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 thehvacguy 06-22-2011 11:59 AM

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?

 OldSingy 06-22-2011 03:53 PM

HVAC Calcs

Quote:
 Originally Posted by thehvacguy (Post 672028) 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

 Technow 06-22-2011 06:42 PM

Quote:
 Originally Posted by OldSingy (Post 672156) ================ 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:jester:

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

 thehvacguy 06-22-2011 08:08 PM

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.

 thehvacguy 06-22-2011 08:12 PM

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:jester: 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?

 OldSingy 06-22-2011 08:34 PM

HVAC Calcs

Quote:
 Originally Posted by thehvacguy (Post 672313) 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.

 thehvacguy 06-22-2011 09:20 PM

Ok so let me ask this, what would a decent velocity be for 10ft ceilings?

 OldSingy 06-23-2011 09:33 AM

Velocities

Quote:
 Originally Posted by thehvacguy (Post 672351) 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.

 beenthere 06-23-2011 08:00 PM

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.

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