What is Fresh Air Intake for HVAC: Design, Calculation, Selection & Example

Fresh air intake for HVAC (Heating, Ventilation, and Air Conditioning) systems is a critical component that ensures indoor air quality and comfort. As the pollution is increased day by across the world, it is now necessary to have healthy fresh air to breathe for healthy & smooth life.

In this comprehensive guide, we will delve into the various aspects of fresh air intake for HVAC systems, including its purpose, design, calculation, selection, and more. By the end of this article, you’ll have a deep understanding of why fresh air intake is essential and how to incorporate it into your HVAC system effectively.

What is Fresh Air Intake for HVAC?

Fresh air intake for HVAC, also known as outside air intake, refers to the process of introducing outdoor air into the HVAC system to enhance indoor air quality. This outdoor air is mixed with the recirculated indoor air, filtered, and conditioned before being distributed throughout the building. Fresh air intake is a vital element of HVAC systems, as it ensures that the indoor environment remains comfortable and healthy for occupants.

What is Fresh Air Intake for HVAC?
What is Fresh Air Intake for HVAC?

What’s the Purpose of Fresh Air Intake for HVAC?

The primary purpose of fresh air intake for HVAC systems is to provide a continuous supply of clean outdoor air, which serves several crucial functions:

  • Indoor Air Quality (IAQ): Fresh air intake helps dilute indoor pollutants, including odors, allergens, and volatile organic compounds (VOCs), ensuring a healthier and more pleasant indoor environment.
  • Comfort: By introducing fresh air, the HVAC system can maintain consistent temperature and humidity levels, improving occupant comfort.
  • Energy Efficiency: Controlled fresh air intake can help reduce the energy consumption of the HVAC system by minimizing the need for extensive heating or cooling to maintain comfort conditions.
  • Compliance: Many building codes and standards mandate a minimum amount of outdoor air for ventilation, which fresh air intake helps achieve.

How Does an HVAC Fresh Air Intake Work?

Fresh air intake for HVAC operates through a series of well-coordinated processes:

How Does an HVAC Fresh Air Intake Work?
How Does an HVAC Fresh Air Intake Work?
  • Air Intake: An intake vent or duct draws outside air into the HVAC system.
  • Filtration: The incoming air is typically filtered to remove contaminants like dust, pollen, and particulate matter.
  • Conditioning: The fresh outdoor air is mixed with the indoor air and passes through the HVAC unit, where it is heated, cooled, humidified, or dehumidified to meet the desired indoor conditions.
  • Distribution: Conditioned air is then distributed through the building’s ductwork, maintaining indoor comfort and air quality.

Design & Selection of HVAC Fresh Air Intake System

The design and selection of an HVAC fresh air intake system are critical for ensuring efficient operation and achieving the desired indoor air quality. Some of the major cosiderations are as follows:

01. Location of Fresh Air Intake

The intake should be situated to draw in clean outdoor air, away from potential contaminants like exhaust vents, smokestacks, or heavily trafficked areas, hydrocarbon gasese, etc.

02. Fresh Air Intake for Smoke, Heat

Smoke detectors or heat detectors shall be considered at fresh air intake, so that, in case of smoke or heat the fresh air intake will be closed with the help of motorized damper.

02. Fresh Air Intake for Hazardous Gas

For example, in case there is H2S gas, or any hydrocarbon gasses, and we are forced to provide fresh air into a building, what to do? It simple!

  • Include H2S or hydrocarbon sensors and add a Gas Tight Damper at fresh air intake so that in case of any hazardous gas detection, sensor will give a signal to the control panel to stop gas tight damper within 1 or 2 second.
  • Length of duct should be considered considering the damper closure time.
  • Closing Gas Tight Damper means no ingression of hazardous gas.
  • Check to the location as it should be farthest from the source.

In this senarios, the entire HVAC system can run with recirculation mode so that cooling load will not have any impact, however, the same depends on the codes as well as client’s requirements.

03. Ventilation Rate

Determining the required ventilation rate is crucial, and it is often based on local building codes and occupancy levels. The ventilation rate helps establish the volume of fresh air required.

04. Filtration Requirements

Select appropriate filters based on the local air quality and desired indoor air quality. High-efficiency filters can capture smaller particles and improve air quality. The requirements of accessory for fresh air intake depends on the qualtity of air partaing to the location and indoor air cleanliness requirement. The following filters are generally used with respect to the project requirements:

Fresh Air Intake with sand trap filter
Fresh Air Intake with sand trap filter
  • Sand trap filter: To remove sand & duct particles
  • Chemical filter: To remove chemical particulate
  • Demister : To remove oil & grease
  • UV filter: Protect from radiation etc.

05. Wind Ditection

Wind direction is one the main criteria for the selection of fresh air intake. It is recommended to locate the intake in the direction of fresh air. Keep the following in mind during the designing of air intake:

  • Never keep the intake and exhaust in the same location
  • Never keep the intake oposite to the wind direction, which reduces the efficiency little bit.
  • Intake and exhaust should not be on the same wall side, unless the wall is very lengthy and all other walls doesn’t have any space.

06. Air Mixing

Ensure that the outdoor air is mixed appropriately with the recirculated indoor air, so that the resulting air blend maintains temperature and humidity comfort levels.

07. Ductwork Design

The ductwork system must be designed to distribute the conditioned air evenly throughout the building, avoiding dead spots or areas with poor air circulation.

08. Redundancy of Air Intake

There are many critical projects where redundant fresh air intakes are considered so that in case any problem with one air intake, other will run successfully without interruption.

09. Fresh Air Intake problem for ice

In cold countries, due to very lower temperature, fresh air path can be blocked by ice formation and fresh air can not be entered into the system as required. To cater this problem, trace heating shall be done which will prevent to form ice inside the fresh air path.

Design Calculation for HVAC Fresh Air Intake System

Calculating the fresh air intake requirements for your HVAC system involves several considerations and these are:

Design Calculation for HVAC Fresh Air Intake System
Design Calculation for HVAC Fresh Air Intake System
  • Determine the Ventilation Rate: Calculate the required ventilation rate based on local building codes, occupancy, and activity levels within the space.
  • Outdoor Air Temperature and Humidity: Consider the local climate to determine the temperature and humidity conditions of the outdoor air.
  • Sensible Heat Load: Calculate the sensible heat gain or loss associated with the introduction of outdoor air, which will impact the HVAC system’s load.
  • Latent Heat Load: Calculate the latent heat gain or loss due to the moisture content of the outdoor air.
  • Equipment Selection: Choose HVAC equipment capable of conditioning the incoming air to the desired indoor conditions while considering the sensible and latent loads.
  • Control Strategies: Implement control strategies that adjust the fresh air intake based on occupancy, outdoor conditions, and indoor air quality.

Parameters to be known for HVAC Fresh Air Intake Calculation:

  • Determine Ventilation Rate or ACPH
  • Room Dimension, Height
  • Nos of people/occupancy
  • Outdoor air requirements as per ASHRAE 62.1
  • Exfiltration through wall, roof, floor, door equation & calculation

How to Calculate Fresh Air & Size of Fresh Air Intake?

Sizing calculation of fresh air steps:

Step-1 Dimension of Room: Get the dimension of room like Room length, width & height. For example, say room lengh, width & height are 20m, 10m & 5m respectively.

Step-2 ACPH & Air Quantity: Find out ACPH or air changes per hour based on the area. For example, waiting room, the ACPH is 1.0. Now, knowing room dimensions & height, calculate the minimum fresh air quantity considering ACPH considering the following formula:

Fresh air requirements

= Room length (m) x Room width (m) x Room height (m) x ACPH

= 20 x 10 x 5 x 1 m3/hr

= 1000 m3/hr or 1000/3.604 L/s = 280 L/s

Step-3 Calculate Fresh Air Considering People outdoor air rate & Area area outdoor air rate as per ASHRAE 62.1: It is clearly given the value of people and area outdoor rate in ASHRAE which needs to be used in this calculation:

People outdoor air rate & Area area outdoor air rate
People outdoor air rate & Area area outdoor air rate
  • Waiting room People outdoor air rate: 3.8 L/s per person
  • Waiting room area outdoor air rate: 0.3 L/s per m2
  • Nos of person: 1

Outdoor air on People outdoor air rate basis (OA-1) = nos of person x People outdoor air rate = 1 x 3.8 = 3.8 L/s

Outdoor air on area outdoor rate basis (OA-2) = area x Area outdoor air rate = 200 x 0.3 = 60 L/s

Total outdoor or fresh air = OA-1 + OA-2 = 3.8 + 60 = 63.8 L/s

Step-4 Exfiltration Calculation: Exfiltration – air flow through external wall,  roof, floor & door need to be calculated

a) First calculate the exfiltration through door formula

Applicable formula for Exfiltration
Q= AxCdx(2∆P/ρ)0.5 
Q= Flow rate through openingm³/s
A=Area of opening
Cd= Discharge Coefficient (0.6)
∆P= Applied Pressure DifferencePa
ρ= Density of air (1.2)Kg/m³

First calculate the exfiltration through door calculation

Ventilation Pressurization / Ex-Filtration Calculations (Doors and windows)
[Pa] Horizontal
[m][m][m][m][m²] [m³/s][L/S][L/S][L/S] 
1Waiting Room30.0Double Leaf door (D2)

b) Ex-Filtration Calculations (Building Envelope (Wall, Roof and floor) formula

  • Over pressure air = 0.839 x Aw x ∆P ½ (m³/s)
  • Kw : Wall Leakage area Ratio (0.50×10‾4)
  • Kf : Floor/ Roof Leakage area Ratio (0.66×10‾5)
Ventilation Pressurization / Ex-Filtration Calculations (Building Envelope (Wall, Roof and floor)
S.No.AreaLeakage area ratioPressure ∆P(Pa)Direction Leakage AreaOver Pressure AirTotal L/S
(m2) (L/S)
1Waiting Room0.0000530WSW138.60.006931.8165

Total exfiltration = exfiltration through door calculation (43 L/s) + Ex-Filtration Calculations (Building Envelope (Wall, Roof and floor) (165 L/s) = 209 L/s

c) Final Summary of Fresh Air requirement is as follows:

MechStudies Sample Project Example
Fresh Air Requirements (Waiting Room)
[m][m][m²][m][m³]  [L/S]NO.L/s per Person[L/S]L/s*m2[L/S][L/S][L/S][L/S][L/S] 
1Waiting Room2010200.05.010001.0ACH28013.83.80.36063.820980280.0 
    200 1000  2801 3.8 6063.8209 280.0 
Fresh Air Requirements =280 L/S

What Should We Do if The Fresh Air Duct is Routed Outside?

In many project, due to the space constraints, fresh air intake many times need to be routed outside the building instead inside the builiding. There are a few measures which should be taken care during fresh air intake design and installation:

Fresh Air Duct is Routed Outside
Fresh Air Duct is Routed Outside
  • Insulation: Proper insulation thickness shall be considered. Normally, fresh air duct is not insulated, however, if there is a chance of heat gain from outside or there is any heat source near to that duct, insulation can be considered.
  • Material: The material of duct shall be selected considering the weather data, area classification etc. For H2S gas prone area, duct shall be made of stainless steel.
  • Protection: As the duct is located at the outside, it shall be routed in such a way that there will not be any chance of physical damage.
  • Location: The Fresh air duct , in case, if located on the roof, the entrace should not be very close to the parapet wall or any other obstruction to allow the air for a smooth entry into the duct.
  • Supports: The support of fresh air duct shall be designed considering earthquake (if applicable), wind load, snow, ice etc.
  • Maintenance: Fresh air intake shall be designed considering suitable maintenance specially filter, damper etc.

What Are the Benefits of Having Fresh Air Intake for Your HVAC System?

Incorporating fresh air intake into your HVAC system offers a range of benefits:

  • Improved Indoor Air Quality: Fresh air dilutes indoor pollutants, creating a healthier and more comfortable living or working environment.
  • Enhanced Comfort: Properly conditioned outdoor air helps maintain consistent temperature and humidity levels, ensuring occupant comfort.
  • Energy Efficiency: Controlled fresh air intake can lead to energy savings by reducing the need for excessive heating or cooling.
  • Compliance: Meeting ventilation requirements ensures your building complies with local codes and standards.
  • Odor Control: Fresh air intake helps remove and dilute unpleasant odors, creating a more pleasant indoor environment.
  • Moisture Control: It can help manage indoor humidity, reducing the risk of mould growth and moisture-related issues.

How to Control Airflow Through HVAC Fresh Air Intake?

Control of airflow through the HVAC fresh air intake is vital for maintaining indoor air quality and energy efficiency. Here’s how it can be accomplished:

Control Airflow Through HVAC Fresh Air Intake
Control Airflow Through HVAC Fresh Air Intake
  • Dampers: Install motorized dampers in the fresh air intake system to control the volume of incoming air. These dampers can be adjusted based on temperature, occupancy, or indoor air quality.
  • Sensors: Implement sensors to monitor parameters such as temperature, humidity, CO2 levels, and VOC concentrations. These sensors can trigger the adjustment of the fresh air intake rate as needed.
  • Variable Frequency Drives (VFDs): Use VFDs to control the speed of fans and adjust the airflow based on demand.
  • Building Automation Systems (BAS): Integrate the fresh air intake control into a BAS, allowing for centralized monitoring and control of all HVAC components, including the fresh air system.

Is Having a Fresh Air Intake Vent a Requirement for HVAC Systems?

In many regions, having a fresh air intake vent is a requirement for HVAC systems, especially in commercial and residential buildings. Local building codes and standards mandate minimum ventilation rates to ensure indoor air quality and occupant health. These regulations may specify the volume of fresh air required based on factors like occupancy, building use, and local climate conditions.

Compliance with these regulations is essential to avoid legal and health-related issues. Failure to meet ventilation requirements can lead to poor indoor air quality, discomfort, and potential health risks for occupants.

How Can You Add a Fresh Air Intake?

If your existing HVAC system lacks a fresh air intake or if you’re planning a new installation, there are several methods to add fresh air or outside air:

How Can You Add a Fresh Air Intake
How Can You Add a Fresh Air Intake
  • Dedicated Ductwork: The most effective approach is to install dedicated ductwork for fresh air intake. This ensures a clean and separate source of outdoor air that can be filtered, conditioned, and distributed efficiently.
  • Air Handling Unit Modification: In some cases, you can modify your existing air handling unit to include a fresh air intake duct, allowing for outdoor air to be integrated into the system.
  • Window or Louver Ventilation: Another option for residential buildings is the installation of window vents or louvered openings to introduce outdoor air. However, this method lacks the filtration and conditioning capabilities of dedicated fresh air systems.
  • HRV/ERV Systems: Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs) are specialized systems that incorporate fresh air intake, exchange heat and humidity, and are ideal for energy-efficient ventilation in homes and some commercial settings.
  • Consult a Professional: For the most effective and compliant fresh air intake solution, consult an HVAC professional who can assess your specific needs and recommend the best approach.

HVAC Air Intake Pros and Cons

Improved Indoor Air Quality: Fresh air intake enhances indoor air quality by diluting pollutants and providing a continuous supply of clean outdoor air.Energy Costs: Operating a fresh air intake system may increase energy costs, particularly if outdoor conditions are extreme.
Better Comfort: Properly conditioned outdoor air helps maintain consistent temperature and humidity levels, enhancing occupant comfort.Installation and Maintenance: Adding or maintaining a fresh air intake system can be costly, and it requires regular maintenance to ensure optimal performance.
Energy Efficiency: Controlled fresh air intake can reduce energy consumption by minimizing the need for excessive heating or cooling.Complexity: Integrating a fresh air system into an existing HVAC system can be complex and may require significant modifications.
Compliance: Meeting ventilation requirements ensures compliance with local building codes and standards.
Odor Control: Fresh air intake helps remove and dilute unpleasant odors, creating a more pleasant indoor environment.

Energy Recovery Ventilators (ERVs)

Energy Recovery Ventilators (ERVs) are advanced systems that not only introduce fresh air into the building but also recover energy from the exhaust air, making them a highly energy-efficient solution. ERVs transfer heat and humidity between the incoming and outgoing air streams, reducing the workload on the HVAC system. They are especially useful in regions with wide temperature swings.

Maintenance and Inspection of Fresh Air Systems

To ensure the continued effectiveness of a fresh air intake system, regular maintenance and inspections are crucial. Filters should be replaced or cleaned at appropriate intervals to maintain air quality, and dampers, sensors, and control systems need to be inspected for proper functioning. Routine maintenance can prevent breakdowns and ensure that the system operates efficiently.

Humidity Control in Fresh Air Systems

Maintaining proper humidity levels within the HVAC system is essential. If not managed correctly, excessive humidity can lead to mould growth and discomfort, while overly dry air can cause respiratory problems and static electricity issues. Many HVAC systems incorporate humidity control features to address these concerns.

Zoning and Airflow Control

Zoning is a strategy that divides a building into different areas with individualized temperature and ventilation needs. This approach allows for more efficient fresh air intake control, as it tailors ventilation rates to each zone’s occupancy and requirements. Implementing zoning and airflow control can further improve energy efficiency while meeting ventilation needs.

Integration with Smart Building Technologies

The integration of fresh air intake with smart building technologies is a growing trend in modern HVAC systems. Smart sensors and automation can adjust ventilation rates based on real-time data, optimizing indoor air quality while minimizing energy consumption. This results in a more responsive and energy-efficient system.

Balancing Indoor Air Quality and Energy Efficiency

One of the key challenges in HVAC design is finding the right balance between indoor air quality and energy efficiency. While introducing a higher volume of fresh air is beneficial for air quality, it can increase energy consumption. Energy-efficient HVAC systems, such as those with high-efficiency filters, heat recoveryOpens in a new tab., and demand-controlled ventilation, help strike this balance effectively.

Monitoring and Air Quality Sensors

Using air quality sensors and monitoring systems allows building managers and homeowners to keep a close eye on indoor air quality. These systems can provide real-time data on parameters like CO2 levels, VOCs, humidity, and particulate matter. With this data, you can make informed decisions regarding the operation of your HVAC system, adjusting it as needed to maintain air quality.

Role of Fresh Air Intake in LEED and Green Building Certification

Fresh air intake systems play a vital role in achieving LEED (Leadership in Energy and Environmental Design) and green building certifications. Meeting specific ventilation requirements is often a prerequisite for such certifications. Additionally, efficient HVAC design and operation contribute to lower energy consumption and carbon emissions, aligning with sustainability goals.


Fresh air intake for HVAC systems is a fundamental element for maintaining indoor air quality, occupant comfort, and energy efficiency. Properly designed and controlled, it ensures that the air you breathe indoors is clean, comfortable, and free from pollutants.

When considering a fresh air intake system for your HVAC, it is essential to consult with professionals to design a system that meets your specific needs and local regulations. With a well-designed and effectively controlled fresh air intake system, you can create a healthier, more comfortable, and energy-efficient indoor environment for your home or workplace.


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