Feeling lost trying to understand those big HVAC boxes? Air Handling Units are key to building comfort and air quality, but often seem complex. Getting it wrong impacts efficiency and costs.
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An Air Handling Unit (AHU) is the core machine in many HVAC systems that conditions and circulates air, controlling temperature, humidity, and cleanliness to keep indoor environments comfortable and healthy.
I talk to many facility managers and technicians, and while they know AHUs are important, the specifics can be fuzzy. Why does selecting the right one matter so much for energy bills and the air people breathe? Let’s break down what these units do, the different types you might encounter, and why they are so vital for almost any medium-to-large building. Knowing this helps make better decisions for your facility.
Confused about what that large metal box connected to your ductwork actually does? It’s more than just a fan; it’s central to your building’s breathing system. Misunderstanding its role can lead to poor air and wasted energy.
An AHU is the central station for treating air in an HVAC system. It pulls air in, cleans it, heats or cools it, maybe adjusts humidity, and then sends it out through ducts to your building spaces.
Let’s dive deeper into the fundamental job of an AHU. Think of it as the lungs and airways of your building. Its main purpose, or what we call air handling, is to condition and move air to create a comfortable and healthy indoor environment. This involves several key tasks managed within the unit. It contributes directly to indoor air quality (IAQ) by filtering out pollutants and bringing in fresh air. It also manages thermal comfort by heating or cooling the air as needed. Proper ventilation, ensuring enough fresh air gets circulated while stale air is removed, is another critical function managed by the AHU. You’ll find these units working hard in all sorts of places – the office building where you work, the hospital providing critical care (where air quality is paramount), the shopping mall you visit, schools, and even industrial facilities like factories or data centers where precise environmental control is needed for processes or sensitive equipment. For someone managing a facility, like Wendy Thompson might, understanding this core function helps pinpoint issues when people complain about being too hot, too cold, or the air feeling stuffy. For a technician like James Rivera, knowing exactly what the unit is supposed to do is the first step in diagnosing a problem or performing effective maintenance. It’s not just about temperature; it’s about creating a healthy, breathable, and productive space.
Think all AHUs are the same basic design? There’s actually a variety, each suited for different situations. Choosing the wrong type can mean headaches with installation, performance, or fitting it into your space.
AHUs mainly differ in how they’re built (modular vs. packaged) and if they’re standard or custom-designed. Each type offers specific pros and cons depending on the building’s needs and budget.
Let’s explore the common types of AHUs you might encounter. Selecting the right configuration is crucial, and it’s something I often discuss with clients at Kaydeli when designing a complete system. The main distinctions usually come down to flexibility, installation ease, and application specifics.
First, we have Modular AHUs. As the name suggests, these are built in sections, or modules. Imagine building blocks – one block for filters, one for coils, one for the fan, etc. These sections are shipped separately and bolted together on site. The big advantage here is flexibility. You can customize the sequence of components, choose specific high-performance parts, and configure the unit to fit into tricky spaces, like a crowded mechanical room. They can also be built to handle huge amounts of air. Because of this adaptability, you often see modular units in large buildings, hospitals (which have very specific needs), labs, universities, and complex industrial sites. For a facility manager like Wendy managing a large or complex site, the ability to tailor a modular unit is a big plus. For James the technician, while assembly takes longer, maintenance can sometimes be easier as individual sections might offer better access to components.
Next are Packaged AHUs. These are the opposite – they come from the factory as a single, complete unit with everything inside one casing. Often, you’ll see these installed on rooftops, and in that case, they’re commonly called Rooftop Units or RTUs. Their main advantage is simplicity and speed. Since they are factory-built and tested, installation is generally faster and potentially less expensive upfront. They are a common choice for smaller to medium-sized commercial buildings, retail stores, or restaurants where a standardized solution works well and maybe indoor space for equipment is limited. However, they offer less flexibility in design and component choice compared to modular units. Wendy might choose packaged units for simpler buildings or when speed is critical. James usually finds the installation straightforward, though accessing internal components for repair might sometimes be tighter than in a modular design.
Finally, there are Custom AHUs. These are designed and built for very specific, often demanding, applications where standard modular or packaged units just won’t cut it. Think about unique size restrictions, the need for special materials (like stainless steel to resist corrosion), extremely high filtration requirements, very unusual operating temperatures or pressures, or integration with other highly specialized equipment. You might find custom units in heavy industrial settings, specialized research labs, marine environments, or unique architectural projects. Designing a custom AHU requires very close collaboration between engineers, the client, and the manufacturer.
Understanding these basic types – Modular for flexibility, Packaged for simplicity/speed, and Custom for unique challenges – helps narrow down the best approach for any given project.
Ever wondered what’s actually inside that AHU box making everything happen? It’s not just empty space; it’s packed with crucial parts working together. Knowing these components helps understand how the unit functions and what needs maintenance.
Key AHU components include fans to move air, coils for heating/cooling, filters for cleaning, dampers to control airflow, and the casing that holds it all together, plus controls to manage everything.
Let’s take a closer look at the main AHU components and what each one does. Understanding these air handling unit parts is essential for both facility managers overseeing operations and technicians performing service.
Component Description Fans/Blowers This is the engine of the AHU. It’s responsible for pulling air through the unit and pushing it out into the ductwork to reach the building spaces. Different fan designs exist (like centrifugal or plenum fans), chosen based on how much air needs to be moved (CFM) and how much resistance it needs to push against (static pressure from ducts, filters, coils). A critical feature on modern fans is the Variable Speed Drive (VSD). This allows the fan speed to be adjusted automatically based on the building’s needs, saving a lot of energy compared to running the fan at full blast all the time. James the technician often works on fan motors and VSD settings. Coils (Heating & Cooling) These are like radiators that change the air’s temperature. Cooling coils typically have chilled water (often supplied by a chiller like ours at Kaydeli) or a refrigerant flowing through them. As air passes over the cold fins, it gets cooled down. This process also naturally removes humidity from the air. Heating coils use hot water, steam, or sometimes electric resistance elements to warm the air up during colder weather. The performance of these coils is vital for maintaining comfortable temperatures, a key responsibility for Wendy. Filters These are absolutely essential for cleaning the air. Filters capture dust, pollen, bacteria, and other particles. They come in different efficiency levels, often rated using the MERV scale. Basic filters might catch large particles, while hospitals or cleanrooms use high-MERV or even HEPA filters to capture very tiny contaminants. Some AHUs might also include carbon filters to remove odors or specific gases. Keeping filters clean and replacing them regularly is one of the most important maintenance tasks James performs, directly impacting air quality and system efficiency. Dampers These are like adjustable gates within the AHU or ductwork that control airflow. Mixing dampers control the blend of fresh outside air and recirculated return air. Other dampers might control airflow across coils or isolate sections. Many dampers are motorized and linked to the control system. Humidifiers/Dehumidifiers (Optional) Some AHUs include equipment to specifically add moisture (humidifier) or remove excess moisture (beyond what the cooling coil does). This is important in climates or applications needing tight humidity control. Energy Recovery Devices (Optional) Components like heat wheels or plate heat exchangers can recover energy from the exhaust air and use it to pre-treat the incoming fresh air, saving significant heating or cooling energy. Casing The insulated metal box that houses all these components, preventing energy loss and protecting the internals. Controls Sensors (for temperature, humidity, pressure), actuators (to adjust dampers, valves), and a controller (often connected to a central Building Automation System) manage the AHU’s operation based on setpoints and sensor readings.All these parts must work together correctly for the AHU to do its job effectively.
We know AHUs handle air, but how specifically do they make the air inside a building better to breathe? It’s not just about temperature; clean air is vital for health and productivity. Ignoring this can lead to sick building syndrome or other issues.
AHUs improve indoor air quality primarily through filtration, which removes particles, and ventilation, which brings in fresh air to dilute indoor pollutants. Humidity control also plays a role.
Let’s dive deeper into how Air Handling Units contribute to better Indoor Air Quality (IAQ). In modern buildings, which are often sealed tightly to save energy, indoor air can sometimes be more polluted than outdoor air. The AHU is our main tool to combat this.
The most obvious way is through filtration. As the AHU draws air in (both return air from the building and fresh outdoor air), it passes through filters. These filters are designed to capture airborne particles. The level of filtration varies greatly depending on the need. Standard office buildings might use MERV 8 to 13 filters, which are good for dust, pollen, and mold spores. Hospitals, labs, or cleanrooms require much higher efficiency, often using MERV 14+ filters followed by HEPA filters. HEPA filters are extremely effective, capturing at least 99.97% of tiny particles like bacteria, viruses carried on droplets, and fine dust. Some AHUs also incorporate activated carbon filters. These don’t capture particles but adsorb gases and odors – things like VOCs (volatile organic compounds) that off-gas from furniture, carpets, and cleaning supplies. Effective filtration is crucial, and as a facility manager, Wendy would rely on this to ensure occupant health. James’s role in regularly replacing filters is critical here.
The second major factor is ventilation. AHUs control the intake of fresh outdoor air. Bringing in outside air is essential to dilute pollutants that build up inside, such as carbon dioxide (CO2) from people breathing, odors, and those VOCs I mentioned. Without enough fresh air, CO2 levels can rise, making people feel drowsy and unproductive. The AHU’s dampers control the mix of fresh and return air, ideally adjusted based on occupancy or CO2 sensor readings (Demand Controlled Ventilation) to provide enough fresh air without wasting energy. Standards like ASHRAE 62.1 provide guidelines for minimum ventilation rates.
Third, humidity control impacts IAQ. AHUs help manage humidity levels. High humidity (generally above 60%) encourages the growth of mold, mildew, and dust mites – all common triggers for allergies and respiratory problems. Low humidity (below 30%) can lead to dry skin, irritated sinuses, and potentially increased transmission of some viruses. By cooling and dehumidifying, or by adding humidity when needed, the AHU helps keep the moisture level in a healthy range.
So, it’s this combination of cleaning the air with filters, diluting pollutants with fresh air, and controlling humidity that makes the AHU a cornerstone of good IAQ, going far beyond basic air conditioning.
Running HVAC systems uses a lot of energy, often being one of the largest operating expenses for a building. Making AHUs work smarter, not harder, is key to saving money and being more sustainable. Ignoring efficiency means higher bills and a larger carbon footprint.
Modern AHUs save energy through features like variable speed drives (VSDs) for fans, high-efficiency motors, energy recovery systems that reuse heating/cooling energy, and smart control strategies.
Let’s explore the main ways we can achieve energy efficiency in AHUs. This is a major focus in the industry today, driven by both cost savings and environmental concerns. For facility managers like Wendy, optimizing HVAC energy savings is often a top priority. Here are the key strategies and technologies involved:
Strategy/Feature Description Variable Speed Drives (VSDs) This is a game-changer. Instead of running the fan at full speed all the time, a VSD allows the motor’s speed to adjust based on the actual demand for heating, cooling, or ventilation. Since the energy used by a fan drops dramatically with even small speed reductions (it follows a cube law relationship), VSDs can cut fan energy use by 50% or more in many applications. This provides exactly the airflow needed, no more, no less. High-Efficiency Motors Using motors with better efficiency ratings, especially Electronically Commutated (EC) motors, directly reduces the electricity consumed. EC motors are particularly good because they maintain high efficiency even when running at reduced speeds (unlike traditional motors which lose efficiency at lower speeds), making them a great pairing with VSD control logic. Energy Recovery This is about recycling energy. When ventilating, we exhaust conditioned air (warm in winter, cool in summer) and bring in unconditioned outside air. An Energy Recovery Ventilator (ERV) or Heat Recovery Ventilator (HRV) built into the AHU captures heat (and sometimes moisture with ERVs) from the exhaust air and transfers it to the incoming fresh air. This pre-heats the fresh air in winter and pre-cools it in summer, significantly reducing the work the main heating and cooling coils have to do. Common types include heat wheels, plate heat exchangers, and run-around coils. The energy savings can be substantial, especially in climates with extreme temperatures or buildings needing lots of fresh air. Smart Controls Modern Building Automation Systems (BAS) enable sophisticated control strategies:Implementing these features makes for sustainable AHUs that significantly cut operating costs and environmental impact.
Selecting a new AHU or replacing an old one is a major decision. With so many options and factors, how do you ensure you get the unit that best fits your building? Choosing incorrectly can lead to years of high energy bills, poor comfort, or inadequate air quality.
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Choosing the right AHU involves calculating heating/cooling loads, determining airflow and filtration needs, considering space and budget, and prioritizing energy efficiency goals for your specific building.
Here’s a practical guide to help navigate the process of selecting an air handling unit. While I always recommend consulting with experienced HVAC engineers for the final design, understanding these key factors will help you participate effectively in the decision-making process, whether you’re a facility manager like Wendy or involved in the specification like James might be.
Step/Factor Description Calculate the Loads First, you need to know how much heating and cooling the building or zone actually requires. This involves detailed calculations considering building size, insulation, windows, occupancy, lighting, equipment heat gains, climate data, and ventilation needs. This "load calculation" determines the required capacity of the heating and cooling coils. Getting this right is fundamental to proper sizing. Determine Airflow (CFM) Based on the loads and ventilation requirements (often set by codes like ASHRAE 62.1), you need to figure out how much air (in Cubic Feet per Minute, CFM) the AHU needs to move. This dictates the fan size. Assess Static Pressure The fan doesn’t just move air; it has to push it through filters, coils, dampers, and the entire length of ductwork. You need to calculate this total resistance, called static pressure , to ensure the selected fan can handle the job. Define Air Quality Requirements What level of cleanliness is needed? A standard office needs good filtration (e.g., MERV 13), but a hospital or lab needs much higher levels (HEPA). Do you need special filters for odors or gases (carbon filters)? This dictates the filter section design. Consider Space and Location Where will the unit go? Is there a large indoor mechanical room? Is rooftop installation necessary? Are there height or footprint restrictions? This heavily influences whether a modular, packaged, or even custom unit is feasible and its physical configuration (horizontal vs. vertical). Set Energy Efficiency Goals Are you aiming for basic compliance, or do you want maximum energy savings? Are there certifications like LEED involved? This guides decisions about VSDs, motor efficiency, and whether to include energy recovery. Higher efficiency often means higher initial cost but lower operating cost – a key calculation for Wendy. Establish the Budget Be realistic about both the upfront cost (unit purchase + installation) and the long-term operating costs (energy + maintenance). Sometimes investing more initially in efficiency pays back quickly. Evaluate Noise Criteria How quiet does the space need to be? Offices, libraries, or performance spaces have stricter noise limits than warehouses. This might require specific fan selections, lower speeds, or sound attenuators. Plan for Controls Integration How will the AHU connect to your existing Building Automation System (BAS)? Ensure compatibility and that the controls can execute the desired energy-saving strategies. James will need to commission and work with these controls. Think About Maintenance Access Ensure the final design allows reasonable access for routine tasks like changing filters, cleaning coils, and servicing the fan and motor. Poor access makes maintenance difficult, costly, and less likely to be done properly.Given these interacting factors, choosing an AHU is a complex task best tackled with expert help to ensure all needs are met effectively and efficiently.
In simple terms, your Air Handling Unit is vital for good air quality, comfort, and efficiency in your building. By understanding its core job, the different types available, and how it works, you can make better choices for your HVAC system needs.
Air Handling Units. How they work and how they’re built.
In this presentation we’ll be learning how Air Handling Unit (AHU) works within various commercial and healthcare buildings. We’ll show you how custom Air Handling Units are built with the selection of various options such as humidifiers, heat wheels, heating and cooling coils, dampers, economizers, UV lighting and the various types of fans being used.
We’ll explain the different types of air handlers used including VAV, CAV, Dual-Duct, Multizone and 100% Outside air units.
For the YouTube video of this presentation, scroll to the bottom.
The main differences between an air handling unit and your typical air conditioner is that the air handler doesn’t provide the source for heating or cooling. Also, air handlers are available in much larger sizes up to and over 400,000 CFM . Air handling units also allow greater customization to fit project specifics.
The air conditioner has the refrigeration cycle as part of the equipment, where the compressor circulates the refrigerant that does the cooling and/or the heating. The air handling unit will do its cooling using chilled water from a chiller or refrigerant from a remotely located compressor, and do its heating using heating hot water or steam from a boiler.
As you can see the Return air system has two options, it can return air back to the inlet of the supply, or it can exhaust the air outside. The rest of the air systems only have one option as shown by the directional arrows.
Let’s build a custom air handler like the manufacture would using the engineers project requirements and the available air handler sections.
The air handling unit manufacture will use their software program to assemble a unit based on the project specifics. Looking at a screen of options the air handlers engineering team will start building the air handler maybe starting with the inlet options.
We’ll build our air handling unit using plenum fans because they provide better acoustics and allow for some redundancy incase a fan burns out. As you can see we added them on the supply inlet and on the exhaust.
Since this is an operating room will add a UV (Ultraviolet) lighting system to kill any bacterial, viral or fungal organism in the air and on or near the cooling coil, including the drain pan where water accumulates before draining.
We added an energy recovery wheel to save energy and capture the heat that is being wasted. Using 100% outside air is energy intensive as we are spending energy to cool the air and then we are exhaust all of that air out of the building. To capture this energy there are several choices like the heat recovery wheel, heat exchanger or a run around coil. We’ll explain these in another video. We also added filters on the exhaust stream to keep the energy wheel clean.
The first section will be our pre filter section to clean the air that’s coming in, and then the Heat Recovery Wheel, then our supply fans. The fans are what pull the air from the outdoors and push the air into the operating room. Then we’ll have a hot water and chilled water coil to temper the air, the UV Lighting system, and finally we’ll have a HEPA filter to clean the air as there’s additional requirements for the operating room, since you’re cutting people open the air has to be super clean. And then on the exhaust side we need an exhaust damper and an exhaust fan to take the air from the room and exhaust it out.
Now we’ll take this air handler that we’ve built and put it in the mechanical room in the hospital. You see we got our two operating rooms. We’ll set our Air Handler that we just built, and then the HVAC contractor will run the main ducts, exhaust and supply mains, and from there they’ll tie into branches through exhaust and supply valves feeding the operating room with laminar Flow grilles and low return.
Then the contractor will rig and set in a boiler, hook up all the hot water piping to the hot water coil, and then make a connection from the chiller to the chilled water coil, or if you’re using a compressor you can run DX refrigerant piping.
We just showed you how you can customize any air handler according to the project requirements, now we’ll show you a few more systems where air handlers are used.
This is a dual duct air handling unit that provides Cold air and warm air to travel down two different main supply ducts to dual-duct VAV mixing boxes. After the Dual Duct mixing box the air leaves in one common duct with the correct mixture of cold and warm air to satisfy the temperature setting of the space.
The air handler supply’s air over the cooling and heating coils simultaneously and the dual-duct boxes decide how much of each to open depending on the requirements of the room temperature sensor. One dual duct mixing box could be providing cooling while another is providing heating.
This is a multi-zone air handling unit. Each of the zones has a hot and cold deck damper at the air handler unit, which is different than the dual duct air handler we just showed you. Each zone has its own supply air damper at the heating coil and another at the cooling coil.
This air handler has a mixing box that allows for return air and outside to mix. The outside air damper will modulate to maintain the minimum amount of ventilation air as required by code. It’s possible to control the outside air using a CO sensor in the space being served by this air handler, to allow for energy conservation.
This is another version of the multi-zone air handler, except instead of a dual duct system with two coils in the air handler, this unit uses only a cooling coil. The heating if required is provided by an in-duct reheat coil.
Here is a Variable Air Volume VAV Air Handler mounted on a roof. The VAV Air Handler is probably the most common system used in medium to large commercial buildings.
This custom air handler starts with the return air section, then travels through the return air fans. The air next enters the economizer, where the air can be exhausted out of the building or returned to the system. The next set of dampers is the outside air dampers that work with the economizer to bring in code mandated ventilation air. As the outside air damper opens to let more ventilation air in, the exhaust damper will open approximately the same amount to let air out.
Economizers save energy by using the outside air to cool down the building when the temperature is lower than the return temperature or some set value.
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