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Old 10-16-2009, 09:38 AM   #1
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duct calculator


i am looking for help on how to use a duct calculator i have a slide duct calculator from carrier. i am put on a 500 sqf addition on my house need to run duct work for a 40000btu furnance 1.5 ton ac. need help on laying out the duct work can anyone help me thanks jim

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Old 10-16-2009, 07:15 PM   #2
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duct calculator


Don't put any faith in a slide duct calculator.
What size of a home is the 40K furnace heating now?
Location is a big factor.

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Old 10-16-2009, 07:30 PM   #3
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duct calculator


What's wrong with a slide duct calculator?
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Old 10-16-2009, 07:56 PM   #4
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duct calculator


Quote:
Originally Posted by Plumber101 View Post
What's wrong with a slide duct calculator?
Don't get me wrong.
Slide duct calculators are very useful.
However, adding another 500 square feet to an existing home requires more than just goofing with a duct calculator.
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Old 10-16-2009, 07:57 PM   #5
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So ture

I just didn't get where you were going with that.
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Old 10-16-2009, 09:13 PM   #6
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duct calculator


the 40000 btus is just for the addition i have a 60000 btus for the old part of the house and it is only only4 years old so i didnt want to replace the 4 year old furance and the 40000 btu is in the basmemet under the addition. and the 60000 btu is far from the addition
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Old 10-16-2009, 09:23 PM   #7
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duct calculator


We need to look at the a/c first. You have a 1 1/2tn so you will need 600 CFM

Set the calculator to 600cfm at .1 friction and this will start you out.

Can you post you e-mail address here since you don't have PM capabilities yet and I will email you a pamphlet on duct sizing how to's
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Old 10-16-2009, 09:26 PM   #8
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my email is dizzy4t6@gmail.com thanks for help
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Old 10-16-2009, 09:28 PM   #9
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Should be in the IN box
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Old 10-17-2009, 05:42 AM   #10
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duct calculator


A .1" friction rate for both the supply and return duct work, would require the blower to work against a .2" static pressure to over come the duct work(if the total equivalent length is only 100").
Plus the added pressure drop of the A/C coil. Which could be .2", so now the blower has to over come .4" of static.

Add on .06" for the supply and return grilles/registers, and your up to .46" of static pressure.
Add another .1" for pressure drop across the air filter, and your at .56" static the blower is working against.

This is only if the total equivalent length is 100".

Bet, by the time you add in all the equivalent lengths for the fittings, the TEL for the longest run is closer to 200'.

A ductulator doesn't determine the size duct, without you knowing the FR you should use for the TEL of the duct/distribution system.

.1" FR is seldom the proper rate.
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Old 11-10-2009, 06:09 PM   #11
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here is my lay out(with math how i came up with cfms ect.) can i get some input on what is right or wrong. thanks
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Old 11-10-2009, 06:23 PM   #12
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I only see 6 supplies. That total 525 CFM, a bit low for a 40,000 BTU 80%.
Unless it never goes to second stage.

14X8 is small for 1.5 tons(if you want it to be quiet), and for that furnace.

How you came up with those CFM's is beyond me. Since you don't indicate how you got your multiplier.
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Old 11-10-2009, 08:39 PM   #13
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i read to determine cfm of air needed to supply the room. cfm means cubic feet per minute and defines the amount of air flowing.
one cfm of air is required to heat or cool 1.00-1.25 sq. ft. of floor area.
example
a 120 sq. ft. room will require between 96 and 120 cfm.
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Old 11-11-2009, 03:30 AM   #14
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You read a rule of thumb somewhere. Or an average.

There are 4,000 sq ft homes with 2 tons of A/C. Meaning they only have .2 CFM per sq ft.
There are others, that need 1 CFM per sq ft.

Kitchens usually take more CFM per sq ft then other rooms for cooling.
That rule of thumb didn't allow for that.

Plus, looks like your using a .1 FR. Another bad rule of thumb, lazy designers/installers use.
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Old 11-11-2009, 04:29 AM   #15
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Quote:
Originally Posted by tinmanrob View Post
Don't get me wrong.
Slide duct calculators are very useful.
However, adding another 500 square feet to an existing home requires more than just goofing with a duct calculator.
Word.

Can't tap into existing system using a ductulator.Gonna need a separate system for it.

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