Zone Airflow Balancing (Single speed AHU / Zone imbalance) 2

[bwilliams86]

Hi All,

Aspiring mechanical engineer here looking for some advice with some zonal airflow questions. I am in CA for code reference;

I am working on designing a 3-zone system using a single-speed AHU. This is a residential home with uneven zone area. I.e. 3 floors with say 700 Sq Ft Top floor, 1500 Sq Ft Middle Floor, and 700 Sq Ft bottom floor. I calculated my Airflow using the equation in Manual J as follows;

Total required airflow for cooling: (Sensible Capacitiy) / 1.1 * TD = CFM
Total Sensible capacity: 27,878 BTU
I am using the assumption of a low latent load and based on manual J tables a TD of 17 is required;

(27,878) / (17)*(1.1) = 1490.80 --> Selecting the next nominal tonnage up I would select a 1,600 Nominal CFM unit.

Now for my predicament;

When in single-zone operation I have a ton of additional airflow on the top floor operation and bottom floor operation. Does anyone have experience dealing with mitigation of this airflow?

I have used bypass but have concerns that using too large of a bypass will cause my coil to freeze. I am also under the impression that new 2013 California Energy Code will make using a bypass difficult if not impossible. I have thought of dumping air into the attic or outside but was told this is counter-intuitive to energy efficiency.

Should I be worried about getting proper air entrainment at my bottom floor? What is an acceptable limit on bypassed airflow? What should I do if bypasses are prohibited in the new code?

Thank you all in advance. I look forward to learning a lot in here!

Best Regards,

Brad

 

[MintJulep]

You could perhaps downsize and try to count on usage diversity.

But really your approach is unlikely to ever satisfy owner expectations.

You need variable flow.

 

[bwilliams86]

Thanks MintJulep! Definitely made that consideration and I agree it is truly the best solution for this situation. Just getting the buy in on that extra $$ has been the biggest hurdle. Do you have any thoughts on dumping extra air to a low-impact area such as a stairwell or to the alternate zones? A large concern with this method has been how the impact changes in heating vs cooling application.

 

[willard3]

I don't think any hammer-and-tongs solutions are a good idea. Bite the bullet and use variable air volume.