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Old 03-24-2011, 01:39 PM   #1
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unequal supply & return


I had a ductwork design done by a professional HVAC contractor. There are individual returns rather than a central return which is what I wanted. The total of the individual supply CFMs equals approximately the total of the return CFMs which makes sense to me. However the cross sectional area of the return trunk entering the furnace is 17% greater than the area of the supply trunk leaving the furnace. Is this ok?

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Old 03-24-2011, 02:32 PM   #2
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unequal supply & return


It is impossible to answer this question with the data given.

In general,

1. Multiple returns are superior to a single return.
2. Return ducts are always larger than supply trunks.
3. Supply air and return air is always equal because its a closed loop system to the equipment. i.e. if 1000cfm enters a furnace, 1000 cfm exits.

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Old 03-24-2011, 03:25 PM   #3
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unequal supply & return


The return duct/plenum entering the furnace is generally larger then the supply.
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Old 03-24-2011, 04:16 PM   #4
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unequal supply & return


You can't have to much return, as you are only going to blow the cfm's that the blower is designed to blow.
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Old 03-25-2011, 01:48 AM   #5
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unequal supply & return


I'll take individual returns over one big return any day of the week
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Old 03-25-2011, 07:15 AM   #6
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unequal supply & return


The total supply CFM is 1762 and the total return CFM is 1705. The return trunk is 12 x 28 and the supply trunk is 18 x 16. Is there a rule to determine how much larger the ruturn should be? I would like to keep it to a minimum to conserve space.
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Old 03-25-2011, 07:22 AM   #7
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unequal supply & return


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Originally Posted by Hermand View Post
The total supply CFM is 1762 and the total return CFM is 1705. The return trunk is 12 x 28 and the supply trunk is 18 x 16. Is there a rule to determine how much larger the ruturn should be? I would like to keep it to a minimum to conserve space.
I am sure the return is the same as supply CFM's, the difference is undoubedly temperature differential. If you correct for temperature they will be fine.

The actual physical size of ducting is to ensure that the system stays within the design static pressure. As supply velocity is critical for proper distribution it is not uncommon to have supply ducting with a smaller cross section than return.

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Old 03-25-2011, 07:45 AM   #8
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hEY mARK YOUR ABSOLUTELY RIGHT! You be surprized how many times Ive gone on jobs and the duct isnt sized and be so far off that by the time it reaches the register its lost all its velocity. This man has individual returns should give him a pretty good balanced system.You always want your return a little bigger.By putting individual returns you dont have to figure any transfer grills.
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Old 03-25-2011, 11:49 AM   #9
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Originally Posted by Hermand View Post
The total supply CFM is 1762 and the total return CFM is 1705. The return trunk is 12 x 28 and the supply trunk is 18 x 16. Is there a rule to determine how much larger the ruturn should be? I would like to keep it to a minimum to conserve space.

How was the CFM measured? Your differences could indicate a slight leakage in the return. Depending how CFM was checked.

The return generally needs to be larger to allow for the air filters resistance to air flow.

Duct velocity has no real bearing on register velocity.
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Old 03-25-2011, 12:20 PM   #10
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WOW Ok you just blew me away there.What do you mean duct velocity has nothing to do with register velocity.I dont agree with that statement at all. A good duct system is designed first from the size of your equipment.Lets say a 5 ton system,its designed at 2000 cfm of air. Than you ck out the amount of different areas and rooms in the home to decide how much air to be delivered to what area and this will determine the duct size for each area as well. The velocity is how much air is gonna be needed.Velocity is basically how many ft per minute the air is traveling.You dont want it to peter out before it hits the registers.What you have left is your static pressure.This is the resistance that is put against the air flow as its going through the duct.Registers do nothing but direct the air flow as its being delivered plus its a resistance to the air flow travel as well. You want to concentrate on how much air is to be delivered to the area{Determined by sq footage} and the size of air duct that will handle it.If your duct system is designed correctly and the registers are put in the right places youll have a even air flow to give you comfort throughout your home!
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Old 03-25-2011, 12:28 PM   #11
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I love this part of our trade because its very challenging and rewarding when its all said and done and I love it when a plan comes together.When you design a duct system theres ways that you can set it up and even go as far as installing air dampers on the takeoffs to adjust your air delivery system so air flow gos exactly where its supposed to go.By using a flow hood it will allow you to balance the airflow to the amount of cfm thats required for each area .This is some tactics you can use on anyjob but mostly on your higher end homes.Ive got it to where if everythings sized correctly im usuall within 50 cfm of my target amount of air thats suppo0sed to be delivered.By adding the dampers allows me to definitly zero in on my targeted cfm amount.This gives you a more even airflow amount throughout your home delivering a very high comfort level!
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Old 03-25-2011, 04:17 PM   #12
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Duct velocity is simply how fast the air is moving in the duct. Has no bearing on how fast it comes through a supply or return register.

The static pressure in the duct has more bearing on register velocity then duct velocity does. Its like a tractor trailer going down the highway at 70 miles an hour. Its speed(velocity) has no bearing on how much is in the trailer.

A Hart&Cooley 4X10 420 floor register will deliver 100 CFM at a face velocity of 600FPM with a PD of .022" wc weather its fed by a 7" duct with a duct velocity of 370FPM, or a 5" duct with a duct velocity of 725FPM.

As long as the volume is there, and the static pressure is higher then the PD of the register, it will deliver that X amount of air at that X FPM, weather the duct velocity is higher or lower then the register velocity.
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Old 03-25-2011, 06:07 PM   #13
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ok i got ya I still dont quite agree with you.All your saying is every register has a certain amount of air that can travel through it .In other words the register rating or specs
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Old 03-25-2011, 06:55 PM   #14
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A register will have what ever velocity at X CFM, no matter how low or high the duct velocity is.

I can supply a Hart&Cooley 420 floor register with 160 CFM, its velocity will be 800FPM, the PD across it will be .04"wc, its throw will be 11', and its spread will be 13'. If I use a 8" pipe, the pipes velocity will be 460FPM.

If I switch to a 6X14 register of the same line instead, the registers velocity will be 500FPM, the PD will be .016"wc, with an 8' throw, and a 9' spread, and the 8" pipes velocity will remain unchanged.

The pipes velocity has no real bearing on register velocity. Velocity is determined by the volume through an area.

Look at a high velocity system. 1800FPM, and only moving 40CFM through the supply duct.

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