How to Calculate Air Change Rate for Basement Car Park Ventilation - Air Change Rate (ACH)

Basement car parks require effective ventilation to remove vehicle exhaust gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburnt hydrocarbons. Without proper ventilation, these pollutants can accumulate and create hazardous conditions for occupants.

One of the key parameters used by HVAC engineers when designing parking ventilation systems is the Air Change Rate (ACH). This metric determines how many times the entire volume of air inside the car park is replaced within one hour.

This guide explains what ACH is, why it matters, and how to calculate the required airflow for basement car park ventilation systems.

What Is Air Change Rate (ACH)?

Air Change Rate (ACH) refers to the number of times the total air volume in a space is replaced within one hour.

ACH = Airflow (m3/hr) / Room Volume (m3)

Where:

• ACH = Air Changes per Hour

• Airflow = Ventilation rate supplied by fans

• Room Volume = Total volume of the basement car park

In ventilation design, ACH is used to determine how much air must be supplied or exhausted to maintain acceptable indoor air quality.

Why Ventilation Is Required in Basement Car Parks (Calculating the Air Change Rate (ACH))

Vehicles produce several pollutants while entering, exiting, or idling inside parking structures.

Major contaminants include:

• Carbon Monoxide (CO) – toxic gas produced by incomplete combustion

• Nitrogen Oxides (NOx) – harmful respiratory irritants

• Particulate Matter (PM) – from exhaust and brake wear

Proper ventilation helps:

• Maintain safe air quality

• Prevent accumulation of toxic gases

• Meet building and fire safety regulations

• Support smoke extraction during fire emergencies

Many international standards such as ASHRAE, BS, and local building codes define minimum ventilation rates for enclosed parking garages. (Calculating the Air Change Rate (ACH))

Typical Air Change Rate Requirements

Most ventilation standards recommend different airflow rates depending on operating conditions.

Condition	Typical ACH Range
Normal ventilation	4 – 6 ACH
Peak traffic ventilation	6 – 10 ACH
Smoke extraction (fire mode)	10 – 12 ACH or higher

However, modern designs often use CO sensor–based demand-controlled ventilation instead of constant ACH operation to reduce energy consumption.

Step-by-Step Method to Calculate Air Change Rate

Designing ventilation for a basement car park generally follows three steps.

Step 1: Calculate the Car Park Volume

First, determine the total volume of the parking area.

Volume=Length×Width×Height

Example:

• Length = 60 m

• Width = 40 m

• Height = 3 m

Volume=60×40×3Volume = 60 \times 40 \times 3Volume=60×40×3Volume=7200 m3Volume = 7200\ m^3Volume=7200 m3

So the total air volume inside the car park is 7200 m³.

Step 2: Select the Required Air Change Rate

Assume the design requirement is:

6 ACH for normal ventilation

This means the entire air volume must be replaced 6 times every hour.

Step 3: Calculate Required Airflow Rate

Now multiply the volume by the ACH value.

Airflow=Volume × ACH

Airflow=7200×6

Airflow=43200 m3/hr

So the ventilation system must provide: 43,200 m³/hr of airflow

Converting Airflow to L/s (Common HVAC Unit)

Engineers often use L/s (liters per second).

1 m3/hr=0.2778 L/s

= 43200×0.2778

= 12,000 L/s

Required airflow:

12,000 L/s

Determining Number of Exhaust Fans

Once the airflow is known, the number of fans can be selected.

Example:

If one fan capacity is:

10,000 m³/hr

Number of fans =Total airflow / Fan capacity

= 43200/10000

= 4.32

So the design would require:

5 exhaust fans (rounded up).

Alternative Method: Airflow per Parking Space

Some standards calculate airflow based on the number of parking spaces.

Typical design values:

Standard	Airflow per parking space
ASHRAE	7.5 L/s per m²
BS / International practice	6–10 ACH
Some local codes	50–75 L/s per car

(Please read the standards for more information.)

Example:

If a car park has 120 parking spaces

120×50 L/s

=6000 L/s

This method is often used for quick preliminary design.

Demand-Controlled Ventilation (Modern Approach)

Modern car parks use CO sensors to reduce energy consumption.

How it works:

1. CO sensors monitor pollutant levels.

2. If CO concentration rises above a set limit (e.g., 25 ppm), fans increase speed.

3. When air quality improves, fans slow down.

Benefits include:

• Lower electricity consumption

• Reduced fan operation time

• Compliance with green building standards

Many systems use variable speed drives (VFDs) with jet fan ventilation systems.

Key Design Considerations

When calculating ACH for basement ventilation, engineers should consider:

1. Ceiling Height

Lower ceiling heights require better air distribution.

2. Airflow Distribution

Jet fans or ducted systems ensure pollutants are properly extracted.

3. Dead Zones

Columns and walls can block airflow, creating pollutant pockets.

4. Fire Mode Ventilation

Smoke extraction requirements may require higher airflow.

5. Sensor Placement

CO sensors should be placed where pollutants accumulate.

Example Summary Calculation

Parameter	Value
Car park size	60 m × 40 m × 3 m
Volume	7200 m³
Required ACH	6
Required airflow	43,200 m³/hr
Fan capacity	10,000 m³/hr
Required fans	5

Final Thoughts

Calculating the Air Change Rate (ACH) is a fundamental step in designing basement car park ventilation systems. By determining the total air volume and applying the required ACH value, engineers can accurately estimate the airflow needed to maintain safe air quality.

Modern ventilation systems are increasingly shifting toward demand-controlled ventilation with CO monitoring, which improves safety while significantly reducing energy consumption.

A well-designed car park ventilation system not only protects occupants from harmful vehicle emissions but also ensures compliance with building codes and fire safety regulations.