There are two primary HVAC fan types: axial and centrifugal. They differ in how air passes through them, and that single difference determines everything else about their performance, application, and where you will find them.
Axial fans move air parallel to the shaft, like a propeller. They deliver high airflow at low pressure. Centrifugal fans draw air in along the axis and discharge it radially, at ninety degrees to the intake. They deliver lower airflow at higher pressure.
This guide covers how each works, which applications suit them, the centrifugal subtypes, and how to maintain both so they operate efficiently across a long service life.
Axial and Centrifugal Compared
| Characteristic | Axial | Centrifugal |
|---|---|---|
| Airflow direction | Parallel to the shaft | Enters axially, exits radially |
| Air volume | High | Moderate |
| Pressure developed | Low | High |
| Handles resistance | Poorly, output falls quickly | Well, holds output against resistance |
| Typical use | Open air movement, short distances | Ducted systems, long runs |
| Physical size | Compact for the airflow delivered | Larger housing required |
| Found in | Heat pumps, condensers, cooling towers, extract fans | Air handling units, ductwork, blowers |
The distinction that decides selection. An axial fan moves a lot of air but gives up quickly when it meets resistance. Put one at the end of a long duct run with filters and bends and its output collapses. A centrifugal fan moves less air but pushes it against resistance, which is why anything ducted uses one. Choosing between them is really a question about how much resistance the air has to overcome, not about how much air you need.
HVAC Fan Types: Axial
Axial fans have blades that rotate around a central axis, similar to a propeller, moving air parallel to the shaft. They suit applications requiring high airflow at low pressure, and they are found in cooling towers, exhaust and extract systems, condenser units, and small ventilation systems.
They are particularly effective at moving air over short distances with little obstruction, which is why they appear on the outside of equipment rather than inside ductwork.
Axial fans in heat pumps and air conditioners
Worth noting because it is the application most readers will have at home.
The fan on an outdoor heat pump or air conditioning unit is axial. Its job is to draw large volumes of ambient air across the heat exchanger so heat can be absorbed or rejected, and there is almost no resistance in that path, so an axial fan is exactly right.
Two design details matter in this application. A DC brushless motor allows the fan to vary its speed to match compressor duty rather than running flat out whenever the system operates, which reduces both energy consumption and noise. And low-noise blade profiles reduce the aerodynamic sound that dominates fan noise in domestic settings.
Legom heat pumps use low-noise axial fans with NIDEC or SiGe brushless DC motors, with the largest unit in the range using twin fans so each runs at lower speed than a single fan moving the same air volume would need to. Our guide to HVAC noise levels covers how fan design affects what you actually hear.
Axial fan maintenance
Isolate the power supply before starting. This is not a formality. A fan that starts unexpectedly while hands are near the blades causes serious injury, and some units restart automatically when a control signal arrives. Isolate at the breaker rather than relying on a switch.
Visual inspection. Check for dirt, dust, or debris around the blades and motor. Clean the blades with a soft brush or microfibre cloth, or a vacuum with a brush attachment. Take care not to bend the blades, since even slight deformation creates imbalance and vibration.
Motor and bearings. Check lubrication and apply oil according to the manufacturer’s recommendation where the design requires it. Many modern fans use sealed bearings that need no lubrication, and adding oil to those does nothing useful.
Airflow path. Ensure the intake and discharge are unobstructed. On an outdoor unit, clear leaves, debris, and encroaching vegetation, since restricted airflow reduces both output and efficiency measurably. Clean or replace the air filter where one is fitted.
Interval. A complete clean every three to six months, more frequently in dry or dusty environments.
HVAC Fan Types: Centrifugal
Centrifugal fans are found in ducted systems, air handling units, and blowers. Air enters along the fan axis, is caught by the rotating impeller, and is thrown outward by centrifugal force before leaving through a discharge at ninety degrees to the intake.
That change of direction is what produces the pressure. The scroll-shaped housing converts the velocity imparted by the impeller into static pressure, which is why centrifugal fans can push air along a long duct run past filters, bends, and dampers without their output collapsing.
This makes them well suited to long-distance air movement and systems with resistance, which describes essentially every ducted installation.
Centrifugal fan maintenance
Isolate the power supply first, as with any fan.
Blade cleaning. Use a vacuum or soft brush on the impeller blades. Take care not to damage or bend them, since incorrect handling affects performance directly. Centrifugal impellers are balanced at manufacture, and dirt accumulating unevenly on the blades causes imbalance and vibration on its own.
Motor inspection. Check for dust build-up on the motor. Use compressed air or a brush on the motor and electrical components. Never spray water or introduce moisture near electrical parts.
Housing inspection. Check the fan housing for dust and dirt accumulation and clean with a damp cloth, keeping moisture away from electrical components. Where the fan uses bearings requiring lubrication, apply according to the manufacturer’s recommendation.
Belt drives. Where a fan is belt-driven rather than direct-drive, check belt tension and condition. A loose belt reduces fan speed and therefore airflow, and a worn belt eventually fails without warning.
Interval. Inspection and cleaning every six to twelve months.
Centrifugal Fan Subtypes
Three common designs share the same general maintenance requirements but differ in performance and application.
Backward-curved
Blades curve backward relative to the direction of rotation, as shown in the diagram above. These are used in commercial and industrial HVAC systems where efficient air movement and higher pressure are required with lower power consumption.
Backward-curved impellers are the most efficient centrifugal type and are described as non-overloading, meaning power draw peaks and then falls as airflow increases rather than climbing continuously. That characteristic protects the motor from overload if system resistance turns out lower than designed, which is a practical advantage in installations where the ductwork may change.
Forward-curved
Blades curve forward in the direction of rotation. Used in smaller air handling units and residential systems, where they operate relatively quietly at moderate pressure.
Compared with backward-curved, forward-curved fans suit lower-pressure systems and are less efficient. They deliver more airflow for a given impeller diameter, which allows a smaller and cheaper unit, and this is why they dominate residential equipment where size and cost matter more than peak efficiency.
The trade-off worth knowing is that forward-curved fans are overloading: power draw rises continuously as airflow increases, so a fan operating with less resistance than designed can overload its motor.
Plug fans
A centrifugal impeller with the motor integrated into the fan wheel, mounted without a scroll housing. This reduces both space and noise, which is why plug fans appear in rooftop units, air handling units, and other applications where a compact arrangement matters.
Because there is no housing, the discharge is not directional, which suits installations where air is delivered into a plenum rather than a specific duct.
Which Fan Is in Your Equipment?
| Equipment | Fan type | Why |
|---|---|---|
| Heat pump outdoor unit | Axial | High airflow across the coil, no resistance |
| Air conditioner condenser | Axial | Same reason |
| Air handling unit | Centrifugal | Must push air through ductwork |
| Furnace blower | Centrifugal, usually forward-curved | Moderate pressure, compact, quiet |
| Fan coil unit | Centrifugal, often forward-curved | Compact with some resistance |
| Cooling tower | Axial | Very high volume, minimal resistance |
| Bathroom extract | Axial, or centrifugal on long ducts | Depends on duct length |
| Rooftop unit | Plug fan or centrifugal | Compact with pressure capability |
That last row is worth expanding. A bathroom extract fan on a short duct straight to an external wall works fine as an axial fan. The same axial fan on a five-metre duct with two bends will move very little air, which is why extract fans on long runs use centrifugal or mixed-flow designs. This is the most common fan selection error in domestic installations.
Signs a Fan Needs Attention
Reduced airflow with no other change usually means dirt accumulation on blades or a restricted intake. On a belt-driven fan it may also mean belt slip.
New vibration indicates imbalance, most often from uneven dirt build-up on the impeller, sometimes from a bent blade or worn bearing. Vibration is worth addressing promptly, since it accelerates bearing wear and transmits noise into the building structure.
Grinding or rumbling points to bearing wear. This is a mechanical fault that will worsen, and continuing to run the fan risks damage to the shaft and motor.
Higher energy consumption with unchanged operation suggests the fan is working harder against restriction, whether from a clogged filter, dirty coil, or blocked intake.
Increased noise generally is worth investigating rather than accepting. A fan that has become louder than when installed has a cause, and identifying it early is cheaper than replacing the motor.
Why Fan Maintenance Affects Efficiency
Fans are easy to neglect because they keep turning even when performing poorly, and the degradation is gradual enough to go unnoticed.
Dirt on blades changes their aerodynamic profile, so the fan moves less air for the same power. A restricted intake forces the motor to work harder for less result. Worn bearings increase friction and draw more current. None of these produces a fault code or an obvious failure; they simply mean the system consumes more electricity to deliver less, indefinitely.
On an outdoor heat pump unit specifically, restricted airflow also reduces heat transfer at the coil, which lowers the coefficient of performance directly. Clearing debris and vegetation from around the unit is one of the highest-value maintenance tasks available and costs nothing.
“The mistake I see in selection is choosing an axial fan because it moves more air on paper, then putting it on a duct run. Axial fans are excellent at moving large volumes with nothing in the way, and they give up almost immediately once they meet resistance. That is why every heat pump has an axial fan on the outside and every ducted system has a centrifugal one inside. The question to ask is not how much air you need but how much resistance that air has to get through, and the answer decides the fan type before you look at any product.”
— Maggie Shen, Director of Legom
Fans in Legom Equipment
Legom air source heat pumps use low-noise axial fans with brushless DC motors from NIDEC or SiGe, allowing speed to vary with compressor duty rather than running at fixed output. Noise ratings across the range run from ≤52 dB(A) on the 5 kW unit to ≤56 dB(A) on the 16 kW, which uses twin fans so each operates at lower speed.
We manufacture the complete hydronic system at our facility in Jiaxing, Zhejiang Province, including manifolds, floor heating pipe, thermal actuators, room thermostats, and HVAC valves, supplying partners in more than 90 countries. Contact the technical team to discuss specification.
Frequently Asked Questions
What is the difference between axial and centrifugal fans?
Airflow direction, and everything follows from it. An axial fan moves air parallel to its shaft, like a propeller, delivering high volume at low pressure. A centrifugal fan draws air in along the axis and discharges it radially at ninety degrees, converting velocity into static pressure. That gives it lower volume but the ability to push air against resistance. Axial fans suit open air movement; centrifugal fans suit ducted systems.
Which fan type is used in a heat pump?
Axial. The fan on an outdoor heat pump or air conditioner unit draws large volumes of ambient air across the heat exchanger, and there is almost no resistance in that path, so an axial fan is exactly suited to it. Quality units use brushless DC motors that vary speed with compressor duty rather than running flat out, which reduces both energy consumption and noise. Larger units may use twin fans so each runs at lower speed.
Why can’t I use an axial fan in ductwork?
Because axial fans lose output rapidly when they meet resistance. Ductwork with filters, bends, and dampers presents substantial resistance, and an axial fan that moves impressive volumes in open air will move very little air through it. This is the most common selection error in domestic extract installations: a fan that works fine on a short duct straight through a wall performs poorly on a five-metre run with bends.
What is the difference between backward and forward-curved fans?
Backward-curved impellers are more efficient and are non-overloading, meaning power draw peaks and then falls as airflow rises, which protects the motor if system resistance is lower than designed. They suit commercial and industrial systems needing higher pressure. Forward-curved impellers deliver more airflow for a given diameter, allowing smaller and cheaper units, which is why they dominate residential equipment. They are less efficient and are overloading, so power draw rises continuously with airflow.
How often should HVAC fans be cleaned?
Axial fans benefit from a complete clean every three to six months, more frequently in dry or dusty environments. Centrifugal fans in ducted systems generally need inspection and cleaning every six to twelve months. Always isolate the power supply at the breaker before starting, since some units restart automatically when a control signal arrives, and a fan starting unexpectedly with hands near the blades causes serious injury.
Why does a dirty fan reduce efficiency?
Dirt accumulating on blades changes their aerodynamic profile, so the fan moves less air for the same power input. A restricted intake forces the motor to work harder for less result, and worn bearings add friction and current draw. None of this produces a fault code, so the system simply consumes more electricity to deliver less, indefinitely. On a heat pump, restricted airflow also reduces heat transfer at the coil, lowering the coefficient of performance directly.
What causes fan vibration?
Most often imbalance from uneven dirt build-up on the impeller, since fans are balanced at manufacture and any uneven accumulation disturbs that. Other causes include a bent blade, frequently from careless cleaning, or a worn bearing. Vibration is worth addressing promptly because it accelerates bearing wear and transmits noise into the building structure, turning a cleaning job into a component replacement if left.
What is a plug fan?
A centrifugal impeller with the motor integrated into the fan wheel and no scroll housing around it. This reduces both space and noise, which is why plug fans appear in rooftop units, air handling units, and other compact applications. Because there is no housing, the discharge is not directional, so they suit installations delivering air into a plenum rather than into a specific duct.
Reviewed by Maggie Shen, Director at Legom, on September 15, 2026. This guide to HVAC fan types was reviewed for technical accuracy, including the radial discharge characteristic of centrifugal fans and the overloading behaviour of forward-curved impellers.