Plenum Box Sizing Calculation for AHU

All calculation for plenum box sizing for all HVAC equipment such as air handling unit and fan coil unit. You can download all the calculation in a pdf file and keep it on your pc or mobile.

What is the plenum box?

A plenum is a distribution box attached to the supply outlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. It is also used to balnce the air and attenuate the sound caused by air movement  in the duct.

See also: McQuay Duct Sizer software

Plenum Box Sizing Calculation

Plenum box sizing for air handling unit (AHU):

Q = V x A --------- (1)
Q = flow rate (Cubic feet per minute) >> CFM
V = Velocity (feet per minute) >> ft/min
A = Area of plenum box (Square feet) >> ft2
As per Ashrae standard required velocity for Air handling unit (AHU) is 800 ft/min
Let’s say according to our calculated heat load tonnage is = 8 TR
As rule of thumb 1TR needs 400 CFM
So, 8 TR needs = 8 x 400 = 3200 CFM
As per equation 1:
A = Q/V
A = 3200/800
A = 4 ft2
Area of Plenum box = W x H --------- (2)
According to site condition we have to assume height of plenum box and try to find width
Let’s say H = 1.6 ft
So, using values in equation 2:
Width of plenum box = Area of plenum box / height of plenum box = A/H
W = 4/1.6
Width of plenum box = 2.5 ft
Depth of plenum can be calculated by using following equation:
D = 2.5 x d --------- (3)
Where d = diameter of fan blower of AHU
Let’s say d = 12”
Using equation 3:
D = 2.5 x 12
D = 30” = 2.5 ft 

Plenum box

Plenum box sizing for fan coil unit (FCU):

As per Ashrae standard required velocity for fan coil unit (FCU) is 500 ft/min
Same procedure as discussed above except velocity i-e 500 ft/min 

Download plenum box sizing calculation in pdf format

We intriduced to you the calculation steps for sizing a plenum box for both AHU and FCU according to SMACNA. Supply air plenum sizing mainly depends on the air flow and the air velocity.