1. ## HVAC Design

I need to redo the duct work in my attic as some rooms get more airflow than others. I have made certain there are no leaks by taping where needed and am sure the unit is large enough to supply the entire house.

I understand that there are several factors involved in designing a proper system and was wondering if there are any tips, links, or other resources someone could provide that would allow me to determine how to best size the duct runs throughout the house. I would love to get my hands on the calculations, equations, schedules and other tools the professionals use. Thanks in advance.

2. ## Re: HVAC Design

Duct work needs to be sized for A/C. You need 400 cfm per ton of cooling load. 2-ton = 800 cfm, that said, a ductulator is the best calcualtor but if you do not have anything, this method will work with 98% accuracy for you. To properly size the duct work, use this formula.

# of heat runs X 2 + 2 = duct size

example: Off the furnace going one way you have 8 heat runs.

8 x 2 = 16 + 2 = 18 you would need 18x8 duct

if 2 heat runs come off that 1st section of duct and you are left with 6 to size duct work with;

6 x 2 = 12 + 2 = 14 you would need 14x8 duct

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Duct size # of Registers 4x10
8X8 .................... 3
10X8 .................. 4
12X8 .................. 5
14X8 .................. 6
16X8 .................. 7
18X8 .................. 8
20X8 .................. 9
22X8 ................. 10
24X8 ................. 11

You need a ductulator to size return air duct and a return air drop.

Last edited by Dan Hagman; 03-01-2011 at 11:19 AM. Reason: spacing and typo

3. ## Re: HVAC Design

Go to Air Conditioning Contractors of America
There is lots of good info and you can order a ductolator and any other materials you might want to. Doing Manual J, D and S load calcs is probably too much for you and your situation, but heh, you never know.
A lot of if and how your system works will depend on many factors. It's easy enough to do overall but there are critical factors that can make huge differences.

4. ## Re: HVAC Design

Wow Dan, thanks for such a quick and detailed reply - exactly what I was hoping for as a starting point. It looks like I have a lot more to study and learn.

3 more questions:

How do you factor in the length of the different runs? For example, wouldn't a 25' run require a different size than a 6' run (all other things being equal)?

How does the calculation change, if any, when one run includes a "y"?

Does the size of the room factor in determining duct size or is that more a function of number and placement of the registers.

Also, I am not sure what you mean by this:

it 2 heat runs come off that duct and you are left with 6
6 x 2 = 12 + 2 = 14 you would need 14x8 duct

Thanks again for the info. It looks like ductulator is my word of the day.

5. ## Re: HVAC Design

Thanks for the link Markus. I have a feeling you're right about the load calcs being a bit much for me but I want to look into it just in case.

6. ## Re: HVAC Design

Lester,

Marcus is correct. Done properly you would load calculations (Manual J), size the unit with (Manual S) and design the duct system with (Manual D). One of the biggest problems in construction today is that many HVAC contractors do not use the required documents. I think they should do load calculations and duct design when they replace existing units!

7. ## Re: HVAC Design

Originally Posted by lester mahoney
Wow Dan, thanks for such a quick and detailed reply - exactly what I was hoping for as a starting point. It looks like I have a lot more to study and learn.

3 more questions:

How do you factor in the length of the different runs? For example, wouldn't a 25' run require a different size than a 6' run (all other things being equal)?

How does the calculation change, if any, when one run includes a "y"?

Does the size of the room factor in determining duct size or is that more a function of number and placement of the registers.

Also, I am not sure what you mean by this:

it 2 heat runs come off that duct and you are left with 6
6 x 2 = 12 + 2 = 14 you would need 14x8 duct

Thanks again for the info. It looks like ductulator is my word of the day.
Question 1
Remember when I said 400 cfm per ton, a small house with a 2-ton A/C only needs 800 cfm of air, the runs will not be very long, a larger house that needs 5-tons of cooling needs 2000 cfm of air, the bigger blower will maintain duct static pressure and the velocity of air will travel the distance with no problem. That's without getting to technical.

Question 2
Heat runs should not use a "Y" they all should come off the main duct trunk. If you did have a "Y" then I would recommend you have a supply round pipe that can carry 220 cfm up to the "Y" because each 6" pipe to a 4x10 register is about 110 cfm.

Question 3
The size of the room does not factor the size of the duct, it does factor how many registers are in that room and that will determind the duct size to accomodate the registers.

Question 4
started with 8 registers, if 2 come off the first section of duct then you are left with 6, if 2 more come off the 2nd section of duct then you are left with 4, if 2 more come off the next section of duct then you are left with 2

example:
The duct sizing would look like this for that side of the furnace.
Plenum / 18x8 / 14x8 / 10x8 / 8x8 , with 2 heat runs off each section.

I hope this helps.

Last edited by Dan Hagman; 03-01-2011 at 11:41 AM. Reason: Typo

8. ## Re: HVAC Design

I wonder if you were to make each register smaller, would it affect your furnace's ability to last longer? Or even maybe increase the airflow around the home and keep the dust from piling up in the ducting. Maybe you would even see a difference when you went to change your furnace filters.

Last edited by Jeremy Holmes; 04-19-2011 at 11:05 PM. Reason: spelling

9. ## Re: HVAC Design

Originally Posted by Bob Spermo
Lester,

Marcus is correct. Done properly you would load calculations (Manual J), size the unit with (Manual S) and design the duct system with (Manual D). One of the biggest problems in construction today is that many HVAC contractors do not use the required documents. I think they should do load calculations and duct design when they replace existing units!
The mechanical code in NC requires a load calculation when a unit is replaced. It has to be available for review in the event of a problem or complaint.

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