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Old 10-12-2011, 11:53 AM   #1
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Main trunk question


I have started the process of sizing and laying out my HVAC system. I still hope to find a competent installer but up till this point in the remote area we are in there haven't been any in the 8 I have had bid the job. This is why I began the Manual J calculation of my home and duct sizing calculations. Now that that is over here is my question. In the main trunk sizing, lets say the furnace is 1700 cfm, this tells me I need approximately 26x10 trunk.(speaking in generals not complete design) Now if the furnace sits in the middle of the house do you adjust the trunk size for total length and takeoffs on each side therefore possibly having one trunk 20 x 8 and one 16 x 8 off either side to total the required airflow off the plenum or would both sides be matched? Thanks for your time.

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Old 10-12-2011, 05:04 PM   #2
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Main trunk question


Sized for the runs on each side. Pretty common for one trunk to be larger then the one going in the opposite direction.

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Old 10-12-2011, 05:36 PM   #3
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Main trunk question


I believe the velocity target you should be designing to is 750 FPM. So yes, once you know the requirements for each branch then size the main trunk accordingly.

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Old 10-12-2011, 07:17 PM   #4
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Main trunk question


Actually the register should be at 500-750, but whatever you like is fine.
You need a ductulator to properly size the system.
This will give you static pressure and velocity based on cfm and duct area. The idea is to keep the static pressure even throughout the system, and to keep it between a certain threshold.
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Old 10-13-2011, 04:17 AM   #5
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Main trunk question


Duculator will tell you static pressure loss from friction, but won't tell you your static pressure.
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Old 10-13-2011, 07:38 PM   #6
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Quote:
Originally Posted by beenthere
Duculator will tell you static pressure loss from friction, but won't tell you your static pressure.
Although the above statement is technically true, the ductulator will help you stay in a range.
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Old 10-14-2011, 03:33 AM   #7
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Main trunk question


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Originally Posted by Master of Cold View Post
Although the above statement is technically true, the ductulator will help you stay in a range.
And you should know what range you need to stay in before using the duculator.

Need to ASP for the CFM you want to use/need.

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