In Variable air volume how do you size the required air flow correctly?

[Teccc]

I have a large amount of VAV's to maintain,and check the operation of, i would like to know how to see if the vav box is sized correctly for cooling for that area, or how to size it ... by air changes per hour or L x W or does a actual heat load calculation need to be done?
Then in heat mode i was always taught that the supply air should not be more then 90 degrees, or 15 degrees hotter than the area served is that correct? I use the 5 degree rule to check heating... from where the air comes past the heat coil, to where it comes out of the grill should not drop more for checking by 5-6 degrees? If it does it is generally not moving enough airflow? Then it should never completely close? I am no expert but just willing to tackle anything am i close?

 

[willard3]

You are asking for confirmation fore rules of thumb, a lousy way to design HVAC systems.

Do a careful heat gain/loss and then size the system.

 

[Teccc]

Copy thank you

 

[Cuby]

If you want rules of thumb you could start with 400 SF per ton and assume 400 CFM/ton - or one CFM per square foot. No one sets the rules on air temp for heating but your general assumptions are where I would design a system. Anything over 90°F gets uncomfortable and tends to cause temperature swings and eats more energy.

 

[Teccc]

They wanted 5 air changes per hour,

I think it is CFM = volume of the room / 60 = some number then times that by 5.... is what I found on google.

From the responses i am seeing do a heat load calculation as much as i can then compare it to cfm, in cooling mode,

BTUH x .75 / 1.08 x 20 = cfm? I got that off the psychrometric web site seems logical?

then in heating mode, try not to get hotter then 90 degrees coming out of the grill, use a temperature probe and check the temperature of the air coming from the heating coil and then the temperature of the air coming out of the grill, should not drop more then 5 degrees? For maximum efficiency?

Then for the minimum airflow I am going to see what the minimum outside air needed for the space at 20 cfm per person, then time that by the number of people in the space? I researched all of this and hoping it is a good approach
again i am not a engineer just trying to get a good approach for balance and checking efficiency. From what I can gather i am going to try and approach it this way.


I am due to start this 10-1 thx for any input

 

[Teccc]

Ansd also thank you for the responses

 

[Cuby]

CFM = BTUH/(T1-T2)x 1.08 (at sea level). Not sure where the 0.75 multiplier comes from. Are you adjusting for gas fired heat efficiency? If you are using a heating coil you don't need to adjust for efficiency.

Your air changes per hour formula works.

Hopefully you are not getting a 5 degree loss between the discharhge of the coil and the temperature at the grille. That is a lot of loss.