Wednesday, April 2, 2025
Home Blog Page 6

Smoke Control Explained

By
MEP Academy Instructor
-
0
smoke exhaust system
smoke exhaust system

In this article, we break down two essential smoke control strategies designed for large commercial spaces. First, we cover a targeted smoke control method tailored for hotel guestroom floors, keeping hallways and exit routes safe during an emergency. Then, we explore how an atrium smoke control system works to keep open, multi-story spaces clear of smoke by using smoke exhaust fans. These are just a couple of the many approaches used to manage smoke effectively and maintain safe evacuation paths.

Hotel Zoned Smoke Control System

A Highrise hotel can use a zoned smoke control pressurization system to keep smoke from spreading across guestroom floors during a fire, ensuring that hallways, stairwells, and elevators remain safe for occupants to evacuate. Here’s how the system typically works:

Dedicated Smoke Exhaust System for Hotel Guestroom Floors
Dedicated Smoke Exhaust System for Hotel Guestroom Floors

When smoke is detected on a guestroom floor, the smoke control system activates automatically. In a dedicated smoke control system, the equipment is only used for smoke control and will require frequent testing.

The supply fans pressurize areas critical for evacuation, such as stairwells, elevators, and corridors, by pumping in fresh air. This pressurization creates a pressure differential between these escape routes and the potentially smoke-filled spaces, which forces smoke back into the affected areas and prevents it from entering escape paths.

The Sandwich Technique

The sandwich technique is a zone smoke control method used in high-rise buildings to contain smoke to the fire floor and prevent vertical spread. It works by exhausting smoke directly from the fire floor while pressurizing the floors immediately above and below with supply fans, creating a pressure “sandwich.” This differential pressure keeps smoke isolated to the fire floor, ensuring that adjacent floors remain smoke-free.

The floor above and below the fire zone is positively pressurized, while the floor with the fire is under negative pressure created by the smoke exhaust fan. There are smoke control dampers on each of the supply and exhaust ducts that are opened or closed depending on the floor or fire zone. The exhaust damper is opened only on the fire floor, and the supply dampers are opened on the floor above and below the fire floor. This is just one strategy. By venting smoke directly from these spaces, the system reduces smoke density at the fire source and improves visibility for both occupants and emergency responders.

This is a dedicated system where the smoke control equipment is used only during emergency and is not part of the regular HVAC system.

Stairwell Pressurization

Most building codes will require that the stairwell in a high rise be pressurized to keep the smoke out. This allows the occupants a safe egress out of the building without having to breathe in smoke. A dedicated fan system supplies clean air into stairwells, creating a pressure differential that prevents smoke infiltration. The pressurization created by the fan must be significant enough to prevent smoke from entering the stairwell but also allow occupants to open the stairwell door.

In a high-rise, a vestibule is often used for smoke control with stairwells or elevator lobbies, to act as a buffer zone between areas with different pressures. These enclosed spaces are pressurized with fresh air during a fire to prevent smoke from entering, ensuring that stairwells and elevator shafts remain safe for evacuation and firefighter access.

Non-dedicated Smoke Control System

Here is how a non-dedicated smoke control system in a high-rise hotel using existing HVAC air handlers that supply and return air on each guestroom floor during normal operation, with each floor designated as a separate fire zone:

Using an Air Handler (HVAC System) in a Smoke Control System

Upon fire alarm activation on any guestroom floor, the smoke control system engages, signaling the HVAC air handler for the affected floor to switch to smoke control mode. The dampers in the air handler switch to smoke control by shutting the return damper and opening the outside air damper. The air handler serving the fire floor closes the supply air damper on that floor to prevent spreading the fire to other areas. The supply dampers on the floors above and below the fire floor open to pressurize and sandwich the fire floor to prevent smoke infiltration.

The exhaust air damper on the fire floor opens, while the exhaust dampers on all other floors close. This will create a negative pressure on the fire floor preventing smoke from exiting the floor. The smoke will be exhausted out the air handler or in this setup a smoke exhaust fan is provided.

The HVAC air handler on the floors directly above and below the fire floor activate in pressurization mode, supplying fresh air to create a positive pressure barrier.

This pressurization prevents smoke from migrating vertically, confining it to the fire floor. Stairwells, corridors, and elevator shafts are pressurized separately to keep them free of smoke, allowing safe evacuation routes for occupants and access for firefighters.

Atrium Smoke Control

When a fire ignites, it begins to consume nearby materials, releasing heat, smoke, and toxic gases. The combustion process produces high temperatures, causing hot air to rise. Flames reach upward, creating a vertical plume of heat and smoke above the fire source. The intensity and spread of the fire depend on fuel type, oxygen availability, and surrounding materials.

Atrium Smoke Exhaust System
Atrium Smoke Exhaust System

The rising heat from the fire generates a vertical column of smoke and hot gases known as the smoke plume. This plume carries particulates, toxic gases, and intense heat toward the ceiling. As the plume rises, it pulls in the surrounding air, growing in size and cooling slightly. The plume’s upward motion spreads smoke and heat vertically, reaching ceiling levels quickly, especially in tall spaces.

A smoke control system for an atrium is specifically designed to manage and exhaust smoke from the large, open space of an atrium during a fire. Atriums are common in modern commercial and institutional buildings like hotels, shopping malls, and office complexes, where they provide open, multi-story gathering spaces that are visually appealing. However, their large volume and vertical open design present unique challenges for smoke control, as smoke can accumulate and spread quickly throughout multiple levels. A well-designed atrium smoke control system prevents smoke buildup, enhances visibility for evacuation, and keeps pathways clear for firefighters.

Ceiling Jet Stream

Upon reaching the ceiling, the hot smoke and gases spread outward, forming a layer just below the ceiling, this is called the ceiling jet. The ceiling jet moves horizontally, carrying intense heat and smoke across the ceiling surface.

When the ceiling jets hits the wall, it turns back thickening the smoke layer. This jet is key to triggering fire detection systems, as smoke detectors and sprinklers respond to the elevated temperatures in this layer. The ceiling jet influences the spread of heat and smoke to other areas and guides smoke control strategies.

Smoke Layer

As the ceiling jet spreads, smoke accumulates, forming a hot, dense smoke layer beneath the ceiling. This smoke layer thickens as the fire burns, moving downward over time and reducing visibility in the space. The smoke layer is hazardous, containing toxic gases and reducing breathable air near floor levels as it descends. Smoke control systems aim to slow or halt the smoke layer’s spread to maintain visibility, protect egress paths, and aid firefighting efforts. The international building code section 909.8.1 states that the lowest horizontal surface of the smoke layer must be maintained at 6 feet (1829 mm) above any walking surfaces that is part of an egress within a smoke zone.

Make-Up Air System

To ensure the smoke exhaust system works effectively, a make-up air supply is often required to replace the air that’s being expelled. Make-up air helps maintain proper airflow and prevents negative pressure that could disrupt smoke control. This air supply can be introduced from adjacent zones, louvers, or through dedicated air intakes positioned lower in the atrium.

The make-up air system is designed to ensure that fresh air enters the space below the smoke layer, reducing the chance that smoke will be pulled downwards, which could impede evacuation and visibility.

What is Plug holing

In smoke control systems, plug holing is a phenomenon where cool air enters and mixes with the hot smoke layer near an exhaust vent, disrupting the effective removal of smoke. Instead of drawing out hot smoke from the upper layer, the exhaust fan begins to pull in the cooler, denser air from below. This reduces the efficiency of the smoke exhaust system, as it allows smoke to accumulate rather than being fully vented out.

Plug Holing causes problems for Smoke Exhaust Systems
Plug Holing causes problems for Smoke Exhaust Systems

Plug holing is particularly problematic in high atriums or large spaces, where temperature differences between the smoke layer and ambient air can cause this mixing. To prevent plug holing, smoke control systems may use carefully designed exhaust vent placements, larger vent sizes, or specific airflow controls to ensure that only the smoke layer is removed, maintaining clear and safe evacuation paths.

Stratification

Stratification occurs when a layer of warm air forms just below the roof due to solar heating. This layer can interfere with smoke detection and smoke layer behavior.

Stratified Layer Interferes with Smoke Layer
Stratified Layer Interferes with Smoke Layer

When the sun heats the roof, the air beneath it warms up and creates a thermal barrier. This warm air layer remains at the ceiling level, preventing cooler air and smoke from rising naturally.

Smoke Exhaust Control Systems

Direct Return vs Reverse Return Piping

By
MEP Academy Instructor
-
0
direct return vs reverse return piping
direct return vs reverse return piping

In HVAC water-based systems, water distribution is critical to ensure that each terminal unit receives the correct amount of heated or cooled water. Two popular piping configurations, Direct Return and Reverse Return, are often used in these systems. Each configuration has its unique benefits, potential drawbacks, and implications for water balancing and system performance. Here, we’ll break down the differences between these two types of piping layouts, discuss their impact on water balancing, and examine the pros and cons of each.

if you prefer to watch the video of this presentation, then scroll to the bottom.

What is Direct Return Piping?

In a Direct Return Piping system, water flows to each air handler coil and returns along the shortest path. This means that the closes air handlers coil to the heating or cooling supply source, such as a boiler or chiller, will have the shortest overall piping length, while the furthest air handler will have the longest piping route.

Direct Return Piping
Direct Return Piping

Since the closes air handlers have less piping, they’ll have less pressure drop, considering the pressure drop for all coils are the same. This creates different pressure drops for the piping circuits to and from each air handler, which creates a water balancing issue. This is why balancing valves are often added to create additional pressure drops to equal out each piping circuit.

Direct Return is often considered simpler and more cost-effective to install, as it requires less pipe and can be laid out with fewer complex fittings.

The layout can differ with all coils in a straight line like this piping diagram which makes it easier to see just how this works. If the pump is cycled on and the water flows through the system, the water molecules leaving the chiller at the same time will reach each air handler coil at different times. If you must run 50 feet and your competition must run 100 feet, you will easily return to the finish line first. The closest coils will get fed first, while the farthest coil will receive chilled or heated water last.

What is Reverse Return Piping?

In a Reverse Return Piping system, water flows to each terminal unit via the shortest path but returns along the longest path. Essentially, the return pipework is designed so that the last unit on the supply line is the first to return to the source. This approach results in equalized pipe lengths for each terminal unit, which is one of the defining characteristics of Reverse Return piping.

Reverse return piping
Reverse return piping

The theory is that each air handler will have the same distance from the source and back which would make their pressure drops equal and balancing valves not required. In theory maybe, but air handler zone differences can affect the pressure drop through each coil as each control valve adjust flow for their current zone demand.

Here we can see that all the water leaving the chiller will return from each air handler to the chiller at approximately the same time as the total round trip piping length is the same distance.

Here is another look at a reverse return where the air handlers are set in a straight line with equal lengths of pipe from the chiller to the coil and back to the chiller. The water for each air handler arrives at different times, but the total length is the same overall, which balances out the pressure drop in a perfect world.

We can see that with the direct return piping layout the water returns to the source much quicker that the reverse return piping layout. The reverse return piping distance for the water to travel is much longer than the direct return method.

Piping Differences

In a Direct Return system, the main pipes remain the same size along the entire supply and return runs until they reach a section where the flow decreases, and a size reduction is needed. This is simpler compared to a Reverse Return system, where the design often requires a gradual reduction in pipe size along the return path as flow is picked up from each terminal unit. This consistency in sizing throughout most of the Direct Return system reduces complexity and makes both material handling and installation faster and easier.

Here are the main ideas to remember about Direct Return vs. Reverse Return piping systems:

Direct return vs reverse return piping
Direct return vs reverse return piping

Direct Return Piping

Simpler Installation: Main pipes stay the same size until a reduction is necessary, streamlining material handling and installation.

Shorter Total Pipe Length: Typically requires less piping, saving on material costs and installation time.

Imbalance in Flow: The unequal pipe lengths to each terminal unit create varying flow resistances, making water balancing more challenging.

Higher Maintenance: More frequent adjustments and use of balancing valves are needed to ensure consistent flow rates.

Lower Initial Cost: Installation is generally less expensive due to reduced pipe material and labor.

Reverse Return Piping

Naturally Balanced Flow: The design ensures equal pipe lengths to and from each terminal unit, aiding in consistent flow and easier balancing.

Higher Installation Cost: More piping material and complex routing increase initial costs and installation time.

Longer Total Pipe Length: Requires additional piping to loop back the return path, impacting space and cost.

Reduced Maintenance: Less need for frequent adjustments due to the self-balancing nature of the system.

These points highlight the trade-offs between the simpler, cost-effective Direct Return and the more balanced but costlier Reverse Return systems.

Direct Return Piping versus Reverse Return Piping

UV-C Light Explained

By
MEP Academy Instructor
-
0
UV-C Light Sanitation
UV-C Light

In this article we’re diving into one of the most popular topics in modern sanitization UV-C light and how it works to fight germs, bacteria, and viruses. UV-C technology is gaining a lot of attention for its ability to disinfect homes, businesses, and even industrial spaces, but with all this interest comes plenty of questions.

In this article, we’ll be answering some of the most frequently asked questions about UV-C sanitizers. Can you safely use UV-C light in your home? Is it effective against viruses like COVID-19? How do you know if a UV-C device is working properly, and what should you look for when choosing one?

Can I use UV-C lights while I’m in the room?

In most cases, it is not safe to be in the room when a UV-C light is in operation, as direct exposure to UV-C rays can harm your skin and eyes. Many devices come with safety features, such as motion detectors that turn off the light when someone enters the room. If you’re using Far UV-C (200 to 230 nanometer), this can be safe for occupied rooms, but standard UV-C lights should only be used in unoccupied spaces.

Portable Shielded UV-C Light Air Purifier

A portable shielded air purifier with an internal UV light won’t kill germs that are on surfaces within the room as the light is shielded. This type of air purifier pulls air into the housing where it’s exposed to the UV light that kills the germs as they pass through. This type of shielded UV light can be used when the room is occupied. If you want full room coverage, you’ll need to use an open lamp type that requires that the room is unoccupied when in use.

Shielded Portable UV-C Light Air Sanitizer
Shielded Portable UV-C Light Air Sanitizer

Open Lamp UV-C Light

This open lamp UV Light has a timer that can be set for 15, 30, or 60 minutes of run time. It will beep for 15 seconds to allow the person to exit the room. This type of open UV lamp will shine in a 360-degree radius effectively sanitizing the area within its reach. You want to set this in the middle of the room if possible and leave the room while the light is on. You don’t want anyone looking at the light while it’s on as this can damage their eyes.

Avoid looking at the UV-C light which can damage your eyes.
Avoid looking at the UV-C light which can damage your eyes.

Hospitals and Healthcare Facilities. UV light systems are installed in operating rooms, patient rooms, and high-touch surfaces to prevent the spread of infectious diseases. Portable UV sanitizers are also used to disinfect medical equipment and tools, providing an extra layer of protection against hospital-acquired infections.

Portable UV-C Light sterilizing an unoccupied Operating Room in a Hospital
Portable UV-C Light sterilizing an unoccupied Operating Room in a Hospital

Food and Beverage Processing. UV-C light is used to sterilize surfaces, packaging materials, and even air in food processing plants. It helps prevent contamination and ensures food safety, commonly applied in meat, dairy, and beverage industries.

HVAC Systems in Buildings. UV lights are installed in HVAC ducts and air handling units to disinfect the air by killing airborne bacteria, viruses, and mold spores. This improves indoor air quality in large commercial buildings, such as offices, hotels, hospitals, and schools.

How does UV-C light work to kill germs?

UV-C light works by emitting ultraviolet light at a specific wavelength (typically 254 nanmeter) that penetrates the cell walls of microorganisms, such as bacteria, viruses, and mold. This light damages their DNA or RNA, which prevents them from reproducing or functioning, effectively killing or inactivating the germs. UV-C lights can inhibit the growth of mold or mildew on cooling coils, ducts, and other moist surfaces in HVAC systems reducing unpleasant odors. This can be useful for people with allergies, asthma, or other respiratory conditions.

Can UV light kill Gems? Yes
Can UV-C light kill Gems? Yes

Is UV-C light safe for humans, pets, and plants?

Direct exposure to UV-C light is not safe for humans, pets, or plants. It can cause skin burns and eye injuries, such as photokeratitis (like sunburn of the cornea). However, certain forms of UV-C, like Far UV-C (200 to 230 nanometer), are considered safer for occupied spaces. Most UV-C devices should only be used in unoccupied areas or enclosed systems to avoid exposure.

A person should not look directly at UV-C light, as it can cause serious damage to the eyes and skin. UV-C light is harmful because it emits high-energy ultraviolet radiation.

How long does it take for UV-C light to disinfect a room or surface?

The time required depends on the power of the UV-C light, the size of the room or surface, and the specific microorganisms being targeted. On average, it can take anywhere from 10 to 60 minutes to disinfect a room or surface. Manufacturers usually provide recommended exposure times based on their devices’ specifications and the area being disinfected. 

What types of pathogens can UV-C light kill?

It is effective against a wide range of microorganisms, including bacteria (coli, MRSA), viruses(influenza, SARS CoV-2), mold, and fungi. It is also commonly used to control airborne pathogens and allergens in HVAC systems and surfaces.

Is UV-C effective against COVID-19?

Yes, UV-C light has been shown to be effective at inactivating the SARS CoV-2 virus, which causes COVID-19. Studies have confirmed that UV-C light can destroy the virus by breaking down its RNA, making it unable to replicate. However, it should be used as part of a comprehensive strategy that includes proper ventilation, cleaning, and other precautions.

Can UV-C light penetrate through surfaces or fabrics?

No, UV-C light does not penetrate through solid surfaces or opaque materials like fabrics, glass, plastic, or metal. It only disinfects what it directly shines on. This means that any objects or areas that are shaded or blocked from the light will not be disinfected.

What is the difference between UV-A, UV-B, and UV-C light?

UV-A (315 to 400 nanometer) and UV-B (280 to 315 nanometer) are longer wavelength forms of ultraviolet light that primarily cause skin aging, sunburns, and DNA damage leading to skin cancer. These are present in sunlight.

Visible and Invisible light Spectrum
UV light Spectrum

UV-C (200 to 280 nanometer) has the shortest wavelength and is the most effective for germicidal purposes because it can inactivate pathogens. Unlike UV-A and UV-B, UV-C is filtered out by the Earth’s atmosphere, so it’s not naturally present in sunlight.

Only UV-C is commonly used for disinfection.

How do I know if a UV-C light is working properly?

Some UV-C devices include indicator lights or built-in sensors that show whether the UV-C lamp is functioning. Since UV-C light is invisible to the human eye, you can’t see it. Regular maintenance, such as cleaning the lamp and replacing bulbs when necessary, will ensure proper functioning. You can also use UV intensity meters to measure the output of the light.

What’s the lifespan of a UV light bulb?

The UV light bulbs typically last between 9,000 and 12,000 hours, depending on the manufacturer and usage conditions. However, even though the bulbs may still light up after this period, their germicidal effectiveness diminishes over time, so it’s recommended to replace them after reaching their rated hours.

Are there any risks of UV-C light damaging materials or electronics?

Prolonged exposure to UV-C light can cause some materials to degrade over time, especially plastics, rubber, and certain fabrics. UV-C light can cause discoloration, brittleness, or weakening of these materials. However, properly installed and used UV-C devices do not typically cause harm to electronics.

Do I need to clean surfaces after using UV-C light?

UV-C light does not leave any residue, so there’s no need to clean surfaces after use. However, UV-C light only disinfects surfaces, so if there is visible dirt or grime, cleaning is necessary before disinfection, as UV-C does not penetrate dirt or organic matter effectively.

How much area can a UV-C sanitizer light cover?

The coverage area depends on the power of the UV-C device and its design. Most devices specify the maximum area they can effectively sanitize. Smaller units may only cover a few square feet, while larger units can disinfect entire rooms or HVAC systems. For general reference, high-powered UV-C units can cover areas of up to several hundred square feet.

The physical area that the UV-C device can effectively disinfect, is often expressed in square feet or cubic feet. Larger spaces require more powerful or multiple UV-C devices to ensure full coverage. Compare devices based on the size of the area they can disinfect effectively, especially if you’re considering them for larger rooms, air ducts, or water systems.

Are UV-C sanitizers portable and easy to install?

Yes, many UV-C sanitizers are portable and designed for easy installation. Portable units are often plug-and-play, allowing you to move them between rooms as needed. Some devices, such as those used in HVAC systems or mounted for air or surface disinfection, may require professional installation.

Is UV-C light energy-efficient?

UV-C lights are relatively energy-efficient, especially when used for shorter disinfection cycles. Most UV-C devices have low power consumption compared to the energy required for constant air filtration or chemical disinfection methods. Devices typically range from 10 to 200 watts, depending on their size and capacity.

Does UV-C light affect air quality or release harmful chemicals?

Properly designed UV-C devices do not release harmful chemicals or fumes. However, some UV-C lamps can produce a small amount of ozone, especially if they emit wavelengths below 240 nanometers. Ozone can be harmful in high concentrations, so it’s important to choose ozone-free UV-C lamps or ensure proper ventilation in the space.

Can UV-C light replace traditional cleaning methods?

No, UV-C light should not completely replace traditional cleaning methods. It is most effective when used as a supplement to cleaning. For example, you should still clean surfaces to remove dirt and debris before using UV-C to disinfect. Similarly, combining UV-C with good ventilation and air filtration enhances overall disinfection.

These answers provide a comprehensive overview of the most common questions about UV-C sanitizer lights.

UV-C Light Sanitation

How to Calculate Ventilation Air

By
MEP Academy Instructor
-
0
Zone Air Distribution Effectiveness
Zone Air Distribution Effectiveness

In this article we’ll cover ASHRAE Standard 62.1 which outlines the ventilation requirements for acceptable indoor air quality (IAQ) in commercial and institutional buildings. The standard uses a combination of the Ventilation Rate Procedure (VRP), which calculates the amount of outdoor air needed based on space type, occupancy, and area. 

If you prefer to watch the Video of this article, then scroll to the bottom.

ASHRAE 62.1 Minimum Ventilation Air Requirements
ASHRAE 62.1 Minimum Ventilation Air Requirements

The ASHRAE 62.1 ventilation rate formula is based on three key factors. the number of people in the space, the square footage of the area, and the zone air distribution effectiveness (Ez). The number of people determines the amount of fresh air needed for occupants, while the square footage accounts for the ventilation required to offset contaminants from the building materials and activities. The zone air distribution effectiveness adjusts the airflow based on how well the ventilation system distributes air within the space, ensuring optimal air quality.

Let’s go through three examples using an office, retail store and classroom to illustrate how the ASHRAE 62.1 ventilation rate calculation works in different spaces. 

Example 1. Office Space

Office Ventilation Air Calculation
Office Ventilation Air Calculation

Given Data.

  • Occupancy Type. Office space
  • Floor Area. 5,000 square feet
  • Occupancy Density. 5 people per 1,000 square feet (as per ASHRAE 62.1 Table)
  • Outdoor Air Rate per Person. 5 CFM per person
  • Outdoor Air Rate per Area. 0.06 CFM per square feet

Step 1. Calculate the total number of occupants.

Number of occupants equals Floor Area Occupancy Density. This equals 5,000 square feet divided by 1,000 square feet, multiplied by 5 people per 1,000 square feet equals 25 people.

Calculation for number of occupants per ASHRAE 62.1
Calculation for Number of occupants per ASHRAE 62.1

Step 2. Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. The Ventilation Rate equals 25 people times 5 CFM per person equals 125 CFM for the people.

Calculation for Ventilation Air required for people in an Office
Calculation for Ventilation Air required for people in an Office

Step 3. Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 5,000 square feet times 0.06 CFM per square feet equals 300 CFM for the area.

Calculation for the ventilation Air required for the Area of an Office
Calculation for the ventilation Air required for the Area of an Office

Step 4. Total ventilation rate calculation using ASHRAE’s additive method.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area). The Total Ventilation Rate equals 125 CFM for the people plus 300 CFM for the area, for a total of 425 CFM.

Therefore, for this office space, the required outdoor air ventilation rate is 425 CFM.

What we haven’t covered is how the layout of the supply and return grilles affect the amount of ventilation air required. We’ll cover this at the end of this article.

Example 2. Retail Store

Retail store ventilation air calculation
Retail store ventilation air calculation

Given Data.

  • Occupancy Type. Retail store
  • Floor Area. 10,000 square feet
  • Occupancy Density. 15 people per 1,000 square feet (as per ASHRAE 62.1)
  • Outdoor Air Rate per Person. 7.5 CFM per person
  • Outdoor Air Rate per Area. 0.12 CFM per square feet

Step 1. Calculate the total number of occupants.

Number of Occupants equals Floor Area Occupancy Density. This equals 10,000 square feet divided by 1,000 square feet, multiplied by 15 people per 1,000 square feet equals 150 people

Step 2. Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. The Ventilation Rate equals 150 people times 7.5 CFM per person, for a total of 1,125 CFM for the people.

Step 3. Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 10,000 square feet times 0.12 CFM per square feet, for a Total of 1,200 CFM for the area.

Step 4. Total ventilation rate calculation.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area). The Total Ventilation Rate equals 1,125 CFM for the people plus 1,200 CFM for the area, for a total of 2,325 CFM

Therefore, for this retail store, the required outdoor air ventilation rate is 2,325 CFM.

Example 3. Classroom

Classroom Ventilation Air calculation
Classroom Ventilation Air calculation

Given Data

  • Occupancy Type. Classroom
  • Floor Area. 1,200 square feet
  • Occupancy Density. 35 people per 1,000 square feet
  • Outdoor Air Rate per Person. 10 CFM per person
  • Outdoor Air Rate per Area. 0.12 CFM per square feet

Step 1 Calculate the total number of occupants.

Number of Occupants equals Floor Area Occupancy Density. This equals 1,200 square feet divided by 1,000 square feet, multiplied by 35 people per 1,000 square feet, for a total of 42 people

Step 2 Calculate the ventilation rate required for occupants.

Ventilation Rate (People) equals Number of Occupants times Outdoor Air Rate per Person. the Ventilation Rate equals 42 people times 10 CFM per person, for a total of 420 CFM for the people.

Step 3 Calculate the ventilation rate required for the area.

Ventilation Rate (Area) equals Floor Area times Outdoor Air Rate. This equals 1,200 square feet times 0.12 CFM per square feet, for a total of 144 CFM for the area.

Step 4 Total ventilation rate calculation.

Total Ventilation Rate equals (Ventilation Rate for the People) plus (Ventilation Rate for the Area) Total Ventilation Rate equals 420 CFM for the people, plus 144 CFM for the area, for a total of 564 CFM

For this classroom, the required outdoor air ventilation rate is 564 CFM.

The Zone Air Distribution Effectiveness (Ez)

Zone Air Distribution Effectiveness (Ez) is a factor used in ASHRAE 62.1 to account for how efficiently an HVAC system delivers and mixes outdoor air within a given space or zone. It reflects how well the ventilation air is distributed to the occupants’ breathing zone, impacting the amount of fresh air needed for adequate ventilation. The effectiveness varies based on how the air is supplied and returned within the space, considering factors like supply air temperature and system design.

Zone Air Distribution Effectiveness
Zone Air Distribution Effectiveness

How Air Distribution Effectiveness Ez Varies

1. Floor Supply, Floor Return, Heating Mode

When warm air is supplied from the floor and mixed into the space, it can be pulled downward and distribute evenly within the breathing zone, leading to a higher effectiveness. ASHRAE indicates a 1.0 Ez for floor supplied and returned warm air. The 1.0 doesn’t add or subtract any CFM from the previous calculations.

Zone Air Distribution Effectiveness
Zone Air Distribution Effectiveness

2. Floor Supply, Ceiling Return, Heating Mode

When warm air is supplied from the floor and mixed into the space, it tends to rise and may not reach occupants in the lower part of the room effectively and may not distribute evenly within the breathing zone, leading to a lower effectiveness. ASHRAE indicates a 0.7 Ez for floor supplied and ceiling returned warm air. The 0.7 will add CFM to our previous calculations. For example, our office space was calculated to be 425 cfm. Taking into consideration that the air distribution effectiveness is 0.7, the new CFM would be calculated to be 425 divided by 0.7 equals 607 CFM

Floor Supply of Warm Air and Ceiling Return
Floor Supply of Warm Air and Ceiling Return

Key Factors Influencing Air Distribution Effectiveness

Supply air temperature vs. room air temperature. Larger temperature differences can reduce the effectiveness.

Supply and return air locations. Proper placement of supply and return ducts can improve Ez.

Ventilation system type. Systems like displacement ventilation or underfloor air distribution (UFAD) often have higher effectiveness due to better air mixing.

In general, the higher the Ez value, the more effective the ventilation system is in distributing air, which allows for less outdoor air to achieve the same indoor air quality, making the system more efficient.

This was a simplified explanation of the basic ventilation calculation, as there are a few other key considerations that can change the total CFM required.

Key Considerations

  1. Diversity Factor. In some cases, ASHRAE 62.1 allows the use of a diversity factor to account for spaces that aren’t fully occupied all the time.
  2. System Ventilation Efficiency. Depending on the distribution system (100% Outdoor Air, Multi-zone recirculating, or VAV systems), system ventilation efficiency (Ev) must be factored in to adjust the total outdoor airflow.

These examples show how ventilation rates are calculated based on both the number of occupants and the size of the space, ensuring adequate indoor air quality per ASHRAE 62.1.

How to Calculate Ventilation Air in accordance with ASHRAE 62.1
1...567...55Page 6 of 55
MEP Books

EDITOR PICKS

ESTIMATING

POPULAR CATEGORY

ABOUT US

The MEP Academy provides articles, training and information for the HVAC, Electrical and Plumbing Industry. Engineers and contractors in the commercial construction industry will find useful information to help them do their jobs.

Contact us: [email protected]

FOLLOW US

Subscribe - Newsletter

Copyright © MEP Academy LLC 2020-2025