What are the Sizes of the Air Handling Units?

Air handling unit (AHU) sizes range from 2,000 to 60,000 CFM (cubic feet per minute) in airflow capacity. Typical physical dimensions vary by CFM:

Small units (2,000–10,000 CFM): 6–12 ft long, 4–6 ft wide, 5–8 ft tall (1.8–3.7 m × 1.2–1.8 m × 1.5–2.4 m). 

Large units (30,000–60,000 CFM): 15–30 ft long, 8–12 ft wide, 10–15 ft tall (4.6–9.1 m × 2.4–3.7 m × 3–4.6 m). 

If you are concerned about the sizing of the air handling units according to your home’s needs then you can consult from our team for air handling unit installation and maintenance.

Air Handling Unit Size Chart

AHU Model/Type	Approximate Dimensions (L x W x H) (ft)	Airflow Capacity (CFM)	Typical Application
Small AHU	4 x 3 x 3	500 – 2,000	Small offices, retail stores
Medium AHU	6 x 4 x 4	2,000 – 6,000	Medium-sized commercial buildings
Large AHU	10 x 6 x 6	6,000 – 20,000	Large commercial buildings, hospitals
Extra Large AHU	12 x 8 x 8	20,000 – 40,000	Industrial facilities, large complexes
Custom/Industrial AHU	Varies (up to 30 x 15 x 15 or more)	40,000+	Heavy industry, large manufacturing

Notes:

Dimensions are approximate and can vary by design and manufacturer.

CFM = Cubic Feet per Minute, a measure of airflow capacity.

AHUs can be modular, so sizes may be flexible based on specific requirements.

Many AHUs have options for custom sizing to fit particular projects.

What are the Factors that Depend Upon the Air Handling Unit Size?

The size of an Air Handling Unit (AHU) is determined by several factors, including the building’s size, occupancy, heating and cooling loads, required airflow volume (CFM), indoor air quality standards, ductwork pressure losses, humidity control needs, fan capacity, and the available installation space.

Heating and Cooling Loads: Heating and cooling loads, measured in British Thermal Units per hour (BTU/hr), indicate how much temperature adjustment your AHU must provide. For example, a building with high heat gain from sunlight or equipment will need a larger unit to handle increased cooling demands

Indoor Air Quality Requirements: You also need to account for indoor air quality requirements. If your building code requires more fresh air intake per person, your AHU must be sized accordingly to deliver sufficient ventilation.

Humidity Control: Humidity control is another factor. If you want to maintain a specific moisture level, the AHU must include components capable of humidifying or dehumidifying the air, which can affect its size.

Ductwork Pressure Loss: Ductwork design causes pressure losses due to friction and bends. Your AHU’s fan size and overall dimensions must compensate for these losses to maintain proper airflow.

Available Space Installation: Finally, physical constraints like the available space for installation and maintenance can limit the size of your AHU. You need to ensure the unit fits comfortably and allows for easy access.

How to Calculate the Size of the Air Handling Unit?

Calculating the size of an Air Handling Unit (AHU) involves determining the airflow (CFM – Cubic Feet per Minute) and the cooling/heating capacity (BTU/hr – British Thermal Units per hour or Tons of Refrigeration). Here’s a step-by-step guide to help you understand this process.

Step 1: Determine the Cooling Load

The cooling load is the amount of heat that needs to be removed from a space to maintain a desired temperature. This is the most critical step and often requires a detailed heat gain calculation. 

Factors to consider for cooling load:

Sensible Heat Gain: Heat that increases the dry bulb temperature of the air. This comes from sources like:

• Heat conducted through walls, roofs, windows, and floors. 

• Heat generated by occupants (body heat). 

• Heat from lighting fixtures. 

• Heat from equipment (computers, machinery, etc.). 

• Solar radiation through windows. 

• Latent Heat Gain: Heat associated with changes in moisture content (humidity) in the air. This comes from sources like:  

• Moisture generated by occupants (breathing, sweating). 

• Moisture infiltration from outside air. 

• Moisture from processes (e.g., cooking, humidifiers). 

Formula for Cooling Load (Simplified approach for initial estimation):

While a detailed heat load calculation is best done with specialized software or by an HVAC engineer, a simplified method for initial estimation can be used:

Where:

Q total  = Total cooling load in BTU/hr

Q sensible  = Sensible heat gain in BTU/hr 

Q latent  = Latent heat gain in BTU/hr 

Typical Estimation Values (Highly variable, for rough estimates only):

Residential: 200 – 400 sq ft per ton (12,000 BTU/hr = 1 ton of refrigeration) 

Commercial/Office: 250 – 500 sq ft per ton 

Restaurants/High Occupancy: 150 – 250 sq ft per ton

Example: If you have an office space of 10,000 sq ft and you estimate 400 sq ft per ton, your cooling load would be 10,000 / 400 = 25 tons.

25 tons×12,000 BTU/hr/ton=300,000 BTU/hr

Step 2: Calculate the Airflow (CFM)

Once you have an estimated cooling load, you can determine the required airflow. The airflow rate depends on the sensible heat gain and the desired temperature difference between the supply air and the room air.

Formula for Airflow (CFM) based on Sensible Heat:

CFM= Qsensible /  (1.08×ΔT)

Where:

CFM = Cubic Feet per Minute 

Qsensible  = Sensible cooling load in BTU/hr 

1.08 = A constant (approx. 0.24 BTU/lb⋅∘F×0.075 lb/ft3×60 min/hr) representing the sensible heat capacity of air at standard conditions. 

ΔT = Temperature difference between the room air and the supply air in ∘F. A common ΔT for comfort cooling is between 18∘F and 22∘F. Let’s assume 20∘F for this example. 

Example: If your sensible cooling load (Qsensible ) is 225,000 BTU/hr (assuming 75% of the total 300,000 BTU/hr load is sensible) and your desired ΔT is 20∘F:

CFM=225,000 / (1.08 x 20) =21.6225,000 ≈10,417 CFM 

Step 3: Determine the Heating Load (if applicable) 

If the AHU will also provide heating, you’ll need to calculate the heating load for the space. This is similar to cooling load but focuses on heat losses. 

Factors to consider for heating load: 

• Heat conducted through walls, roofs, windows, and floors to the colder outdoors. 

• Heat lost due to infiltration of cold outside air. 

Formula for Heating Load (Simplified): 

Qheating =UAΔTdesign  / R 

Qheating  = Heating load in BTU/hr 

U = Overall heat transfer coefficient of the building envelope (U=1/Rtotal ) 

A = Area of the surface 

ΔTdesign  = Temperature difference between the desired indoor temperature and the outdoor design temperature. 

R = Thermal resistance of the material 

Step 4: Consider Ventilation Requirements (Outside Air) 

Building codes and standards (like ASHRAE 62.1) specify minimum outdoor air requirements for ventilation to maintain indoor air quality. This fresh air needs to be conditioned by the AHU. 

Formula for Outdoor Air (OA) CFM: 

This is typically calculated based on the occupancy and area of the space. 

CFMOA = (CFM per person × number of people) + (CFM per sq ft × area in sq ft) 

You will find the specific CFM per person and CFM per sq ft values in relevant building codes or ASHRAE standards based on the space type. 

Example: For a typical office, ASHRAE 62.1 might require 5 CFM/person and 0.06 CFM/sq ft. If you have 100 occupants and 10,000 sq ft:

CFMOA =(5×100)+(0.06×10,000)=500+600=1,100 CFM

The total AHU CFM should be the larger of the cooling/heating CFM or the ventilation CFM, but often the cooling/heating load drives the primary CFM. The AHU must be able to handle the conditioning of this outside air.

Step 5: Select the AHU based on Calculated Values

With the calculated CFM and BTU/hr (or Tons), you can now select an AHU. 

Airflow Capacity: The AHU’s fan section must be capable of delivering the required CFM against the system’s static pressure (resistance from ducts, coils, filters, etc.). 

Coil Capacity: The cooling coil and heating coil within the AHU must have sufficient capacity (BTU/hr or Tons) to handle the calculated loads. 

Features: Consider other features like filtration levels, humidification/dehumidification, energy recovery, and controls based on your specific needs.