9 Core Components of an Air Handling Unit
This guide walks the full air path through an AHU, component by component, then covers filtration and air quality, the BMS and PLC logic that runs the unit, how the conditioned air actually reaches the space, maintenance, energy efficiency and where AHUs are applied across India and the GCC.
An AHU with its access doors open. Reading left to right along the air path is how the whole unit becomes intelligible. [REPLACE with your own project photograph or a licensed image.]
- TL;DR
- What Is an Air Handling Unit?
- Interactive AHU Components Diagram
- The 9 Core Components in Order
- Coil Load and AHU Sizing Calculator
- Air Filters and Air Quality
- Filter Energy Penalty Calculator
- AHU Controls: PLC and BMS Logic
- AHU Alarms and Safeties
- Air Distribution and Circulation
- Maintenance and Repair
- Energy Efficiency Considerations
- Applications Across India and the GCC
- FAQs
- Sources and Further Reading
TL;DR
Key takeaways
- An AHU is a sequence of components along one air path: casing, mixing box and dampers, filters, cooling coil, heating coil, drain pan, eliminators, fan section, controls.
- Order is not arbitrary. Filters sit before the coil so the coil stays clean, and the drain pan sits under the coil because that is where condensate forms.
- Filtration is normally staged: a cheap pre-filter protects an expensive fine filter, which in turn protects the coil.
- ISO 16890 replaced EN 779 as the mandatory filter classification on 30 June 2018, using ISO Coarse, ePM10, ePM2.5 and ePM1 groups instead of G, M and F grades.
- HEPA is not on that scale. ISO 16890 explicitly excludes EPA, HEPA and ULPA filters, which are classified separately under EN 1822 as H13, H14 and so on.
- Coil capacity is a total enthalpy calculation, not a temperature one, so it automatically splits into sensible and latent parts.
- The sensible heat ratio changes the coil selection completely by climate: about 0.81 for Delhi conditions against 0.64 for Mumbai on the same unit.
- Fan power is proportional to pressure, so a dirty filter bank is a permanent energy charge, and fan static must be calculated at the dirty condition.
- A freeze stat is hard wired, averaging, and manual reset by design, so a genuine freeze risk cannot be silently cleared by the BMS.
- Efficiency comes from three levers: VFDs (80% speed gives about 51% power), economiser cycles, and energy recovery.
What Is an Air Handling Unit?
An air handling unit is the equipment that takes air, conditions it, and moves it into a building. It filters the air, cools or heats it, controls its moisture, and pressurises it enough to travel through the ductwork to the spaces it serves.
Where it sits in the wider system is worth being precise about, because it is the handover point between two halves of an HVAC installation. Upstream, a chiller produces chilled water and pumps deliver it — see our guide to HVAC chilled water systems for how that side works. That water arrives at the AHU's cooling coil. Downstream, ductwork carries the conditioned air out to diffusers, and return air comes back. The AHU is where the water side becomes the air side.
Everything in this guide is a consequence of that role. The unit needs a coil because water must meet air somewhere. It needs a drain pan because cooling humid air produces condensate. It needs filters because a fouled coil cannot transfer heat. It needs a fan because air will not move through a duct on its own. Nothing in an AHU is decorative.
Interactive AHU Components Diagram
The diagram below is a section through a typical draw-through AHU, with air flowing left to right. Click any numbered component to see what it does, why it sits where it does, and what the design engineer specifies.
Figure 1: Interactive AHU components diagram. This is a draw-through arrangement, meaning the fan sits downstream of the coil and pulls air through it. In a blow-through unit the fan comes first and pushes air across the coil instead.
Master AHU Selection & MEP Design
Learn coil sizing, filter selection, fan static pressure, BMS sequences, and full AHU schedule production — structured for India and GCC MEP careers.
The 9 Core Components in Order
The same list as a reference table. The order column is the one worth memorising, because it is what makes the unit diagnosable.
| # | Component | Function | Why it sits here |
|---|---|---|---|
| 1 | Casing and panels | Houses everything, forms the air path | Encloses the whole unit |
| 2 | Mixing box and dampers | Blends outdoor and return air | First, so everything downstream treats the mixed stream |
| 3 | Pre-filter | Captures coarse dust and debris | Before the fine filter, to protect it |
| 4 | Fine filter | Captures fine and respirable particles | Before the coil, to keep the coil clean |
| 5 | Cooling coil | Removes heat and moisture | After filtration, so fins do not clog |
| 6 | Drain pan | Collects and drains condensate | Directly under the coil, where water forms |
| 7 | Heating coil / reheat | Raises supply temperature | After cooling, for humidity control |
| 8 | Drift eliminator | Catches carryover droplets | Downstream of the coil, before the duct |
| 9 | Fan section | Generates system pressure | Draw through: after the coil, pulling air across it |
| + | Controls and sensors | Runs the sequence, raises alarms | Distributed throughout the unit |
Optional sections you will meet on larger units
Energy recovery wheel or plate exchanger at the outdoor air intake, pre-treating incoming air using the exhaust stream. Humidifier, steam or evaporative, in applications needing minimum humidity such as some healthcare and manufacturing. Sound attenuator where the unit sits near occupied space. Return or relief fan on larger systems needing positive control of building pressure. HEPA final filter downstream of the fan in cleanroom and critical healthcare applications, positioned after the fan so that any leakage past the fan casing cannot bypass it.
Coil Load and AHU Sizing Calculator
Two calculations set the whole unit: how much air, from the sensible load, and how much cooling, from the enthalpy difference across the coil. Everything else follows from those.
Enthalpy captures both temperature and moisture, so total capacity automatically contains the sensible part, which changes temperature, and the latent part, which condenses water out. The split between them is what SHR describes, and it is a property of the climate, not of the unit.
The number that changes the coil, and most articles never mention
Switch the location selector between Delhi and Mumbai and watch the sensible heat ratio move from about 0.81 to 0.64 on the identical unit. Delhi is hotter in dry bulb terms, but Mumbai's coil has to do roughly twice as much dehumidification. Same airflow, same room condition, completely different coil selection: more rows, a lower apparatus dew point, and a different chilled water temperature. This is why an AHU schedule that lists only total capacity in TR is incomplete, and why a unit that performed perfectly on a Delhi project can leave a Mumbai building at 65% relative humidity while hitting its temperature setpoint exactly.
Air Filters and Air Quality
Filter selection directly determines indoor air quality outcomes — the grade specified at design becomes the ceiling on what the building can achieve in service.
Filtration is staged for a reason that is economic as much as technical. A cheap filter protects an expensive one, which protects the coil, which is the most expensive thing to clean.
The Classification Standard Changed
This trips up a lot of otherwise current material. ISO 16890 replaced EN 779 as the mandatory classification basis on 30 June 2018. The old G1 to G4, M5 to M6 and F7 to F9 grades were replaced by four groups based on how well a filter captures the particulate fractions that public health actually tracks: ISO Coarse, ISO ePM10, ISO ePM2.5 and ISO ePM1.
The change matters because the old system rated filters against a single synthetic particle size, while the new one rates them against the same PM1, PM2.5 and PM10 fractions used by the WHO and environmental agencies. A filter is classified in the highest group where it achieves at least 50% minimum efficiency, and the reported efficiency is rounded down to the nearest 5%. So a filter labelled ISO ePM1 70% captures 70% of PM1 particles.
| ISO 16890 group | Roughly replaces | Captures | Typical AHU role |
|---|---|---|---|
| ISO Coarse | G1 to G4 | Larger dust, lint, fibres, insects | Pre-filter, first stage |
| ISO ePM10 | M5, M6 | Particles up to 10 micron | Second stage in less demanding applications |
| ISO ePM2.5 | F7, F8 territory | Fine particles up to 2.5 micron | Common fine filter for commercial buildings |
| ISO ePM1 | F8, F9 territory | Ultrafine particles up to 1 micron | Fine filter where outdoor air quality is poor |
| HEPA and ULPA are outside ISO 16890 entirely, classified under EN 1822 | |||
| EN 1822 H13 | Not comparable | 99.95% at most penetrating particle size | Cleanrooms, operating theatres, isolation rooms |
| EN 1822 H14 | Not comparable | 99.995% at most penetrating particle size | Higher grade cleanrooms and containment |
The middle column says "roughly replaces" deliberately. There is no direct conversion between EN 779 and ISO 16890, because the two systems measure different things. Treat old class equivalences as orientation only, and specify to ISO 16890 on new work. In North American practice, MERV ratings under ASHRAE 52.2 are a third parallel system, which is again not directly convertible.
The HEPA point worth getting right
It is common to see G4, F7 and H13 presented on one continuous scale, as though HEPA were simply the top of the same ladder. It is not. Neither EN 779 nor ISO 16890 covers EPA, HEPA or ULPA filters, which are tested and classified separately under EN 1822 against the most penetrating particle size. They are different standards measuring different things by different methods. On a cleanroom or healthcare project, specifying "H13 equivalent" against an ISO 16890 class is not a specification, it is an ambiguity that a supplier will resolve in their favour.
Filter Energy Penalty Calculator
Filter condition is not just an air quality issue. Because fan power is directly proportional to the pressure the fan must overcome, a loaded filter bank is a continuous energy charge for as long as it stays in place.
- Q airflow in m³/s
- Δp total pressure the fan develops, in Pa
- η combined fan and motor efficiency
AHU Controls: PLC and BMS Logic
The components are hardware. What makes an AHU a system is the control logic coordinating them, normally a PLC or a BMS field controller running a defined sequence of operations.
The Four Control Loops
-
Supply air temperature
A sensor in the supply duct compares against setpoint, and a PID loop modulates the chilled water valve to hold it. In a full sequence the controller modulates cooling valve, economiser dampers and heating valve in sequence, never simultaneously, so the unit is not heating and cooling at once. Advanced sequences reset the setpoint upward as load falls, which saves reheat and chiller energy.
-
Duct static pressure
On a VAV system, a pressure sensor in the duct drives the fan VFD through a PID loop. As zone boxes throttle back, duct pressure rises, and the controller slows the fan. Better sequences reset the pressure setpoint based on the most open zone damper, so the fan runs only as fast as the hardest served zone actually requires.
-
Damper position and ventilation
The outdoor air damper holds a minimum position that guarantees the ventilation rate, and opens beyond it only for economiser operation. The minimum position is a code requirement and must be verified by the balancing contractor in the field, not just written on a drawing.
-
Humidity, where controlled
Where the space has a humidity requirement, the controller overcools to reach the apparatus dew point and then reheats to the supply setpoint. Energy intensive by nature, which is why sequences that find free reheat or use a dedicated outdoor air unit are worth the design effort in humid climates.
AHU Alarms and Safeties
A controls engineer is judged on the alarm list as much as the control loops. Select a tab to see what each alarm means and what to check.
Why the freeze stat is hard wired and manual reset
Both design choices exist for the same reason: a frozen and burst coil floods a plant room and can write off a unit. Hard wiring into the fan starter means the safety operates even if the controller has crashed, lost power or been left in a test mode. Manual reset means nobody can clear the trip from a workstation without physically attending, seeing the condition and thinking about the cause. An auto reset freeze stat would cycle the unit repeatedly against the condition that tripped it, which is exactly how a coil freezes solid overnight with a clean alarm log.
Air Distribution and Circulation
Conditioned air leaving the AHU is only useful if it reaches occupants and returns. The path is a closed circuit.
- Supply duct
The fan pushes air into the supply ductwork, sized so that friction loss and velocity stay within acceptable limits — the full sizing methodology is in our HVAC duct design guide. Every metre of duct and every fitting is pressure the fan must overcome, permanently.
- Terminal units and diffusers
VAV boxes modulate flow per zone where fitted, and diffusers deliver air into the room. A diffuser is selected on airflow, throw, pressure drop and noise criteria together, not on airflow alone.
- Room mixing
Supply air entrains and mixes with room air before it reaches head height. Get the throw wrong and occupants feel either a draught or a dead spot.
- Return path
Return grilles collect room air into a ducted return or a ceiling plenum and carry it back to the AHU mixing box, where a proportion is exhausted and replaced with fresh outdoor air. The circuit closes.
Why balancing decides whether any of this works
A duct system distributes air by the path of least resistance, not according to the drawing. Without proportional balancing at commissioning, zones near the fan take more than their design airflow and distant zones are starved, which produces the classic complaint pattern of one floor too cold and another stuffy. The usual response, dropping the setpoint for everyone, overcools the already cold zones and raises energy consumption to solve a problem that was never about capacity. An unbalanced system makes a correctly selected AHU look faulty.
Maintenance and Repair of Air Handlers
Routine Checks
- Filter inspection and replacement. Change on measured differential pressure reaching the final resistance, not on a fixed calendar. A calendar throws away good filters early and leaves loaded ones in place during a dusty season.
- Coil cleaning, both faces. Fouling on the air side raises pressure drop and cuts capacity. Comb straightened fins and check the coil is clean between the rows, not just on the entering face.
- Drain pan and trap. Confirm the pan is draining and holding no standing water, that the trap is correctly filled and correctly sized for the unit's negative pressure, and that the pan itself has not corroded through.
- Belt and bearing inspection. On belt driven units, check belt tension, alignment and wear, and listen and feel for bearing condition. Direct drive plug fans remove this task entirely, which is a real part of their appeal.
- Damper linkages and actuators. Confirm each damper strokes fully and that the outdoor air minimum position has not been disturbed. Linkages seize, and actuators get left in hand after a service visit.
- Sensor verification. Compare BMS readings against a calibrated instrument. A drifted supply air sensor makes the whole control sequence chase a temperature that is not real.
Three Failure Patterns and Their Early Signs
| Failure | Early warning | Consequence if ignored |
|---|---|---|
| Fouled cooling coil | Rising air side pressure drop, falling temperature drop across the coil, chilled water valve driven fully open at part load | Loss of capacity, higher fan energy, eventual complete inability to meet setpoint on a design day |
| Blocked or unsealed drain trap | Water in the base of the unit, gurgling from the drain, damp insulation downstream | Pan overflow into the plant room, microbial growth, ceiling damage below |
| Bearing or belt failure | Change in noise, increased vibration, belt dust in the fan section, rising motor current | Fan seizure and complete loss of air supply to everything the unit serves |
Energy Efficiency Considerations
Three levers do most of the work, in descending order of how reliably they pay back.
1. Variable Frequency Drives
Fan power varies with roughly the cube of speed. That single relationship is why VFDs dominate the efficiency conversation for air systems.
| Fan speed | Airflow | Fan power | Saving |
|---|---|---|---|
| 100% | 100% | 100% | baseline |
| 90% | 90% | 72.9% | 27% |
| 80% | 80% | 51.2% | 49% |
| 70% | 70% | 34.3% | 66% |
| 60% | 60% | 21.6% | 78% |
Theoretical fan law values. Real savings are somewhat lower because drive losses, motor efficiency at part load and any static pressure that does not vary with flow all erode the ideal figure. The direction and magnitude hold: a modest speed reduction produces a large power reduction.
2. Economiser Cycle
When outdoor conditions are more favourable than return air, the controller opens the outdoor air damper beyond its minimum position and cools with outside air instead of chilled water, up to 100% outdoor air. In temperate climates this is worth a great deal of annual energy.
An honest note for this region. A dry bulb economiser has limited application in most of India and the GCC, because the hours when outdoor air is genuinely cooler than return air are few, and admitting humid outdoor air can add more latent load than the sensible saving is worth. Enthalpy based economiser control, which compares total heat rather than temperature alone, is the correct form here, and even then the benefit is seasonal and location specific. Model it rather than assuming it.
3. Energy Recovery
A rotary wheel or plate exchanger uses the exhaust air stream to pre-treat incoming outdoor air. Because outdoor air is the most expensive air in the building, and in this region it arrives hot and often very humid, recovery equipment has a strong case. The distinction that matters is heat recovery, which transfers temperature only, against energy recovery, which transfers moisture as well. In humid coastal climates the moisture is most of the load, so an enthalpy wheel recovers far more than a sensible only device.
Four more that cost nothing at design stage
Low face velocity. A physically larger coil and filter section at lower velocity means lower pressure drop for the life of the unit, and lower fan energy every hour it runs. Direct drive plug fans. No belt losses, no belt maintenance, and better part load behaviour than a belt driven centrifugal. Supply air temperature reset. Raising the setpoint as load falls reduces both reheat and chiller work. Static pressure reset from zone demand. Letting the most open damper set the fan pressure target rather than a fixed number, which exploits the cube law far more of the time.
Applications Across India and the GCC
| Application | What drives the design | Typical filtration |
|---|---|---|
| Commercial offices, malls, IT parks | Comfort, ventilation rate, part load efficiency | Pre-filter plus ISO ePM2.5 or ePM1 fine filter |
| Hotels and hospitality | Acoustics, humidity control, guest zone flexibility | Pre-filter plus fine filter, attenuators fitted |
| Hospitals, general areas | Pressure relationships between zones, infection control | Multi stage, with HEPA in critical areas |
| Operating theatres and isolation rooms | Air change rates, directional airflow, terminal HEPA | Terminal EN 1822 H13 or H14 |
| Pharmaceutical and industrial cleanrooms | Particle count classification, room pressure cascade, validation | Terminal HEPA, often H14 |
| Data centres | High sensible load, close temperature control, redundancy | Pre-filter plus fine filter, high sensible coils |
What the Climate Changes
Two regional realities shape almost every AHU decision in India and the GCC, and both come back to the same underlying variable.
Latent load dominates in coastal and monsoon climates. As the coil calculator above shows, a Mumbai or Chennai coil spends a third or more of its duty on dehumidification, while a Delhi or Riyadh coil is mostly sensible. That drives deeper coils, lower apparatus dew point, and often reheat for humidity control. A design copied from a dry climate project will hold temperature and miss humidity.
Outdoor air quality drives the filter specification. In cities with high ambient particulate, the fine filter class is not a comfort refinement but a basic requirement, and the pre-filter change interval shortens considerably. This also raises the fan static allowance, which feeds back into fan and motor selection.
One more that gets missed: freeze protection is largely irrelevant here. Freeze stats appear on units built to international specifications and are correct to include, but they will essentially never operate in most of the region. Understand what they do, do not let them distract from the humidity control problem that will actually determine whether the building is comfortable.
Where This Knowledge Leads
AHU selection sits at the point where load calculation, psychrometrics, hydraulics, controls and coordination all meet, which is why it is such a good indicator of whether an engineer understands a building or only a spreadsheet. Knowing the components is the entry ticket. Being able to select a coil on both sensible and total duty, build a fan static pressure at the dirty filter condition, specify a filter class against a real outdoor air condition, and write a sequence of operations a controls contractor can implement, is the job.
Frequently Asked Questions
Sources and Further Reading
- ASHRAE Handbook: HVAC Systems and Equipment, ASHRAE. Chapters on Air Handling and Distribution, Air Cleaners for Particulate Contaminants, and Fans, covering component arrangement, coil selection and filtration.
- ASHRAE Handbook: Fundamentals, Psychrometrics chapter, for the enthalpy and moisture relationships behind the coil load calculation used in this article.
- ISO 16890, Air filters for general ventilation. The classification standard that replaced EN 779 as the mandatory basis on 30 June 2018, defining the ISO Coarse, ePM10, ePM2.5 and ePM1 groups.
- EN 1822, High efficiency air filters (EPA, HEPA and ULPA). The separate standard governing HEPA classification, which ISO 16890 explicitly does not cover.
- ANSI/ASHRAE Standard 52.2, the MERV rating method used in North American practice, a third parallel system not directly convertible to either of the above.
- ASHRAE Guideline 36, High Performance Sequences of Operation for HVAC Systems. The reference for AHU control sequences, including supply air temperature reset, static pressure reset from zone demand, and economiser logic.
- ANSI/ASHRAE Standard 62.1, for the outdoor air rates that set the minimum damper position.
- Manufacturer selection software and spec sheets from AHU and coil suppliers. The certified selection for the specific unit always governs over any generic figure in this article.
Calculation basis for this article's tools
The coil calculator computes moist air properties from the Magnus relation for saturation vapour pressure, evaluates the outdoor air state on the wet bulb line, and mixes outdoor and return air by mass proportion at the stated outdoor air fraction. Leaving air is assumed saturated to 90% relative humidity at the supply temperature, which approximates a real coil but is not a substitute for a manufacturer selection at a calculated apparatus dew point. Standard air density of 1.2 kg/m³ and specific heat of 1.006 kJ/kg·K are used throughout. Design conditions listed are representative summer values for illustration; use actual ASHRAE design day data for your site. The filter calculator computes fan power as airflow multiplied by pressure divided by combined efficiency, and considers the filter section only, not total unit static. Fan law values in Table 4 are theoretical. These are teaching and first pass tools. Issued designs should use manufacturer selection software and validated load calculations.
Standards are revised on fixed cycles. Confirm the edition adopted by your project before issuing a specification. This article was last verified against the sources above on 1 August 2026.
Ready to start your engineering career journey?
Investing in structured training is one of the best ways to set yourself apart in MEP engineering. Our Courses cover HVAC, electrical and plumbing design in depth, plus tools like HAP software — the exact skillset that sets you apart in a competitive field.