Duct Sizing - HVAC - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum > HVAC duct sizing
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

06-22-2011, 11:59 AM   #1
Still Learning

Join Date: Oct 2009
Location: Los Angeles
Posts: 338
Rewards Points: 250

## duct sizing

Question... I know the rule of thumb is 1 cfm per square foot. But some are telling me I need to determine velocity, and some tell me I need to determine pressure loss... can someone give me some advise on using my ductulator?

06-22-2011, 03:53 PM   #2
Newbie

Join Date: Sep 2010
Posts: 28
Rewards Points: 27

## HVAC Calcs

Quote:
 Originally Posted by thehvacguy Question... I know the rule of thumb is 1 cfm per square foot. But some are telling me I need to determine velocity, and some tell me I need to determine pressure loss... can someone give me some advise on using my ductulator?
================

HVACGUY,

While 1 cfm/sqft is a guideline for residential work, it's important to run a load calc to be sure you take all factors into consideration like construction type, glass, insulation, exposures, shade trees, any special conditions, etc. Also, another guideline is 400 CFM per recirculated ton for residential A/C.

Example - 3 tons = 1200 SCFM (Standard CFM). Using your ductulator, assume .1" per 100 foot pressure drop. Start with the total cfm (1200) and then read the values for the duct sizes, velocities, round duct, and friction per 100 feet of duct.

Round duct - 16"
Square/rectangular duct - 15 x 13 (will depend on how much room you have; adjust accordingly)

There is a website that can help with residential duct sizing, cfm, velocities, etc. It's:

http://efficientcomfort.net/jsp/ResDuct_Web.jsp

06-22-2011, 06:42 PM   #3
HVAC Tech/Owner

Join Date: Nov 2010
Location: VA
Posts: 640
Rewards Points: 604

Quote:
 Originally Posted by OldSingy ================ HVACGUY, While 1 cfm/sqft is a guideline for residential work, it's important to run a load calc to be sure you take all factors into consideration like construction type, glass, insulation, exposures, shade trees, any special conditions, etc. Also, another guideline is 400 CFM per recirculated ton for residential A/C. Example - 3 tons = 1200 SCFM (Standard CFM). Using your ductulator, assume .1" per 100 foot pressure drop. Start with the total cfm (1200) and then read the values for the duct sizes, velocities, round duct, and friction per 100 feet of duct. Round duct - 16" Square/rectangular duct - 15 x 13 (will depend on how much room you have; adjust accordingly) There is a website that can help with residential duct sizing, cfm, velocities, etc. It's: http://efficientcomfort.net/jsp/ResDuct_Web.jsp

And when you take it to the next level and really learn what the ductulator does you will know that the little sharpee mark at .1 doesn't mean a thing

Rules of thumb are for people who dont know what they are doing

Last edited by Technow; 06-22-2011 at 06:45 PM.

 06-22-2011, 08:08 PM #4 Still Learning   Join Date: Oct 2009 Location: Los Angeles Posts: 338 Rewards Points: 250 The way they taught me in the trade school was with the .1 for flex and .5 for spiral, and then 1 cfm per square foot to determine the duct size. That does.t seem like the proper way to do it though. The purpose for this question is my brother has a warehouse with 3, 5 ton package units. He is making some offices in there, and I wanna make sure it is sized somewhat decent. This got me thinking about the velocity factor since there will be 10ft ceilings. And I know comfort can be compromised if there is too much or not enough static pressure.
06-22-2011, 08:12 PM   #5
Still Learning

Join Date: Oct 2009
Location: Los Angeles
Posts: 338
Rewards Points: 250

Quote:
 Originally Posted by Technow And when you take it to the next level and really learn what the ductulator does you will know that the little sharpee mark at .1 doesn't mean a thing Rules of thumb are for people who dont know what they are doing
Where can I learn to do this properly? Do I have to get myself a copy of manual D?

06-22-2011, 08:34 PM   #6
Newbie

Join Date: Sep 2010
Posts: 28
Rewards Points: 27

## HVAC Calcs

Quote:
 Originally Posted by thehvacguy Where can I learn to do this properly? Do I have to get myself a copy of manual D?
=============

As I said in my previous post, a "rule of thumb" is ok for general informaion, but it's important to run a load calc based on the actual design conditions. I would disagree that rules of thumb mean you have no idea what you're doing.

A baseline starting point is always helpful. For a complete load calc, and in today's conditions, there are software programs available to assist as opposed to the older manual methods.

 06-22-2011, 09:20 PM #7 Still Learning   Join Date: Oct 2009 Location: Los Angeles Posts: 338 Rewards Points: 250 Ok so let me ask this, what would a decent velocity be for 10ft ceilings?
06-23-2011, 09:33 AM   #8
Newbie

Join Date: Sep 2010
Posts: 28
Rewards Points: 27

## Velocities

Quote:
 Originally Posted by thehvacguy Ok so let me ask this, what would a decent velocity be for 10ft ceilings?
==============

Is this going to be a constant volume or VAV system? VAV is generally more efficient, but requires additional controls, considerations and has a higher first cost.

Anyway....

Somewhere in the 600-700 fpm range would be acceptable. If noise is an issue, you can use internally insulated ductwork (commercial use), but be sure to compensate for the size difference that the insulation will take up. Also, you didn't say if you were going to use a plenum return system or ducted.

At any rate, try to keep the returns (ducted or egg crate) from being too close to the supply diffusers so you don't get short-cycling.

06-23-2011, 08:00 PM   #9

Join Date: Oct 2008
Location: Somewhere
Posts: 34,608
Rewards Points: 8,202

First. You need to establish teh size unit you need, and then the CFM you need for each room. this is done with manual J. Weather you use software or do it by hand doesn't matter. Then you should use manual S to determine the size of the equipment you need to meet the load at the designt emps you used in the load calc.

Then manual T to determine what registers to use for the throw you need. then manual D, to determine what friction rate to use with the duculator.

Manual D, will show you why your duct system is more then 200' of total equivalent length, instead of just the 60 or so foot you first thought you had. A warm air start collar can have 65 foot of total equivalent length. You return ell at the furnace/air handler can be as much as 135 total equivalent foot. manual d will show you this, and show you ways to lower the resistance/total equivalent length of those attachments. So yes, you do need to go out and buy the manuals, if you want to do it right. And if you test the static on a few systems, you'll see that .1" friction rate is almost always wrong to use. And .08" is often still to0 high of a friction rate to use.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post kilosos2 HVAC 28 01-26-2011 09:21 AM rjordan392 HVAC 3 05-18-2009 08:06 AM C44 HVAC 3 03-23-2009 04:24 PM Swordfish215 HVAC 1 01-15-2007 03:22 PM Stu HVAC 2 12-22-2006 05:36 PM

Top of Page | View New Posts