How to calculate AHU CFM

 How to calculate AHU CFM

To calculate AHU CFM (Cubic Feet per Minute), you determine the airflow required based on either the room volume and air changes per hour (ACH), or by using thermal load and temperature difference. Both methods are widely used in HVAC design and commissioning.

🧩 Methods of Calculating AHU CFM

🔹 Method 1 – Room Volume & ACH

This is the most common method for ventilation design.

$$CFM=\frac{RoomVolume\times ACH}{60}$$

• Room Volume = Length × Width × Height (ft³)

• ACH (Air Changes per Hour) = Number of times air is replaced per hour

Example: Room size = 20 ft × 15 ft × 10 ft = 3,000 ft³ ACH = 6 (typical for offices)

$$CFM=\frac{3000\times 6}{60}=300CFM$$

➡️ The AHU must supply 300 CFM to meet ventilation requirements.

🔹 Method 2 – Thermal Load (BTU/hr)

Used when sizing AHUs for cooling/heating loads.

$$CFM=\frac{BTU\mathrm{/}hr}{1.08\times \mathrm{\Delta }T}$$

• BTU/hr = Heat load of the space

• ΔT = Temperature difference between supply and return air

Example: Heat load = 24,000 BTU/hr ΔT = 20°F

$$CFM=\frac{24000}{1.08\times 20}=1111CFM$$

➡️ The AHU must deliver 1111 CFM to handle the thermal load.

🔹 Method 3 – Differential Pressure & K‑Factor

Used in Testing, Adjusting, and Balancing (TAB) procedures with installed airflow measuring devices.

$$CFM=K\times \sqrt{\mathrm{\Delta }P}$$

• K = Device constant (provided by manufacturer)

• ΔP = Differential pressure measured across the flow grid

Example: K = 1250, ΔP = 0.64 in. w.c.

$$CFM=1250\times \sqrt{0.64}=1000CFM$$

➡️ The AHU supplies 1000 CFM, verified against design specs.

📊 Comparison of Methods

Method	Formula	Best Use Case
Room Volume & ACH	(Room Volume × ACH) ÷ 60	Ventilation design (offices, labs, hospitals)
Thermal Load (BTU/hr)	BTU/hr ÷ (1.08 × ΔT)	Cooling/heating load calculations
Differential Pressure & K‑Factor	K × √ΔP	Commissioning, TAB, field verification

✅ Key Takeaway

• Use Room Volume & ACH for ventilation design.

• Use Thermal Load method for cooling/heating sizing.

• Use Differential Pressure & K‑Factor for commissioning and field balancing.