HVAC
Air handling unit (AHU) components and operation field guide
What every section of an air handler does, in the order the air hits it: mixing box, filters, coils, condensate, fan, and the controls that tie it together.
Direct answer
An air handling unit (AHU) is the indoor central air handler that conditions and moves air through a building's duct system. Air flows in as return plus outside air through a mixing box, filters, heating and cooling coils, then the fan into the supply duct. The manufacturer's data and the project design govern the sizing and setpoints.
Key takeaways
- An AHU's airflow order is fixed: return plus outside air mix, then filters, then coils, then fan, then supply duct.
- Filters sit ahead of the coils so air is cleaned before reaching the fins; change them on measured pressure drop, not the calendar.
- Only the cooling coil has a condensate pan; the drain line needs a trap sized to the unit's static, deeper for negative-pressure draw-through units.
- Draw-through (fan after the coil) is the common arrangement; it spreads airflow evenly across the coil face and runs the coil section at negative pressure.
- Minimum outside-air ventilation comes from ASHRAE Standard 62.1; verify actual outside-air flow, since a damper position does not equal a flow.
What an air handling unit is and what it does
An air handling unit is the central station box that conditions air and pushes it through a building's ductwork. It is the indoor machine, usually sitting in a mechanical room or a penthouse, that takes return air from the spaces, mixes in fresh outside air, cleans it, heats or cools it, and sends it back out through the supply duct to the zones. Strip away the sheet metal and it is a series of sections in a line, each doing one job to the air as it passes.
The AHU is the heart of a built-up central system. On a large building it feeds a network of VAV terminals that throttle the air to each zone, so the unit makes a steady supply of cold air and the boxes downstream decide how much each space gets. See the VAV box commissioning guide for the terminal side of that system.
What it does not do is make its own cooling, at least not the chilled-water version. A chilled-water AHU has a coil fed by a central plant, so the chiller and pumps live elsewhere and the AHU just moves air across the coil. A DX air handler carries its own refrigerant coil tied to a remote condenser. Either way, the unit's job is to condition and move air, reliably, against the resistance of the duct system it serves.
What is the difference between an AHU, an RTU, and an FCU?
An AHU is the indoor central air handler, often built up from modular sections, that serves a large area or a whole building through ductwork. An RTU is a packaged rooftop unit with everything, cooling, heating, and fans, in one weatherproof cabinet sitting on a curb outside. An FCU is a small fan coil unit that serves a single zone, usually with just a fan and a coil and no real outside-air handling.
The line between an AHU and an RTU is mostly where the equipment lives and how it ships. An AHU is indoor and frequently arrives in sections that bolt together on site, which lets it grow as large as the application needs and carry features a packaged unit cannot fit. An RTU is the all-in-one outdoor version of the same idea, factory-assembled and dropped on the roof. For the packaged rooftop side, see the RTU installation and startup guide.
An FCU is the small cousin. One fan, one or two coils, a filter, and not much else, serving a room or a small zone and often recirculating the space air rather than bringing in much fresh air. When a job needs ventilation air, dehumidification, and serious filtration for a large area, that is AHU territory. When it needs a quiet box over a hotel room ceiling, that is the FCU.
| Unit | Where it sits | Scope | Outside air |
|---|---|---|---|
| AHU | Indoor, mechanical room or penthouse | Floor or whole building via duct | Yes, through a mixing box |
| RTU (packaged) | Outdoor, on a roof curb | Zone to a floor, packaged | Yes, built-in economizer common |
| FCU | In or above the zone | Single room or small zone | Often little to none |
The airflow path through the unit
Air moves through an AHU in a fixed order, and knowing that order tells you where to look when something is wrong. Return air comes back from the building and meets outside air at the mixing box. The combined stream hits the filters first, then the coils, then the fan, and leaves through the supply duct to the zones. Return plus outside, mix, filter, coil, fan, supply. That is the line.
The order is not arbitrary. Filters sit ahead of the coils on purpose, so the air is cleaned before it reaches the fins. A dirty filter is cheap to change. A coil packed with the dirt the filter should have caught is a day's work to clean and a loss of capacity in the meantime. Put the clean section first and the expensive section stays clean longer.
Where the fan lands in that order is the one variable. In most units the fan sits after the coil and pulls air through everything ahead of it, which is the draw-through arrangement. Some units put the fan before the coil and blow through it. The sections before the fan are the same either way. Walk the unit in airflow order, not in the order the access doors happen to open, and the sequence makes sense.
The mixing box and the dampers
The mixing box is the first section, where return air and outside air come together before anything is done to them. It carries three dampers: an outside-air damper that lets fresh air in, a return-air damper that admits air back from the building, and a relief or exhaust damper that dumps the air the building cannot keep when the outside-air damper opens up. They move together, opposed, so as one opens the other closes and the total airflow stays roughly constant.
Those dampers are also the economizer. When the outside air is cool and dry enough, the controls swing the outside-air damper wide and the return damper shut, and the building cools itself on free air instead of running the chiller. That is real energy, and it is also the first thing that quietly stops working, because a damper actuator that has failed or a linkage that has seized leaves the unit stuck on minimum outside air with the economizer doing nothing. See the economizer material by topic for the control side.
The blunt failure here is a stuck damper nobody noticed. An outside-air damper frozen open in January overworks the heating coil and can freeze it. Frozen shut in a mild fall, and the building runs mechanical cooling on a day it should have been free. Cycle the dampers and watch the blades move, do not trust the actuator's position feedback alone.
The filter section
The filters are the first thing the air hits after mixing, and they protect everything downstream. Most commercial AHUs run two stages: a low-cost pre-filter that catches the big stuff and a final filter that does the real cleaning. Filter efficiency is rated by MERV, where a higher number catches finer particles, and the design picks the MERV the application needs against the static the fan can afford to spend pushing air through it.
Filtration is a trade between clean air and fan energy. A finer filter cleans better and costs more static pressure, which the fan has to overcome, so you do not just fit the highest MERV on the shelf. The design sets the MERV; the field keeps them changed. See the air-filtration material by topic for selecting media and MERV.
The thing that gets skipped is the filter PM, and it costs more than people think. A loaded filter raises the pressure drop across that section, the fan works harder to hold airflow, and on a unit without a VFD chasing setpoint, the airflow simply falls off. Read the filter gauge, the differential-pressure gauge across the filter bank, and change on pressure, not on the calendar alone. A unit that lost half its airflow to clogged filters looks like a coil problem from the zone, and it is not.
The coils: cooling, heating, and preheat
The coils are where the air actually gets conditioned. A cooling coil is a finned-tube heat exchanger fed with chilled water from a central plant, or a DX coil carrying refrigerant straight from a compressor. The air gives up heat to the coil as it passes through the rows of tubes and the fins between them, and the more rows deep and the tighter the fin spacing, the more capacity and the more static the coil costs. See the chilled-water and DX coil material by topic for the refrigerant and water side.
The cooling coil also dehumidifies. When the coil surface runs below the dew point of the air, moisture condenses on the fins and runs off, which is why the cooling coil, and only the cooling coil, has a condensate pan under it. That is the wet coil. The heating coil stays dry.
Heating comes from a separate coil: hot water from a boiler, steam, or an electric element. Many units also carry a preheat coil ahead of the main coils, sized to temper cold outside air before it can freeze the chilled-water coil behind it. In a cold climate that preheat coil is freeze protection as much as comfort, because a chilled-water coil that sees subfreezing air with no flow will split a tube, and a split coil floods the unit and the room below it.
The maintenance line on coils is short and it matters: clean the coil. A coil fouled with dirt that slipped past worn filters loses capacity row by row, and from the zone it reads as an undersized unit or a plant that cannot keep up. Comb the fins straight, wash the coil per the manufacturer's method, and check the air side and the water side both before you blame the chiller.
The condensate pan, trap, and drain
Under the cooling coil sits a drain pan that catches the water the coil pulls out of the air, and a drain line that carries it away. The pan has to slope to the outlet so it drains dry, and the line needs a trap, because the pan sits in an airstream that is either below atmospheric pressure on a draw-through unit or above it on a blow-through unit, and without a trap that pressure either holds the water in or blows it out.
The trap is sized to the unit's static pressure, and it is the single most-mangled part of an AHU. On a draw-through unit the fan pulls a negative pressure on the pan, so the trap has to be deep enough to hold a column of water taller than that negative pressure, or the unit sucks air back up the drain and the pan never empties. Get the trap depth wrong and the pan overflows even though the drain line is clear.
A clogged pan floods. Algae and biofilm grow in the standing water, the drain plugs, the pan fills, and it spills into the unit and through the ceiling. Half the water-damage calls on an AHU are a condensate pan nobody serviced. Pour water in the pan and watch it leave, clear the trap, check the slope, and confirm the overflow switch trips before the water finds the drywall.
The supply fan and motor
The fan is what moves the air, and it sits in its own section with a motor that drives it either through belts and sheaves or directly on a common shaft. The fan has to develop enough pressure to push the design airflow through the filters, coils, and the entire duct system, so it is sized to a flow and a pressure together, not to flow alone.
Fan type is a real choice with real consequences. Forward-curved wheels move a lot of air at low pressure and low cost, but they are less efficient and lose ground at higher static. Backward-inclined and airfoil wheels are more efficient and handle higher pressure, with airfoil the most efficient and the quietest of the centrifugal family. Many modern units use a plenum fan, a direct-drive wheel with no scroll housing that sits in the fan section and discharges into the cabinet, which suits variable-volume work well.
More units now run a fan array, also called a fan wall: several smaller direct-drive plenum fans in a grid instead of one large fan. The array gives redundancy, since one fan dropping out does not stop the unit, and it shortens the cabinet. On belt-drive units the belts and bearings are the recurring service item, and a slipping or worn belt loses airflow quietly. On a variable-volume unit the fan rides a VFD that ramps speed to hold duct static, which is both the efficiency story and the way the unit follows the VAV boxes downstream. See the VFD material by topic.
What is the difference between draw-through and blow-through?
In a draw-through AHU the fan sits after the coils and pulls air through the mixing box, filters, and coils ahead of it. In a blow-through AHU the fan sits before the coils and pushes air through them. The sections are the same; the fan's position relative to the coil is the whole difference, and it changes the pressure in the coil section and the way the supply air temperature behaves.
Draw-through is the common arrangement, and for good reason. With the fan downstream the air is pulled evenly across the full face of the coil, so the coil sees uniform airflow and performs to its rating. The fan also adds a little heat to the air, and on a draw-through unit that heat lands after the cooling coil where it barely matters. The coil section runs at negative pressure, which is why the condensate trap has to be deep.
Blow-through puts the fan first, which means the fan heat hits the air before the cooling coil and the coil has to remove it. It can also throw air unevenly onto the coil face unless the cabinet is built to spread it, which hurts coil performance. Blow-through earns its place where layout demands it or where a downstream component needs the fan ahead of it. Know which one you are standing in front of, because it tells you the pressure in the coil section and how to set the condensate trap.
The fan and external static pressure
The fan has to overcome the resistance of everything the air passes through, and the part outside the unit, the duct, the diffusers, the dampers, is the external static pressure, or ESP. Add the internal resistance of the filters and coils and you have the total static the fan develops. A fan does not produce one airflow; it produces a curve of flow against pressure, and where that curve meets the system's resistance curve is the operating point.
This is why a fan that hit design airflow on the test stand can fall short in the building. The duct has more resistance than the design assumed, the operating point slides up the fan curve to a higher pressure and a lower flow, and the zones starve. Dirty filters and a fouled coil push the internal static up and do the same thing from inside the unit.
On a variable-volume system the fan rides a VFD that modulates speed to hold a duct static-pressure setpoint as the VAV boxes open and close. Set that static setpoint too high and you waste fan energy and make noise all day; too low and the far boxes cannot get their air. See the static-pressure material by topic for measuring ESP and reading the fan curve.
The humidifier section
Some AHUs carry a humidifier section to add moisture to dry supply air, common in winter and in spaces with a humidity requirement like museums, labs, and printing. It usually sits after the heating coil, because warm air holds the added moisture without it condensing back out, and it injects steam or atomized water into the airstream through a manifold or a bank of nozzles.
The detail that bites is absorption distance. The moisture needs a length of duct downstream to fully absorb before it hits a turn, a coil, or a filter, and if that distance is short the water lands on metal and pools, which grows biology and corrodes the cabinet. A humidifier crammed too close to a downstream component wets it instead of the air.
Not every unit has one, and where humidity is not a requirement, it is one less thing to maintain. Where it is required, treat the humidifier and its controls as part of the unit's job, not an add-on. See the humidification material by topic.
The energy recovery section
Where a unit brings in a lot of outside air, an energy recovery section preconditions that air using the energy in the building's exhaust before it reaches the coils. The common device is an energy recovery wheel, a slowly rotating wheel that sits between the outside-air and exhaust streams and carries heat, and on an enthalpy wheel moisture too, from one to the other. In summer it cools and dries the incoming air with the building's cool exhaust; in winter it warms it.
The payoff is real load reduction. The coils see air that is already part way to setpoint, so the chiller and boiler do less work, and on a high outside-air unit the recovery section can cut the outside-air conditioning load substantially. A device that transfers only heat is a heat-recovery wheel or an HRV; one that transfers heat and humidity is an enthalpy wheel or an ERV.
Maintenance is the wheel's seals, its drive belt, and keeping the media clean. A wheel that has stopped turning, from a snapped belt or a seized bearing, looks fine from outside while the coils quietly carry the full outside-air load again. See the energy-recovery material by topic.
The casing, cabinet, and access
The cabinet holds it all together and keeps the conditioned air in and the weather and noise out. A good commercial AHU uses a double-wall, insulated panel: two skins of metal with insulation between them, which holds temperature, deadens noise, and gives a wipe-clean inner surface instead of exposed fiberglass shedding into the airstream. The insulation also stops the cold cabinet from sweating in a humid mechanical room.
Built-up and modular units ship as sections that bolt together on site, which is how an AHU gets large enough to handle the air a building needs. Each section is its own box with its own access door, and the joints between them have to seal, because a leaky section joint pulls unconditioned room air into the unit on the negative-pressure side and loses conditioned air on the positive side.
Access is not a luxury on an AHU. Every wet section needs a drainable floor that slopes to a drain, and every section needs a door big enough to pull a filter, comb a coil, or change a belt. A unit you cannot get into is a unit that does not get maintained, and the components that do not get serviced are exactly the ones that fail.
Vibration isolation and sound attenuation
The fan is a spinning mass, and left rigid it sends vibration into the cabinet, the duct, and the building structure as noise and wear. The fan and motor sit on spring or rubber isolators sized to the assembly's weight and speed, and a flexible connector joins the fan discharge to the duct so the spinning fan does not telegraph straight into the sheet metal. A flex connector pulled tight or a collapsed isolator passes the vibration right through.
Sound is its own problem on an AHU, because the fan makes noise and the duct carries it to the occupied space. Sound attenuators, lined sections of duct or packaged silencers, sit in the supply and sometimes the return to absorb fan noise before it reaches the zones. They cost static pressure, so they are part of the fan's pressure budget, not free.
The field tell is a unit that got loud after a repair. A flex connector replaced with something rigid, an isolator bolted down solid during a fan swap, and the whole building hears the fan. Check that the fan still floats on its isolators after any work in that section.
The controls and the sequence
The AHU runs on a sequence of operation, usually executed by the building automation system, that decides what every section does moment to moment. The main jobs are holding the supply air temperature with the coil valves, holding the duct static pressure with the fan VFD, swinging the economizer dampers when the outside air is favorable, and keeping the outside-air flow at or above the ventilation minimum. See the building management system material by topic for the controls layer.
These loops interact, which is where commissioning earns its keep. The economizer and the cooling coil both cool, so the sequence has to hand off between them cleanly instead of fighting, opening the dampers for free cooling first and only adding mechanical cooling when the free air runs out. A sequence that lets both run at once burns energy and chases its own tail.
Most AHU problems that read as equipment problems are really the sequence. A unit that short-cycles, hunts, or never reaches setpoint usually has a control loop tuned wrong or a sensor reading wrong, not a broken coil. Trust the sensor only after you have checked it against a real reading.
Supply air temperature and reset
The supply air temperature, the temperature of the air leaving the unit and heading to the zones, is the number the AHU is built to control. On a system feeding VAV boxes the unit holds a steady cold supply, often near the mid-50s in degrees F, and the boxes throttle that air to each space. The controls modulate the cooling coil valve to hold whatever leaving-air setpoint the sequence calls for.
Holding a fixed cold setpoint all the time is simple but wasteful, so many sequences use supply air temperature reset. When the zones are not calling for much cooling, the sequence lets the supply temperature drift up, which lightens the coil load and, on a chilled-water plant, lets the chilled water run warmer and the plant run more efficiently. The reset is bounded so the supply never rises so far that it cannot dehumidify or cannot satisfy the worst zone.
The catch with reset is humidity. Push the supply temperature up too far in a humid climate and the coil stops pulling enough moisture, and the building gets cold-and-clammy complaints even while the thermostat reads on setpoint. The reset logic has to respect dehumidification, not just the dry-bulb zones.
The AHU serving a VAV system
On a variable-air-volume system the AHU and the VAV terminals are one machine working together. The unit makes a steady stream of cold supply air at a controlled temperature and pressure, and the VAV boxes out at the zones open and close their dampers to take only the air each space needs. As the boxes throttle down, the duct static would rise, so the fan VFD slows to hold the static setpoint, and the unit's airflow falls to match the real demand.
Reheat lives at the boxes, not the unit. A zone that has throttled to its minimum airflow for ventilation but still overshoots cold gets its air reheated at the terminal box, by a hot-water or electric coil, so the central unit keeps making one cold supply and the zones tune temperature locally. See the VAV box commissioning guide for setting the box minimums, maximums, and the reheat sequence.
This is why the AHU and the boxes have to be commissioned as a system. A unit holding the wrong static setpoint starves the far boxes; boxes calibrated wrong make the unit chase a load that is not real. Neither side runs right alone.
Outside air and ventilation
The outside-air damper does more than feed the economizer. It is also how the building gets its required fresh air. Every occupied space needs a minimum outside-air flow for indoor air quality, and that minimum comes from ASHRAE Standard 62.1 and the project ventilation design, which set how much fresh air the spaces need based on occupancy and floor area. The AHU has to deliver at least that minimum whenever the building is occupied, economizer or not.
Holding the minimum is harder than it sounds on a variable-volume system. As the supply fan slows with falling load, the same damper position lets in less outside air, so the unit can drop below its ventilation minimum without the dampers ever moving. Good sequences measure or calculate the actual outside-air flow and hold it, rather than parking the damper at a fixed position and hoping.
The blunt version: a damper stuck at minimum, or a minimum set too low to save energy, undersupplies fresh air and the building gets stuffy and the carbon dioxide climbs. Confirm the unit actually delivers the design outside-air flow, do not assume a damper position equals a flow. The adopted code and the project mechanical design control the required number.
The AHU preventive maintenance
Most of what kills an AHU's performance is maintenance that did not happen, and the list is short and predictable. Filters loaded past their pressure drop choke the airflow. Coils fouled with the dirt that got past the filters lose capacity. A condensate pan and trap nobody serviced floods. Belts slip and bearings dry out. Damper linkages seize. None of it is exotic, and all of it is on a schedule the unit will not remind you about.
Work the sections in order. Change or clean the filters on pressure drop, not just the calendar, and fix whatever let the coil get dirty. Wash the coils per the manufacturer's method and comb the fins. Flush the condensate pan, clear the trap, confirm the slope and the overflow switch. Check belt tension and sheave alignment, grease the bearings on schedule, and cycle the dampers to confirm they fully stroke. Verify the actuators against the actual blade position, because position feedback lies.
The pattern across all of it: small neglect upstream shows up as a big symptom downstream. A skipped filter change becomes a fouled coil becomes a comfort complaint becomes a chiller someone wants to replace. Catch it at the filter.
Testing and balancing the unit
Test and balance, TAB, is where the unit's design airflow becomes a measured fact instead of a drawing. The TAB technician sets the supply fan to deliver the design airflow against the real system static, balances the air among the zones so each gets its share, and verifies the outside-air flow at the unit. On a variable-volume unit that includes confirming the fan tracks its static setpoint across the range, not just at full flow.
Balancing starts by finding the resistance the design ignored. A duct system always has more leakage and more fittings than the drawing assumed, so the fan's real operating point is rarely the design point until someone measures it and adjusts. The honest TAB report says what the unit actually delivers, the dampers actually set, and the outside-air flow actually measured, not what the schedule hoped for.
Skip a real TAB and the unit runs wrong quietly for years. The zones fight, the energy is high, and nobody can say whether the air is even meeting the ventilation minimum. See the air-balancing material by topic for the procedure and the tolerances.
The data-center CRAH as an air handler
A computer room air handler, a CRAH, is a chilled-water AHU built for a data hall. It is the same idea, a fan pulling hot return air across a chilled-water coil and pushing cold supply back out, sized and packaged for the rack environment instead of an office. The hot aisle returns warm air, the coil cools it, and the fan delivers it under the floor or to the cold aisle.
Two things set a CRAH apart from a comfort AHU. It usually runs closed-loop on the room air with little or no outside air, because the goal is tight, clean control of temperature and humidity rather than ventilation, and it is sized for sensible cooling, removing heat without much dehumidification, since the racks add heat but not moisture. A CRAH differs from a CRAC, which uses its own DX refrigerant coil instead of chilled water.
The components are familiar: fans, often on VFDs and arranged in arrays, a chilled-water coil, filters, and a controls layer holding supply temperature. Once you see the CRAH as a chilled-water air handler for the data hall, the rest of this guide applies. See the data-center cooling material by topic for the thermal guidelines that drive the setpoints.
Selecting and sizing an AHU
An AHU is selected on airflow, coil capacity, and static pressure together, against the application. The airflow in CFM comes from the building load and the supply temperature the design will hold. The coil capacity has to meet the sensible and latent load at the design conditions. The fan has to develop the total static, internal resistance plus external duct resistance, at that airflow.
The trade in selection is face velocity. Push too much air through too small a coil face and the velocity climbs, the static rises, and on a wet cooling coil the airstream starts carrying water droplets off the coil, called carryover, which wets the section downstream. Slow the face velocity and the cabinet grows. The selection software balances coil rows, fin spacing, face area, and fan against the load and the space available, and the manufacturer's certified data is what governs the final numbers.
Size for the real load and the real duct, not the rule of thumb. An oversized unit short-cycles and dehumidifies poorly; an undersized one never catches up on the design day. The application, the load calculation, and the manufacturer's selection control the call, and the certified ratings carry the AHRI and AMCA performance basis behind the catalog numbers.
What to document
An AHU handed over without a record is a unit nobody can troubleshoot later. Capture what each section is, what it does, and what its maintenance is, so the next technician is not reverse-engineering the unit from the nameplate.
At minimum, record the unit's design airflow and the measured airflow from TAB, the design and measured external static, the supply air temperature setpoint and any reset schedule, the outside-air minimum and how it is held, the coil types and their design conditions, the fan type and drive, the filter sizes and MERV, and the condensate trap configuration. The table below is the field version: section, function, and the maintenance that keeps it working.
| Section | Function | Maintenance |
|---|---|---|
| Mixing box / dampers | Mix return and outside air; economizer | Cycle dampers, confirm full stroke, check actuators |
| Filters | Clean air before the coils | Change on pressure drop, not calendar alone |
| Cooling coil | Cool and dehumidify the air | Wash coil, comb fins, check air and water side |
| Heating / preheat coil | Heat air; temper cold OA, freeze protection | Check flow and valve, verify freeze protection |
| Condensate pan / trap | Carry off the water the coil removes | Flush pan, clear trap, confirm slope and overflow switch |
| Supply fan / motor | Move air against system static | Belt tension, bearings, isolators, VFD tracking |
| Energy recovery wheel | Precondition outside air from exhaust | Drive belt, seals, keep media clean |
| Controls / sequence | Hold SAT, static, economizer, OA minimum | Verify sensors against real readings, retune loops |
Common mistakes
- Letting dirty filters and a fouled coil kill capacity, then blaming the chiller or calling the unit undersized.
- Ignoring the condensate pan and trap until standing water floods the unit and the ceiling below.
- Setting the condensate trap depth wrong for the section's static, so the pan never drains on a draw-through unit.
- Leaving the economizer dampers stuck, so the building runs mechanical cooling on days it should cool for free.
- Neglecting belts, bearings, and isolators until the fan loses airflow or shakes the building.
- Treating a damper position as a flow, so the unit quietly drops below its ventilation minimum at low fan speed.
- Pushing supply air temperature reset too far in a humid climate and creating cold, clammy complaints.
- Ignoring external static pressure, so a fan that hit design on the stand starves the far zones in the building.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The ventilation side comes from ASHRAE Standard 62.1, which sets the minimum outside-air rates for acceptable indoor air quality by occupancy and floor area; the project mechanical design applies it to the unit. Energy provisions, including economizer requirements, supply air temperature reset, and fan power limits, come from ASHRAE Standard 90.1 and the adopted energy code, which vary by jurisdiction and edition.
Fan and unit performance ratings trace to AMCA for fan air performance and sound, and to AHRI for certified coil and unit ratings, which is the basis behind the manufacturer's catalog numbers. Duct construction and air leakage classes come from SMACNA. Test and balance procedures follow the TAB standards from NEBB or AABC and the project specification.
Cite the standard that controls the point, and let the manufacturer's certified data and the project documents govern the actual numbers. Standards are adopted and amended by jurisdiction, so confirm the edition the project is held to and any local amendments before you treat any figure as fixed. The numbers in this guide are typical practice, not a substitute for the design or the equipment data.
Units, terms, and conversions
An air handler crosses a few unit systems, so the same quantity reads differently across a drawing set, a submittal, and a controls screen.
Airflow is in cubic feet per minute (CFM) on US drawings and liters per second or cubic meters per hour in metric documents. Static pressure is in inches of water column (in. w.c. or in. wg) in the US and pascals (Pa) in metric, where 1 in. w.c. is about 249 Pa. Filter efficiency is MERV in North America and a separate ISO 16890 rating internationally. Temperature is degrees F or degrees C, and capacity is in tons or Btu/h in the US and kilowatts in metric.
- AHU
- Air handling unit, the indoor central air handler that conditions and moves air through duct
- ESP
- External static pressure, the duct-side resistance the fan must overcome, in in. w.c. or Pa
- MERV
- Minimum Efficiency Reporting Value, the filter efficiency rating; higher catches finer particles
- Draw-through / blow-through
- Fan after the coil (draw-through, common) or before it (blow-through)
- Economizer
- The damper control that cools the building on cool outside air instead of the chiller
- SAT
- Supply air temperature, the conditioned air leaving the unit, often reset with load
- CRAH
- Computer room air handler, a chilled-water AHU built for a data hall
FAQ
What is an air handling unit?
An air handling unit (AHU) is the indoor central air handler that conditions and moves air through a building's ductwork. It mixes return and outside air, filters it, heats or cools it across coils, and a fan pushes it to the zones. A chilled-water AHU uses a central plant; a DX air handler carries its own refrigerant coil.
What is the difference between an AHU and an RTU?
An AHU is the indoor central air handler, often built up from sections that bolt together, serving a building through ductwork. An RTU is a packaged rooftop unit with cooling, heating, and fans in one weatherproof cabinet on a roof curb. The RTU is essentially the all-in-one outdoor version of the same air handler.
What are the parts of an air handler?
An air handler's main sections, in airflow order, are the mixing box with outside, return, and relief dampers, the filter section, the heating and cooling coils with a condensate pan under the cooling coil, and the supply fan and motor. Larger units add energy recovery, humidification, and sound attenuation, all in an insulated cabinet.
What is the difference between draw-through and blow-through?
In a draw-through AHU the fan sits after the coils and pulls air through them; in a blow-through AHU the fan sits before the coils and pushes air through. Draw-through is more common because it spreads airflow evenly across the coil face and puts the fan heat after the cooling coil. The coil section runs at negative pressure.
Why is my AHU not cooling enough?
Most AHU capacity loss is a dirty filter or a fouled coil before it is a plant problem. Loaded filters choke airflow; a coil packed with dirt loses capacity row by row. Check filter pressure drop, wash the coil, confirm chilled-water flow and valve operation, and verify the airflow against design before blaming the chiller.
Why does the condensate pan on my air handler keep overflowing?
A pan overflows when the drain or trap is clogged, the trap depth is wrong for the unit's static, or the pan does not slope to the drain. On a draw-through unit the negative pressure pulls air up a too-shallow trap and the water cannot leave. Clear the trap, check the slope, and size the trap to the fan static.
What does the economizer on an AHU do?
The economizer is the damper control that cools the building on cool, dry outside air instead of running the chiller. When conditions are favorable, the controls open the outside-air damper and close the return damper for free cooling. A stuck damper or failed actuator quietly leaves the unit on minimum outside air with no free cooling.
What is a CRAH and how is it different from a CRAC?
A CRAH is a computer room air handler, a chilled-water AHU built for a data hall, pulling hot return air across a chilled-water coil and pushing cold supply to the racks. A CRAC uses its own DX refrigerant coil instead of chilled water. Both run closed-loop on room air and are sized for sensible cooling, not ventilation.
How often should AHU filters be changed?
Change AHU filters on measured pressure drop across the filter bank, not the calendar alone. As a filter loads, its pressure drop rises and the fan loses airflow or burns energy holding it. Read the differential-pressure gauge and change at the manufacturer's final pressure drop. A clogged filter looks like a coil problem from the zone.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.