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Old 02-28-2010, 04:03 PM   #1
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How to determine trunk size? Questions.


With the recent results of the man. j load calculations, I've been TRYING to understand how duct work is designed. What I gather, is that duct work is designed around the a/c unit cfms. My house needs a 1.5 ton air conditioner (18k heat gain) and at 400 cfm/ton is 600cfms. My current sized trunk would have the 600 cfms in mind and then reduce from there correctly. My load cal program also gives the cfm/room and suggested duct sizes. Many of the suggested round ducts should have been 4". Based on per room cfm calcs.
What I don't understand is why so many 6" supply branches? Could the system just have been designed around heating only, or could have there been additional 6" ducts added to this beast of a furnace by some random home owner? I think the a/c unit was never a considered when the home was built in the early 70's. The current condenser is tagged with a 1985 label.
There are 13 total 6" supply branches and two basement trunk cutouts. Could the oversized furnace have been added because of the 13 branches carrying 100cfm each? I think that would put me over 1300 cfm flow and better matched to the furnace, but not the main trunks.
I could replace the furnace and switch out any branches to the correct size from the basement view, but anything up the walls to the second floor would be untouchable. The positive is that there are no return branches to the 2nd floor, so the panned spaces t the 1st could be corrected. I want to stop throwing good money on top of bad.
I'm trying to figure out what went wrong first and why. I also want to get back to the correctly sized system for my house and with as little band aides as possible. I realize this is difficult in an existing home.

Thanks as always.


Last edited by turfgrass; 02-28-2010 at 06:01 PM.
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Old 02-28-2010, 09:53 PM   #2
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How to determine trunk size? Questions.


All residential duct systems are based on 6" round as a means of standardization. That way a tech does not have to spend time doing calculations on diff sized systems, adding more time to trouble shooting.

In a custom home you may find deviation from the 6" standard but mostly they're all six inch.

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Old 02-28-2010, 10:56 PM   #3
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How to determine trunk size? Questions.


HAVAC Calc's duct sizing module isn't good for sizing duct work. it will undersized your duct system.

Duct size is based on the mode that requires the most CFM. Weather its the heating mode or cooling mode does not matter.

600 CFM is good for furnace BTU outputs between 30,000 and 38,000.
Is your furnace that small, or is your home's heat loss low enough that you can install a furnace that small?
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Old 03-01-2010, 03:23 AM   #4
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How to determine trunk size? Questions.


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HAVAC Calc's duct sizing module isn't good for sizing duct work. it will undersized your duct system.

Duct size is based on the mode that requires the most CFM. Weather its the heating mode or cooling mode does not matter.

600 CFM is good for furnace BTU outputs between 30,000 and 38,000.
Is your furnace that small, or is your home's heat loss low enough that you can install a furnace that small?
Been, kindly address my comments on six inch take offs. I am sure you can add something significant.
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Old 03-01-2010, 04:12 AM   #5
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How to determine trunk size? Questions.


6" supplies is just something that rule of thumb brought around.

Most of the time, often it works out better to use 6" as a min, instead of stocking 4 and 5"(for bathrooms and walk in closets). Unfortunately though. Too many installers forget to increase to 7" when they should. Or to atleast run a second 6" supply to the rooms that need more air.

Nothing like finding a master bedroom that needs 130CFM, but only has a single 6" moving 110CFM.
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Old 03-02-2010, 07:22 PM   #6
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How to determine trunk size? Questions.


My computer has been messed, so sorry for the delay. I think some of these 6" branches need to be dampered, because of the unbalanced heat throughout the house. Also, the 2.5x10 registers are small so there is some noise. I assume a damper would kill some noise as well?

The 600cfm was for the 1.5 ton a/c unit the HVAC Calc program provided. I based the 600 on the 400cfm/ton rule. My house has more of a heat loss than gain issue according to the program. I gain about 18k and lose about 50k. I don't know how to size the trunk/cfm for the furnace based on heat loss. Is that just a matter of selecting the proper furnace and then looking at the spec sheet? The furnace I have is oversized but it tells me the cfms at certain degree wc and at certain blower speeds. For example, hi 1,700 cfm @ .2wc and med low 1,400 cfm. I don't remember the med high speed.

I know enough to go with the higher cfm between heating and a/c to choose duct work, but how do you select for heating? Is there a rule of thumb like the 400 cfm/ton with a/c or just by looking at the furnace spec sheet? Thank you.
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Old 03-02-2010, 07:58 PM   #7
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How to determine trunk size? Questions.


Generally. You look at the allowable temp rise span. And select a CFM that gives the middle of that allowable range(preferably as the max rise you design for, usually a bit better if you can design for a little lower rise, allows for longer air filter life).

EG: 50,000 BTU input, 80% gives you 40,000 BTU's output. If its allowable temp rise range is from 40 to 70. Then 55 rise is mid point. So you would want to size for a min of 675 CFM.

Your static will NOT be .2", don't even bother looking at that low of a static scale.
Your coil will be a .13 PD, your supply registers will be a min of .03, your return grille/registers will also be a min of .03, and your air filter will be .1" or more.
So you have at least a .29" static, and there isn't any duct work on it yet.
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Old 03-03-2010, 03:06 AM   #8
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How to determine trunk size? Questions.


Hey Been would you put on a brunette wig and repeat all that in a husky voice for me
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Old 03-03-2010, 03:57 AM   #9
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We're not that kind of site.
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Old 03-03-2010, 04:00 AM   #10
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We're not that kind of site.

You got a web cam???
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Old 03-05-2010, 05:26 AM   #11
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How to determine trunk size? Questions.


Many of the suggested round ducts should have been 4". Based on per room cfm calcs.


Me thinks your J Calc's are slightly off...or you have a lot of rooms the size of bathrooms (5x10 or less?). I would highly advise you sending your house plans to beenthere and letting him do a proper J calc and duct sizing for you. Just my opinion.

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