Need an opinion on duct sizeing.

[Chuck 999]

Originally posted by seatonheating
Here's an off the wall question guys. And I do believe there was a thread on this before. But, isn't it impossible to have too large of a return, theoretically? I mean, if this guy is borderline on his return he should by all means upsize it, can't hurt anything. At least that's the way I see it. For every 100 instances of problems due to return I would think 99 of them are due to it being too small.

Why not?....Dont you want to draw exactly the proper amount of air from each room based on cfm per room? You cant do that if you grossly oversize your return.Return air duct sizing is still based on TEL, I think your creating more air stratification with an improperly sized return air duct.I think this question would be more of a problem with Commercial work more than residential due to the similar sizes of most residential homes..Commercial work we are dealing with a 1 1/2 ton to 60 ton roof top unit so sizing is more critical and more to go wrong.You can almost be a rookie and a residential system will fairly work fine.

[sadlier]

[QUOTE]Originally posted by cedermechanical

by restricting you will actually increase your airflow...

Aside from a VS motor, can you post any furnace spec sheet which shows increased airflow as static pressure increases?

[dan sw fl]

Originally posted by cedermechanical

Originally posted by seatonheating

But, isn't it impossible to have too large of a return, theoretically?

I mean, if this guy is borderline on his return he should by all means upsize it, can't hurt anything.

At least that's the way I see it.

..Dont you want to draw exactly the proper amount of air from each room based on cfm per room?
You cant do that if you grossly oversize your return.

Ceder, Your post is NONsense.

Balancing is performed with adjustment of the diffuser dampers adjustment, not by changing the return air duct size.

For Example,
What about the application ( imagine ... industrial
... or residential where AHU is in a closet with a louvered door) where the air handler is
within the conditioned air space and
___ NO return air duct is needed___ ?!

No Return Air duct _=_ Infinitely Large Duct

[dash]

Originally posted by re2ell
rug,

You might ask for a second opinion on the need to increase tonnage. Have another load calculation done for sure if you feel it is necessary to increase size. If it turns out you can stay with 3 ton, your supply is within reason, (not your return) like Bald Loonie said, increase it. If you decide that 3 1/2 tons will be installed, then increase your supply and return.

Here is a short anlysis of your supply

3 ton a/c, 1200 cfm of air
1200 cfm air traveling in a 10 x 20 duct yields .085 in. w.c. pressure loss and has a velocity of 950 fpm.

3 1/2 ton a/c, 1400 cfm of air
1400 cfm air traveling in a 10 x 20 duct yields .14 in. w.c pressure loss and has a velocity of 1100 fpm.

Concerning the return

1400 cfm of air @ 700 fpm velocity yields 20" round
1200 cfm of air @ 700 fpm velocity yields 18" round

The .085 and .14 in. wc ,is from a ductolator ,and yes it would be taht if and only if you had 100' TEL of duct .If you had 50' TEL,it would be half of that.

Ductolator is in FR ,not in wc..

No way to if those duct sizes would be correct,by just using a ductolator ,instead on Man. D .

[dash]

Oversizing the entire return system,the same amount will just reduce the ESP.

Now with several return trunks/branchs,oversizing just some ,will return more cfm from that area.

Residential sizing may be easier then commercial,but is not for the untrained.

[Edited by dash on 03-19-2006 at 10:08 AM]