Need help on how to run return air ducts back


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Old 08-16-09, 12:05 PM
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Exclamation Need help on how to run return air ducts back

Well, first nice to meet everyone. Kinda a newbie on the home air conditioning ducting. I am replacing the ducts at my mom's house and starting with the return ducts first, because it will clear out alot of space. Now the house 1,800 sq. ft. with a 4 ton unit, all the ducts are 12x12 square sheet metal with some or no insulation left on them(because of the cable guys) and all the 14 returns are about 6" off the ground. I am going to replace all the ducts with the 12" round insulated ducts to 10x24 or 10x30. Now the real question i have, should the ducts be run in series or should it have one common line that connects to a box and splits to the two returns. Any help would be greatly appreciated.
 
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Old 08-16-09, 01:31 PM
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Should have a common return. Have a manual D Done to get it correct the first time.
 
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Old 08-17-09, 11:17 AM
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Arrow You need to do a Manual D for Duck System Sizing

Originally Posted by marquis00
Well, first nice to meet everyone. Kinda a newbie on the home air conditioning ducting. I am replacing the ducts at my mom's house and starting with the return ducts first, because it will clear out alot of space. Now the house 1,800 sq. ft. with a 4 ton unit, all the ducts are 12x12 square sheet metal with some or no insulation left on them(because of the cable guys) and all the 14 returns are about 6" off the ground. I am going to replace all the ducts with the 12" round insulated ducts to 10x24 or 10x30. Now the real question i have, should the ducts be run in series or should it have one common line that connects to a box and splits to the two returns. Any help would be greatly appreciated.
First, you need to do what is possible to reduce the heat-gain of your home. Then do a manual J heat-gain-calc to know the BTUH required for each room.

A 4-Ton unit requires 1600-cfm to 1800-cfm of airflow.
Yes, you need to do a manual D!


Keep the Supply-Side main trunk velocities at 1000-fpm down to 800-fpm.

The Return Trunks need to be sized a lot larger to reduce velocity & the entry Return Filter Racks should be really large to drop airflow velocity through the filters!

Size to get Return-Air velocities down below 600-fpm, 500-fpm would be better.

That 1600-CFM to 1800-cfm if 450-cfm per-ton were used, means for optimal efficient airflow it ought to have
around a 1000-sq.ins., of Return air filter grille area.

Just the filter grille is only 75% free-air-area, then there are the filter's pressure drops that continue to increase as they load!

That would be two 500-sq.in., return filter grille racks, exterior to the air handler which would NOT then use a filter.

At 1600-cfm, it could get by with the recommended one sq.in of free air filter grille to 2-CFM. It's just as easy to use two filter racks at 500-sq.ins., each.

A rough idea ballpark layout.
1600 / by 12= 133-CFM per BR. Run.
If you go for 450-cfm per-ton; 1800/12= 150-cfm each duct run, so you'd have to use 7" duct runs at 150-cfm each.

Probably go with 12 or 13, > 6" takeoff branch runs.

Velocity in those 12-runs will be a bit high at around 677-FPM.

Some runs may be 7" one or two may be 4 or 5" runs. -HVAC RETIRED
 

Last edited by HVAC RETIRED; 08-17-09 at 11:45 AM. Reason: 1000-fpm down to 800-fpm - RA fpm
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Old 08-19-09, 06:07 PM
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thanks for the info. but what is a manual d??
 
 

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