16, 10:38 PM
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You need to do a REAL "Manual J" heat loss calculation. I suspect the true heat loss in your home is probably around 50k BTU/hr. at design temperature of 20 degrees F. Also remember that ALL heat loss programs have a built-in "fudge factor" so it is not necessary to "add a bit for good measure".
 
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Old 08-12-16, 07:02 AM
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The manual J load calculation came out to:
18957 total btu's cooling
17956 sensible load
1001 latent load
38185 total heating btu's

When designing the supply ductwork, does the cfm of the blower (1200) need to match the cfm of the trunk? A quick guide I read said to not exceed 900 cfm for the trunk. Here is the link to the guide: http://efficientcomfort.net/Charts/R...ct_Systems.pdf
 
 

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