HVAC fan array and fan wall systems field guide

What a fan array is, and why it replaced the single big fan

A fan array is a grid of small, direct-drive plenum fans mounted in a wall inside an air handler, doing the job that one large belt-driven housed fan used to do alone. Picture a bulkhead across the cabinet with six or nine or sixteen identical fans set in it, each with its own motor, all pushing in parallel. That bulkhead is where the name fan wall comes from. The air handling unit field guide covers the rest of the box. This guide is about the fan section, and how it changed.

For decades the supply fan in a big air handler was a single centrifugal wheel in a scroll housing, driven by a motor through belts and sheaves. It worked, and it still does in plenty of units. The problem is everything that single fan represents: one wheel, one shaft, one set of bearings, one belt drive, and one point of failure. When it goes down, the unit goes down. It needs a tall cabinet to fit the housing and its inlet, and it dumps air out of a scroll in a way that is anything but even across the coil behind it.

The array breaks the one big fan into many small ones and gets four things in return: redundancy, because the others keep running when one quits; part-load efficiency, because you can turn fans off instead of throttling one; a shorter cabinet, because the small plenum fans fit in less depth; and a flatter velocity profile into the coil. None of that is free, and the array does not fit every application, but on large units those four wins are why it took over.

The single housed fan versus the array

The traditional fan is one large centrifugal wheel in a scroll, belt-driven off a motor sitting beside it. The array is a set of small, unhoused plenum fans, each direct-coupled to its own motor, sharing the cabinet as a common plenum. Same total airflow, very different machine.

The single fan has fewer parts to buy and one motor to wire, and it can develop high static pressure that a wall of small fans cannot match. That is its real edge and it still matters on high-pressure systems. Against that, it is all-or-nothing: lose the fan, the belt, or the motor and the unit is dead until someone fixes it. It runs at one point of efficiency, so at part load you throttle the whole wheel down a curve and pay for the inefficiency. And it needs a deep cabinet plus clean inlet and outlet conditions to perform anywhere near its rated curve.

The array trades the single high-pressure wheel for many low-pressure ones and wins on the things that drive operating cost and uptime in a typical commercial unit. You get redundancy built in, you get to stage fans for the load, you get a short cabinet, and you get even discharge. The reason the array took over so many AHU selections is not that it is better at everything. It is that it is better at the things a building actually pays for over twenty years of running.

The built-in redundancy: one fan down, the unit stays up

Redundancy is the array's biggest selling point. With a single fan, a failed belt or a seized bearing takes the whole unit offline, and a building that depends on that air handler is now scrambling for a temporary fix. With an array, one fan quitting is a non-event for the occupants. The controller sees the loss, the surviving fans ramp up to make up the airflow, and the unit holds its supply while a technician schedules the repair.

That is the difference between an emergency and a work order. On a single-fan unit the clock starts the moment it fails, because the space starts drifting off setpoint right away. On an array sized with a spare fan, you can lose one at 2 a.m. and deal with it during the next maintenance window, because the others covered the load.

The catch is that redundancy only exists if two things are true. The array has to be sized with at least one extra fan, the N+1 idea covered below, and the failed fan has to be sealed off so air does not just recirculate backward through it. Skip either and you bought the hardware for redundancy without getting the benefit. The backdraft damper section is where that second condition gets its own treatment, because it is the detail most often missed.

The part-load efficiency that pays the bill

Most air handlers run at part load most of the time. Design airflow is what you size for, but the building rarely calls for it, so the real energy story is what the fan does at 60 or 70 percent. This is where the array pulls ahead. Each small fan can be picked to sit near its best efficiency point, and at low load the array turns fans off and runs the remaining ones near that point instead of throttling everything down an inefficient part of the curve.

The fan laws are the reason this matters, and the fan laws field guide works through the math. Shaft power tracks the cube of speed, so slowing fans saves power fast, and turning a fan off entirely takes its share of the power to zero rather than dragging it along at poor efficiency. Run four fans hard or eight fans easy to move the same air and the eight-fan case can land in a better spot on the curve. Staging plus speed control is what turns that into real kilowatt-hours saved.

Performance numbers here belong to the manufacturer's selection software and the fan curves, not to a rule of thumb. The array's part-load advantage is real, but how much you save depends on the fan model, the motor type, the system curve, and how the staging is set up. Confirm the projected energy with the manufacturer's selection at the actual operating points, not a brochure headline.

Direct-drive: no belts, no sheaves, less to maintain

Every fan in the array is direct-coupled to its motor. No belts, no sheaves, no belt-driven shaft bearings to grease and align. That removes a whole category of maintenance and a whole category of failure. The single belt-driven fan needs belt tension checked, belts replaced as they wear and shed dust onto the coil, sheaves kept aligned, and bearings lubricated on a schedule. Miss any of it and you lose airflow quietly or grind a bearing loudly.

Direct-drive fans run on either electronically commutated (EC) motors or induction motors on a variable frequency drive. EC motors carry their speed control on board and run efficiently across a wide turndown, which is why they show up so often in arrays. Either way, the maintenance picture is the same: no belt dust on the coil, no realignment, and the motor bearings are the main wear item. The AHU maintenance routine gets shorter when the belt drive disappears from it.

Belt slip is also a hidden airflow thief on the single-fan unit. A loose belt lets the wheel turn slower than the motor, so the unit moves less air than the drive ratio says it should, and nobody notices until a balance check finds the registers low. Direct-drive removes that gap entirely. The fan turns at the speed the motor turns, full stop.

The footprint: a fan wall fits in a short cabinet

Small plenum fans need far less depth than a large housed fan with its scroll and inlet clearance, so the fan section of an array AHU is shorter. On a built-up unit that buys floor space in a tight mechanical room. On a packaged or modular AHU it can shave a whole section off the length of the box.

That shorter footprint is a big part of why arrays win retrofits. When an old single-fan unit dies and the mechanical room was built around it, there is rarely room to slide in a new housed fan and its inlet box. A fan wall assembles inside the existing cabinet footprint and sometimes inside the existing cabinet itself, because the small fans go in through the access door and bolt to a bulkhead rather than needing the unit opened up for a wheel and shaft.

The depth saving is real but it is not the headline. A building owner does not replace a working fan to gain a foot of mechanical room. The footprint matters most when it makes a retrofit physically possible that a housed fan could not, or when it lets the AHU fit a space the design otherwise could not serve.

Even airflow across the coil and filters

A single fan discharges out of a scroll, so the air leaves with a strong velocity on one side and dead spots on the other. Whatever is downstream, the coil, the filters, the next coil, sees that uneven profile and performs to it. The array spreads many small fans across the whole cross-section, so the velocity leaving the fan wall is far more uniform across the face of the coil behind it.

Uniform face velocity matters more than it sounds. A coil fed unevenly has air racing through part of the face and crawling through the rest, which hurts heat transfer and can drive carryover off a wet cooling coil where the velocity is high. Filters loaded unevenly load up fast in the high-velocity band and barely work in the dead zone, so you change them sooner and clean less air in between. Even the air the array delivers gives the coil and the filter bank the flat profile they were rated against.

On a blow-through unit, where the fan sits ahead of the coil, the even discharge is a direct gain because the array is feeding the coil. On a draw-through unit the fan pulls air across the coil from behind, so the benefit shifts to the inlet side, but the principle holds: many small fans pulling evenly beat one big fan pulling hard through one corner. Draw-through and blow-through are covered in the AHU field guide.

Reduced fan system effect

System effect is the performance you lose when a fan's inlet or outlet sees disturbed airflow instead of the clean conditions the rating assumed. A housed fan crammed into a cabinet with a tight inlet, a close turn, or an obstruction in the discharge never makes its rated curve, and the gap, the system effect, is easy to underestimate and hard to fix after the unit is built.

The array softens this in two ways. Each small plenum fan draws from the open cabinet plenum, so its inlet conditions are cleaner and more consistent than a big wheel fighting a cramped inlet box. And because the fans discharge across the full cross-section into whatever is downstream, the outlet is not forced through one constrained path. The result is a fan section that behaves closer to its rated performance in the real cabinet.

This is not a free pass. The array still has to be applied within the manufacturer's guidance on cabinet dimensions, fan spacing, and clearance to the coil, and a fan wall jammed too close to a downstream component will develop its own system effect. AMCA publishes the fan performance and system-effect framework that underlies all of this, and the manufacturer's selection accounts for the specific cabinet. The point is that the array starts from a better position than a housed fan in the same box, not that it cannot be installed badly.

How the array is controlled

The fans in an array run off a common speed signal so they act as one fan to the building automation system. The BAS asks for a duct static pressure or an airflow, a controller works out the speed and the number of fans needed, and every running fan gets the same speed command. To the rest of the control sequence the wall of fans looks like a single variable-speed supply fan.

Two layers sit under that. Speed control sets how hard the running fans work, by an EC motor's 0 to 10 V input or a VFD's frequency reference. Staging sets how many fans run, turning fans on as the load climbs and off as it falls. A good array controller blends the two so the running fans stay near their efficient speed while the stage count handles the big swings in load.

Tie the array to the BAS the way you would any variable-volume supply fan: a duct static pressure setpoint with static pressure reset where the sequence allows it, and a hard high-static safety to protect the ductwork. The reset and the safety logic are no different because it is an array. What is different is that the controller also has to decide the stage count, so the commissioning has to verify both the speed loop and the staging behave, not just one.

VFD and EC motors on the array

The array's part-load efficiency only shows up when the fans actually slow down, so variable speed is part of the package. The two ways to get it are EC motors, which build the variable-speed electronics into the motor, and induction motors driven by a VFD. Both let the fans run at the speed the load needs instead of full speed all the time.

EC motors are common in arrays because each fan carries its own drive, the turndown is wide and efficient, and you skip the separate VFDs and their wiring. The trade is that the electronics live on every motor out in the airstream, so a failure is a motor swap rather than a drive swap at a panel. VFD-driven arrays put the drives in a panel where they are easy to service, sometimes one drive feeding a group of fans, but then a single drive failure can take a group of fans down at once, which works against the redundancy. How the drives are grouped matters to how much redundancy you actually have.

Either way, do not run an array at fixed full speed. A constant-volume array on across-the-line motors throws away the part-load efficiency that justified the array in the first place, and it gives up the smooth turndown that makes staging clean. If the application is truly constant volume, the economics of the array get a lot harder to defend.

Staging fans for the load

Staging is the array trick the single fan cannot do. At low load, instead of running every fan slowly down an inefficient part of its curve, the controller turns some fans off and runs the rest closer to their best efficiency point. Fewer fans, working at a better spot, moving the same reduced airflow.

There is a band where this is a real win and a band where it is not. Down near minimum airflow the math favors fewer fans run harder. Near design airflow you need all the fans anyway. In between, a good staging sequence has hysteresis so fans do not chatter on and off around a threshold, and it spreads runtime across the fans so they wear evenly instead of always cycling the same units.

Staging also interacts with redundancy, and that is where it gets missed. If the control is set up all-or-nothing, every fan running together at one speed, you get none of the staging efficiency and you may also lose the clean redundancy response. The staging logic and the redundancy logic have to be designed together so a failed fan triggers a ramp-up while a low load triggers a stage-down, and the controller never confuses the two.

The backdraft damper on every fan, and why it is not optional

Here is the install detail that decides whether an array works: a fan that is off, whether it failed or was staged down, is an open hole in the fan wall. The running fans pressurize the cabinet downstream, and that higher pressure will push air backward through the idle fan, spinning it in reverse and short-circuiting flow right back to the inlet. You lose the airflow you were trying to deliver and the surviving fans fight their own recirculation.

The fix is a backdraft damper on each fan, a gravity or actuated damper that opens when the fan runs and closes when it stops. When a fan goes down or stages off, its damper shuts and seals that opening so the running fans see a closed wall, not a leak. Without it, the redundancy you paid for evaporates: a failed fan does not just stop helping, it actively bleeds the array. With it, the array isolates the dead fan and the others carry the load the way the selection promised.

Backdraft dampers are favored over fixed blank-off plates because they are automatic. A blank-off plate seals a fan you have removed for service, but it cannot respond when a running fan fails on its own, and it cannot follow the staging. The dampers do cost a little static pressure, since the running fans pull through the damper frames and blades, so that loss belongs in the fan selection. Low-leakage backdraft dampers with blade and edge seals keep the penalty small while still sealing the idle fans. If an array is specified without backdraft dampers, that is a red flag to raise before it is built, not after the redundancy fails to show up in the commissioning test.

Sizing the array: N+1 and capacity with one fan down

Redundancy is a sizing decision, not a feature that comes for free with the array. An N+1 array carries one more fan than the design airflow strictly needs, so that with any single fan out, the remaining fans can ramp up and still hit design. Some critical jobs go further, N+2 or more, but N+1 is the common target where uptime matters.

The sizing question is what airflow the array makes with one fan failed and its damper closed. The surviving fans speed up to cover the gap, which costs power and pushes them up their curves, so the spare has to be real headroom, not a number on paper. If the array was sized so all the fans run near maximum at design, there is nothing left to ramp into when one drops, and N+1 is a label without the capacity behind it.

Match the redundancy to what the space needs. A general office AHU may not justify a spare fan. A hospital, a lab, a clean process, or a data hall usually does, because the cost of losing airflow dwarfs the cost of one extra fan and its damper. Decide the N+1 versus straight-N question on the consequence of an outage, and size the fans so the survivors can actually carry the load when one is gone.

The array's sound is different, not just quieter

An array does not simply make less noise than a single fan. It makes a different noise. Many small fans turning at their own speed shift the sound to a different spectrum than one large wheel, often with the energy higher up where it attenuates more easily in duct and lining, and without the strong low-frequency blade-pass tone a big fan can put into the structure.

Whether that reads as quieter depends on the space and the path. The smaller fans can be smoother to live with, but a wall of fans also has its own discharge turbulence, and backdraft dampers add some noise of their own as air works past the blades. The honest answer is that the acoustic result is a manufacturer selection number, tied to the specific fans, speeds, and cabinet, not a blanket claim that arrays are quiet.

For a noise-sensitive job, get the sound power data from the manufacturer at the actual operating points and run it through the acoustic analysis like any other fan. The array's spectrum is usually friendlier, but design it on the numbers, not on the reputation.

Serviceability: swap one fan without shutting the unit down

The maintenance story is where the array quietly earns its keep. The fans are modular. A failed fan comes out as a unit, motor and wheel together, often through the cabinet access door, and a replacement bolts back to the same opening. There is no wheel to pull off a shaft, no bearings to press, no belts to set.

Better, on a properly sized array you can often do that swap while the unit keeps running. The failed fan's backdraft damper is already closed sealing it off, the other fans are carrying the load, and a technician isolates the dead fan electrically and changes it while the building stays conditioned. Try that on a single-fan unit and you are shutting the air handler down and finding temporary cooling for whatever it serves.

The flip side is that there are more motors to track. An array has many small motors instead of one big one, so the maintenance program counts and trends them, watching for the fan that is drawing more current or running warmer before it fails. That is a different rhythm than one annual belt-and-bearing check, but it is lighter work, and a single fan dying is no longer the event it was.

Commissioning and TAB: prove the airflow and the redundancy

Test and balance on an array starts the same as any AHU: traverse the airflow, confirm the unit makes design CFM against the real system static, and set the duct static pressure setpoint. The array adds two things to verify that a single fan never had. First, the staging: walk the load up and down and confirm fans stage on and off where they should, without chattering around the thresholds, and that the running fans hold the setpoint through each transition.

Second, and this is the one crews skip, test the redundancy. Drop a fan, by command or by pulling its power, and confirm three things happen: its backdraft damper closes, the surviving fans ramp up, and the unit holds design airflow with one fan down. That test is the only proof that the N+1 you paid for actually exists. A balance report that says the array hit design CFM with every fan running has not tested the thing that justified the array.

The TAB standards from AABC and NEBB give the airflow measurement and balancing procedures, and AMCA underlies the fan performance side. The redundancy and staging verification usually live in the commissioning plan and the sequence of operations rather than the balance report, so make sure someone owns it. If the spec does not call out a one-fan-down test, add it, because it is the difference between assuming redundancy and knowing you have it.

Retrofitting an array into an existing AHU

Replacing a tired single fan with an array is one of the array's strongest uses. The old housed fan, its motor, and the belt drive come out, and a fan wall goes in where they were. Because the small fans assemble inside the cabinet, the retrofit often fits the existing footprint and sometimes the existing cabinet, which is exactly the case where dropping in another housed fan would be a fight against the mechanical room.

The retrofit buys the building everything the array offers, applied to a unit that already exists: redundancy where there was none, part-load efficiency from EC or VFD fans where the old fan ran flat out, and the end of belt maintenance. For an aging air handler with a worn fan and a high energy bill, the array retrofit is often a better answer than a new fan of the old kind.

The work is not trivial, though. The structure inside the cabinet has to carry the fan wall bulkhead, the electrical has to feed many motors or a VFD instead of one starter, the controls have to gain staging logic, and the backdraft dampers and their effect on static pressure have to be in the new selection. Treat it as a re-engineered fan section, sized by the manufacturer to the existing cabinet and ducts, not a like-for-like swap.

Data-center and critical AHUs: fan walls for CRAH units

Data centers leaned into fan arrays hard, and for good reason. A data hall cannot tolerate an air handler dropping offline, and it runs at part load and varying load constantly as the IT load shifts, so the array's redundancy and part-load efficiency line up exactly with what the room needs. Computer room air handler (CRAH) units and fan-wall units serving data halls are built around walls of EC plenum fans for this reason.

The pattern is usually a fan wall paired with a cooling coil wall, pulling warm air from the hot side, pushing it through the coil, and pressurizing the supply path under EC control. N+1 at the fan level means a single fan failure is automatically covered while the room never notices, and the even airflow helps feed the coil and the containment evenly. ASHRAE TC 9.9 sets the thermal envelope these rooms are held to, and the Uptime Institute tier framework is where the redundancy expectations come from.

Critical AHUs outside data centers, in hospitals, labs, and clean processes, use arrays for the same logic. Where the cost of an airflow outage is high and the load varies, the array's two headline benefits, no single point of failure and efficient turndown, are worth the extra fans and controls.

Where fan arrays fit

Arrays earn their place on large air handlers where redundancy, efficiency, or footprint actually drives the decision. Big commercial and institutional AHUs, central units serving variable-air-volume systems, hospitals and labs, and data-center CRAH and fan-wall units are the heartland. The common thread is size and consequence: enough airflow that the part-load efficiency adds up to real money, and enough riding on the unit that losing it is a problem worth spending to avoid.

They fit best where the load varies, because the staging and variable speed pay off most when the unit spends its life below design. A unit that runs at a steady flow around the clock gets less of the efficiency benefit, though it can still want the redundancy. And they fit retrofits well, where the short footprint solves a physical problem a housed fan cannot.

Where they fit poorly is the small unit and the high-pressure unit. A small AHU does not move enough air to justify the controls and the extra motors, and the single fan is cheaper and simpler. A high-static system can run past what the small plenum fans can develop. Those limits get their own section, because applying an array outside its range is the most expensive way to learn where the range ends.

The economics: more hardware up front, paid back in energy and uptime

An array costs more in hardware than a single fan for the same airflow. You are buying many motors instead of one, the controls to stage and speed them, and a backdraft damper on every fan. On the purchase order, the single housed fan usually wins.

The case for the array is in operating cost and risk, not first cost. The part-load efficiency cuts the energy bill every hour the unit runs below design, which is most hours, and over a unit's life that can swamp the higher purchase price. The redundancy turns a fan failure from an emergency outage into a scheduled work order, and on a building where downtime has a price, that avoided outage is real money the spreadsheet often leaves out. The end of belt maintenance trims the labor line too.

So the honest framing is that the array is more expensive to buy and usually cheaper to own, and how much cheaper depends on how many hours it runs at part load and what an outage would cost. Run the energy at the real operating points from the manufacturer's selection, put a number on the avoided downtime, and compare that against the hardware premium. On a large, variable, consequential unit the array usually pencils out. On a small steady one it often does not.

The limits: where a single fan still wins

The small fans in an array each develop modest pressure. Gang enough of them and you get the airflow, but the static pressure the wall can develop is bounded by what the individual fans can make, and that ceiling is lower than a big single centrifugal wheel can reach. On a high-static system, a long high-resistance duct run, heavy filtration, deep coils, or high external pressure, the array can run out of pressure where a housed fan would not.

That is the clearest place a single fan still wins. When the system needs high static pressure more than it needs redundancy or turndown, the large wheel is the right tool, and forcing an array onto a high-pressure system is asking the small fans to do something they were not built for. The manufacturer's selection will tell you, but you should walk in knowing the array has a pressure ceiling.

The other limits are practical. The array adds control complexity, more motors to maintain and trend, and the backdraft dampers as parts that have to work. On a small unit none of that is worth it. And a constant-volume application at fixed speed strips out the part-load efficiency that justifies the array, leaving you paying for hardware you are not using. Know the limit before the selection, not after the unit underperforms.

Choosing array versus single fan

The decision comes down to three questions, weighed against the system's static pressure. Does the application need redundancy, the no-single-point-of-failure that keeps the unit up when a fan dies? Does it run enough hours at part load that staging and variable speed save real energy? Is the cabinet or the mechanical room tight enough that a short fan section solves a real problem?

Answer yes to the redundancy or efficiency questions on a large unit and the array is usually the call. Answer yes to footprint on a retrofit and it often is too. But hold all of that against the static pressure: if the system needs high pressure beyond what the small fans develop, the single housed fan wins regardless of the other answers, because an array that cannot make the pressure does not make the airflow either.

For a small, low-pressure, constant-volume unit where downtime is not costly, the single fan is simpler and cheaper and there is no reason to complicate it. The array is not a default and it is not a status symbol. It is the right answer for large, variable, redundancy-sensitive units within its pressure range, and the wrong answer outside that. Let the manufacturer's selection confirm the fans can hit the airflow and the static at the operating points before the choice is final.

What to document

An array has more to record than a single fan, and the record is what the next technician needs when a fan fails or the unit drifts. Capture the array configuration, how it is controlled, and the redundancy proof, so the redundancy is a documented fact rather than an assumption.

Write down the number and size of fans and whether they are EC or VFD-driven, the design and one-fan-down airflow, the staging sequence and thresholds, the duct static setpoint and reset, the backdraft damper type, and the result of the redundancy test. If it was a retrofit, record the original fan it replaced and the new static pressure the selection is built around. The person who opens the cabinet in five years should be able to see how the array is supposed to behave before they touch it.

Common mistakes

• Specifying or installing the array with no backdraft dampers, so air recirculates backward through any off or failed fan.
• Controlling the array all-or-nothing at one speed, throwing away both the staging efficiency and the clean redundancy response.
• Calling it N+1 without sizing real headroom, so the survivors cannot reach design airflow with one fan down.
• Forcing an array onto a high-static system past what the small fans can develop, so it never makes the pressure or the airflow.
• Skipping the one-fan-down redundancy test at commissioning and assuming the redundancy is there.
• Treating the array like a single fan in maintenance, ignoring that many small motors need counting and trending.
• Running a constant-volume array at fixed full speed, paying for hardware whose efficiency benefit you never use.

Field checklist

Standards and references

AMCA is the body behind fan performance and the system-effect framework, so fan curves, ratings, and the way inlet and outlet conditions degrade performance trace back to AMCA methods. Hedge any specific performance, airflow, pressure, sound, or efficiency, to the manufacturer's certified selection at the actual operating points and the AMCA ratings, not to a rule of thumb. Array numbers belong to the selection software for the specific fans and cabinet.

On the air handler itself, ASHRAE and AHRI are the references for AHU performance and rating, and ASHRAE 90.1 carries the fan power and energy limits the design has to meet. For data-center work, ASHRAE TC 9.9 sets the thermal guidelines and the Uptime Institute tier framework drives the redundancy expectations that make N+1 arrays the norm.

Test and balance follows the AABC and NEBB procedures for measuring airflow and setting the system. The staging and redundancy verification usually live in the commissioning plan and the sequence of operations rather than the TAB report, so confirm who owns the one-fan-down test. Across all of it, the manufacturer's selection and the project specification govern the actual numbers, and the adopted code edition and local amendments control where energy limits apply.

Units, terms, and synonyms

The array goes by a few names and shares its vocabulary with the rest of the air-side world, so the same idea reads differently across a submittal, a controls drawing, and a balance report.

A fan array is also called a fan wall or a fan-array fan section, and the individual fans are direct-drive plenum fans or unhoused plenum fans. Airflow is in cubic feet per minute (CFM) or, in metric documents, cubic meters per hour or liters per second. Pressure is external static pressure or duct static, in inches of water column (in. w.c. or in. wg) or pascals (Pa). Redundancy is written N+1, one fan beyond the design need so a single failure still meets full airflow; some specs instead state it as N-1, meaning the array must still carry the load with one fan out.

Fan array / fan wall

A grid of small direct-drive plenum fans in an AHU bulkhead, replacing one large housed fan

Plenum fan

An unhoused centrifugal fan that discharges into the cabinet plenum rather than a scroll

Direct-drive

Motor coupled straight to the fan wheel, no belts or sheaves

EC motor

Electronically commutated motor with on-board variable-speed control, efficient across turndown

N+1 redundancy

One fan more than the design airflow needs, so any single failure still meets design

Backdraft damper

A damper on each fan that closes when it stops, sealing off the idle or failed fan

Staging

Turning fans on and off with the load so the running fans stay near best efficiency

System effect

Performance lost when inlet or outlet conditions differ from the fan's rating basis

Related tools

Calculators and readiness checks for this work

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