Intro to Ventilation
We’re constantly told that ventilation is important and we need to improve it. Here’s a full rundown on what it is, how it works, how to check it, common problems and how to improve it.
What is ventilation and why is it needed?
Ventilation is the process of moving air into and out of a space and typically refers to supplying outdoor air.
The air quality inside buildings can be poor because of harmful airborne pollutants, which can have negative impacts on the health and comfort of occupants. Indoor pollutant concentrations are often higher than those found outdoors, making it crucial to reduce their concentration to maintain healthy indoor air quality.
The most straightforward way to reduce indoor pollutant concentrations is by exhausting indoor air and replacing it with outdoor air. However, this approach can be problematic if outdoor air is heavily polluted. In such cases, filtration can be used to address outdoor air pollution, but it is only effective in removing particulate matter. Ultraviolet (UV) light can help eliminate microbes, but it is not effective against all indoor air pollutants.
In commercial settings, ventilation is often the only method used to maintain adequate indoor air quality. Thus, ensuring adequate ventilation is crucial for protecting the health of occupants.
Health Effects
Inhaling high concentrations of indoor air pollutants can have severe health effects over time. However, by ensuring that outdoor air is supplied to indoor spaces, these health effects can be mitigated. Studies have documented improvements in cognitive function, respiratory problems, and fewer sick days when ventilation rates are increased. This is a good summary of the research for this on schools.
How much do you need?
In general, places with higher concentrations of indoor pollutants need higher ventilation rates. For example, nail salons with high levels of volatile organic compounds (toxic gases released from the beauty products) would require high ventilation rates. Gyms, where people sweat and smell, would also require higher rates than places where people are not engaging in those activities, like a library. These rates are codified in building codes. These codes usually reference a ventilation standard. The most commonly referenced one is ASHRAE 62.1. The required ventilation rates change depending on the type of space, the size of the space and the amount of people. Often values end up being 2–3 air changes per hour or 4–8 litres/second/person).
If your concern is mitigating the spread of airborne diseases like COVID-19, ventilation is important, but filtration or UV can be equally effective. I deal with how much is required here.
Types of ventilation
Ventilation can be classified into three categories: natural, mechanical and hybrid.
Natural ventilation refers to air that enters a building through openings, such as windows, doors, and vents, without the use of fans to supply the air. Temperature differences, wind, and buoyancy determine the rate of airflow in natural ventilation systems. Uncontrolled natural ventilation, also called infiltration, occurs when air leaks into a building through the building envelope. Natural ventilation is typically more energy-efficient since fans are not required to ventilate a space. However, because the airflow is not well controlled, it can often lead to insufficient ventilation rates. Additionally, since air is not filtered or tempered, high natural ventilation rates can be problematic in areas with air pollution or very cold outdoor temperatures. Many old natural ventilation buildings were designed with very low ventilation rates.
Mechanical ventilation, on the other hand, uses fans to control the amount of air supplied to the building. The most significant advantages of mechanical ventilation are that the amount of air supplied to the space is controlled, the air is filtered, and the temperature can be adjusted to ensure that it is comfortable. Heat recovery can also be used to save energy where heat from the air that is exhausted is transferred to the outdoor air that will be supplied to the space. Although mechanical ventilation is generally more expensive to install, operate, and maintain, it provides a more reliable ventilation rate to ensure acceptable indoor air quality.
Hybrid ventilation refers to a variety of methods used to manage air circulation in buildings, which can differ in their implementation. One type of hybrid ventilation simply refers to mechanical ventilation in buildings with operable windows, allowing occupants to adjust air supply. Another type of hybrid ventilation involves switching between mechanical and natural ventilation based on weather conditions. A third type, known as exhaust-driven natural ventilation or exhaust-only ventilation, employs forced infiltration through the use of exhaust fans to continuously exhaust air from each room, creating negative pressure that draws in outdoor air through openings. Older exhaust-only systems often have low airflow rates, which leads to poor indoor air quality.
Mechanical Ventilation Equipment
Mechanical ventilation equipment serves several functions in supplying air to a space. First and foremost, it facilitates ventilation by bringing in outdoor air. Additionally, the air can be adjusted for temperature and humidity, collectively known as climate control, to ensure optimal comfort levels. Recirculating air for climate control is also an option, eliminating the need to rely solely on outdoor air.
A typical air handling unit (AHU) is a common mechanical ventilation system used in commercial buildings. The AHU is composed of a series of components, including filters, heating or cooling coils, and a fan. The system operates by mixing outdoor air and recirculated air from the building through dampers that control the amount of air from each source. The mixed air is then filtered, heated or cooled to the desired temperature and humidity, and supplied to the space.
Another mechanical ventilation system is the unit ventilator. It is a wall-mounted unit that supplies outdoor air to the space through an opening in the wall. The unit ventilator typically contains a fan, filters, and heating or cooling components, allowing it to adjust the temperature of the supplied air.
The Packaged Rooftop Unit
If you are located in a strip mall or a small building, the ventilation equipment is likely a packaged rooftop unit (RTU). The RTU is a cheap air handling unit that has dampers, filters, heating, cooling and a fan, just like a typical air handling unit. A significant issue is that it usually does not carefully control the supply air temperature which means it is limited in how much outdoor air it can supply to a space, or it will supply very cold air in the winter. Usually it can only supply around 20% outdoor air, when minimum requirements are often closer to 35%. RTUs are often controlled by thermostats. The fan setting should always be ‘on’ when the space is occupied and left in ‘auto’ when unoccupied. Because it is located outdoors, the equipment is exposed to the elements and dampers often fail and are left broken. If it is controlled by a thermostat, there is no monitoring to ensure the dampers are working properly or outdoor air is being supplied to the space. Contractors who are responsible for maintaining the units would be the ones to diagnose this, but it is usually ignored. These units are usually lower quality and have difficulty handling MERV-13 filters.
HVAC System Classifications
Ventilation & Climate Control vs. Ventilation Only
Supplied air in HVAC systems serves two primary purposes: ventilation and climate control. In cases where climate control is already in place in each room, such as with perimeter radiators, only outdoor air is required for ventilation. However, when air is needed for both ventilation and climate control, recirculated air is used. For example, if a space requires 100 cubic feet per minute (CFM) of outdoor air for ventilation and a total of 200 CFM of hot air to maintain warmth during the winter, 100 CFM of return air can be used. Dampers are used to control the amount of return air, and in this case, the dampers will be open to 50% return air and 50% outdoor air. If a space gets too cold at night, the HVAC unit can turn on to keep it warmer and only use recirculated air (0% outdoor air) since it would be wasteful to provide outdoor air to an empty building.
100% Outdoor Air vs. Recirculated Air
Some air handling units only supply outdoor air and have no recirculated air. These units are often called Makeup Air Units (MAUs) and are generally used for ventilation-only systems. As previously mentioned, HVAC systems that are designed for ventilation and temperature control use recirculated air. Dampers can modulate to determine how much outdoor air is supplied to the space. These systems can employ a strategy called demand control ventilation, where the amount of outdoor air supplied to the space is adjusted based on the number of people in the space. As the number of people decreases, less air can be supplied to the space, saving energy. Systems with recirculated air ensure that the air supplied to the space isn’t too hot or too cold by mixing outdoor air with recirculated air before resupplying it. These systems also have an economizer mode or free cooling mode, where extra outdoor air can be supplied to the space to cool it down when it is cool outside and hot inside. This is also known as free cooling.
Constant Volume vs. Variable Volume
Most older HVAC systems are constant volume, meaning the amount of air supplied to the space remains constant. Many modern HVAC systems, however, are designed to be variable volume, meaning the amount of air supplied to the space varies. A minimum amount of air should be maintained for ventilation, and extra air can be added if heating or cooling is necessary. By reducing the amount of air supplied to the space, energy can be saved.
Mixing vs. UFAD vs. Displacement ventilation
Mixing ventilation: This is the most common type of ventilation system where air is supplied from diffusers located on the ceiling or high up on the wall. The supplied air mixes with the room air and creates a relatively uniform concentration of contaminants throughout the space.
Underfloor air distribution (UFAD): In this system, air is supplied from the floor and extracted from the ceiling. The supply air pushes contaminants upwards towards the ceiling where they can be extracted. UFAD systems can be more energy-efficient than mixing ventilation systems, but they require a raised floor system to accommodate the ductwork.
Displacement ventilation: This type of system is even more effective in removing contaminants and preventing air from being shared between occupants. The cool supply air is supplied near the floor and spreads out across the floor. As it heats up, it rises to the ceiling where it is extracted, creating a gentle upward airflow. The use of fans or HEPA filters in the space can create cross-currents, which can disrupt the upward airflow in displacement ventilation systems. However, these systems are not as common as mixing systems.
Air distribution
Air distribution measures how much of the air from the air handling unit makes it to the breathing zones of building occupants. Several factors can prevent this clean air from reaching its intended destination.
One factor is ductwork problems. After leaving the air handling unit, the air must travel through ductwork to reach the various rooms and spaces of the building. Ductwork can become damaged, leaky, or separated over time, causing air to be lost or diverted away from its intended destination. This can result in insufficient ventilation reaching certain areas of the building.
Another possible air distribution issue occurs when supplying multiple zones. When an air handling unit supplies air to multiple spaces, it is crucial to ensure that each space receives the appropriate amount of air. In newer installations, manual balancing dampers can be adjusted to ensure the correct amount of air is supplied to each zone. However, in older buildings, these manual dampers may not have been installed or may have shifted over time, leading to inconsistent air distribution throughout the building. Zones closer to the air handling unit could have high airflow rates while zones further away could have very low airflow rates.
In a mixing ventilation system, the goal is for clean air to be evenly distributed throughout the space. This is typically achieved through the use of ceiling diffusers, which help to disperse air evenly throughout the room. The diffusers also ensure the air is supplied with the correct velocity (called throw) so that it will be properly mixed with air in the room.
However, if air supply and return vents are placed too close together, or if the supplied air is too warm or too slow, the air may not properly mix and may instead travel directly from the supply diffuser to the return grill, bypassing the breathing zone entirely. This is called short-circuiting.
Unit ventilators are designed without any air distribution and just supply outdoor air to the side of the room.
Measuring and checking ventilation
There are several tools available to check and ensure proper ventilation.
- CO2 Monitoring: CO2 monitoring is an indirect way to measure the ventilation rate in a space. It works by calculating the difference between indoor and outdoor CO2 levels, as we know the rate at which people exhale CO2. By monitoring the indoor CO2 concentration, one can estimate the ventilation rate and determine if the space meets requirements for adequate ventilation. This method is advantageous as it can be done by occupants and accounts for air distribution. However, there are caveats with how it should be used and interpreted. A detailed description of CO2 monitoring can be found here.
- Airflow Checking: Checking the airflow rate in a space is a straightforward way to determine if the HVAC system is functioning. One method is to attach a tissue to a diffuser and observe whether it flaps continuously while occupied. Alternatively, a broomstick with a tissue taped to the end can be used to see if air is being delivered to a space. However, this method does not provide information on the quality of the supplied air or its distribution throughout the space. For example, if dampers are broken and all the air is recirculated, monitoring the airflow will not identify this issue.
- Pollutant Study: Pollutant studies can be done by using a measurable tracer gas or aerosol which is placed in the air. Any sources of that tracer is then removed and an instrument is used to measure how quickly that tracer is removed from the room. The air change rate in the room can then be calculated. This method can identify low ventilation problems or possibly air distribution issues, depending on where the measurement instrument is located. It only performs a measurement at one point in time. This method will tell you the air change rate, but that is not the proper metric when assessing the risk of a space, so these measurements still require proper interpretation.
- Fog study: Fog generators can help identify problems with air distribution, such as stagnant or poorly ventilated areas, and pinpoint problem areas that require adjustments. It can also give an idea of how quickly pollutants can be removed from the room. This method is also done at a single point in time and does not observe the system continually while occupied.
- Professional Commissioning: Professional commissioning involves observing the air handling unit to ensure that it is functioning correctly. It includes observing if the fan is turning on, if the dampers are moving correctly, if outdoor air is being brought in, and if air is being exhausted. Air balancers have tools to measure the exact airflow and confirm that the system is complying with the design. Airflow can also be measured in the rooms by air balancers. When air handling units alone are commissioned, air distribution issues might still exist in the space. Commissioning only provides a one-time snapshot and can assume that the air from the air handling unit is being delivered perfectly to the space, which is usually not the case.
- Building Automation System (BAS): A building automation system (BAS) is a computerized and automated system that controls the entire HVAC system in a building. Micro-controllers communicate with equipment to turn fans and pumps on and off, open and close valves, modulate dampers and more. Sensors placed throughout the building identify air and water temperatures, pressures, and equipment status. Facility managers can monitor equipment operation from a computer screen to ensure that everything is operating as designed. The BAS can provide continuous monitoring and alerts when equipment is not functioning as it should. It generally cannot identify air distribution issues and is not accessible to occupants.
Most common problems, diagnosis and solutions
When it comes to HVAC systems, there are several common problems that can arise. Understanding how to diagnose and solve these issues is crucial to maintaining the efficiency and safety of the system.
Fan not turning on
If the fan is not turning on, there are several possible causes, including a blown fuse, a broken motor, a broken fan belt, a tripped low limit safety, or a schedule set incorrectly. Additionally, the thermostat may be in auto mode or someone may have turned it off. If the fan is not running, CO2 levels will increase, and tissues taped to the diffusers will stop moving. This is easily identifiable through any method of checking ventilation. The fan should be fixed immediately to ensure proper ventilation.
Dampers not allowing in outdoor air
If the dampers that control outdoor air are not opening, it can significantly limit the amount of outdoor air supplied to the space. Dampers can seize or the actuator that controls them can be broken. CO2 levels will increase, and the unit will need to be inspected to determine if the dampers are moving properly. A BAS should be able to identify this issue, and the dampers need to be fixed immediately.
Insufficient air distribution
When air is not being properly distributed throughout a space, the only way to know is to measure the pollutants in the space using CO2 monitoring or a pollutant or fog study. Checking airflow from the diffusers or investigating the unit supplying the space may not identify the issue. To solve this problem, fans or HEPA filters can be used to improve air distribution.
Insufficient ventilation at minimum values
Variable volume systems are often only measured at maximum ventilation levels and not at minimum levels. Sometimes, these systems are set to shut off ventilation completely when run at minimum, or they ramp down too low. Many modern air handling units are designed by the manufacturer to slow down when heating or cooling are not required, even though it should be a constant volume system. This can violate the minimum ventilation requirements.
The best way to diagnose this problem is through CO2 monitoring. Knowing the expected maximum CO2 concentrations can identify if the minimum airflow rates are insufficient. The expected steady state CO2 concentrations can be found here. Increasing the minimum ventilation levels can help solve this problem, and a technician may need to adjust the equipment to ensure it does not reduce the airflow too much.
Broken BAS Sensors
Many sensors break over time or wires get disconnected. Once these sensors break, the system no longer works properly. It’s a common issue and when ventilation or HVAC systems are neglected, this can go unnoticed. These sensors need to be fixed or replaced.
Improving ventilation
To enhance natural ventilation, windows should be kept open as much as possible. Even in winter, cracking them open can make a difference. Installing a fan to exhaust air out of one window while leaving another open can significantly increase airflow. For maximum effectiveness, windows on opposite sides of the building should be opened.
For mechanical ventilation, if the system is controlled by a thermostat, the fan should be set to “on” when the space is occupied and switched to “auto” when unoccupied.
An important first step to improve ventilation, is to identify possible air distribution problems. If these issues exist, they can be mitigated through the use of HEPA filters or fans to mix the air in the room better. Adjusting the BAS to lower the temperature of the air when supplied from the ceiling can also be done, provided it does not cause thermal comfort issues.
These are other methods to improve mechanical ventilation:
- Systems that recirculate air can be improved by increasing the percentage of outdoor air and decreasing the amount of recirculated air. This can impact energy consumption.
- Demand controlled ventilation, which reduces ventilation when CO2 levels are low, can be disabled or limited to ensure high airflow rates when the space has partial occupancy.
- By replacing an air handling unit that recirculates air with one that includes heat recovery and provides more outdoor air, energy consumption can be kept to a minimum while greatly improving ventilation rates. This would have a higher capital cost, but provide significantly improved ventilation with little additional operating costs.
- If the ventilation system is variable volume, the amount of air supplied can be increased when operating at minimum ventilation levels.
A summary of available options for occupants can be found in this infographic:
The other tools to provide clean air are filtration and UV. To improve filtration, it can be done in the air handling unit by upgrading the filters or by using portable HEPA filters. A full introduction to filtration can be found here. Advice on using HEPA filters while addressing noise can be found here. Common questions about HEPA filters can be found here.
The other tool that can be used is UV light. A full introduction to UV light can be found here. The most effective uses of UV light are upper room UV and far-UV. An introduction to upper room UV can be found here and an introduction to far-UV can be found here.
