What is an Air Handler? How Does It Work?


An air handler is a key component of an HVAC (Heating, Ventilation, and Air Conditioning) system responsible for circulating conditioned air throughout a building. It contains a blower fan, an evaporator coil, and a filter. Here’s a brief summary of what an air handler is and how it works:

1. Function:

  • An air handler’s primary function is to distribute conditioned air. It works in conjunction with other HVAC components, such as a heat pump or furnace, to circulate heated or cooled air into the living spaces of a building.

2. Components:

  • The key components of an air handler include:
    • Blower Fan: The blower fan draws in air from the return ducts and pushes it over the evaporator coil, creating airflow.
    • Evaporator Coil: This coil cools or heats the air, depending on the system’s mode. In cooling mode, it removes heat and humidity from the air. In heating mode, it adds heat to the air.
    • Filter: The air handler typically has an air filter that captures dust, allergens, and pollutants, improving indoor air quality.

3. Air Circulation:

  • Air handlers draw in untreated air from the return ducts, pass it over the evaporator coil (where heating or cooling occurs), and then distribute the conditioned air through the supply ducts into various rooms and spaces within the building.

4. Cooling and Heating:

  • The air handler can work in both cooling and heating modes, depending on the HVAC system it’s a part of. In cooling mode, it helps remove heat and humidity from indoor air, while in heating mode, it assists in raising the air temperature.

5. Filtration:

  • Air handlers can improve indoor air quality by filtering out airborne particles like dust, pollen, and allergens as air passes through the system. Regular filter maintenance is crucial to maintain efficient operation.

6. Fan Speed Control:

  • Many air handlers have variable-speed blower fans that can adjust their speed to match the heating or cooling demand, improving energy efficiency and comfort.

7. Location:

  • Air handlers are typically installed indoors, often in utility rooms, basements, attics, or closets. They are connected to the HVAC ductwork that distributes conditioned air throughout the building.

Components of an Air Handler

Air handler for industrial HVAC systems
Air handler for industrial HVAC systems

Depending on who you ask, some will tell you that an air handler does not heat or cool air but rather it just moves air. Other will say that it does everything: move air, heat, cool and also help in ventilation.

The truth of the matter however is that an air handler cannot work on its own. It has to be connected to an air conditioner or a heat pump.

Depending on the time of the year, the air handler’s blower will force the indoor air across an evaporator coil (for cooling) or electric heat strips (for heating). It will then push the air conditioned or heated air through the ductwork to be supplied to the house.

Let us now look at the components of an air handler which allow it to work as it does.

1. Evaporator Coil

Both heat pumps and air conditioners need an evaporator coil. This is the coil where heat exchange happens inside the house.

When it is hot outside and the air needs to be cooled, a very cold refrigerant is pushed through the evaporator coil. The air handler’s blower pulls hot air from the house through return vents and ductwork and forces it to pass across the coil.

The refrigerant absorbs heat from the indoor air and that is how it is cooled. Again, the blower fan pushes the cooled air inside the supply air ducts which is then distributed indoors.

If the air handler is connected to a heat pump, the direction of flow of the refrigerant is reversed when it is time to heat the indoor air. A superheated refrigerant is thus supplied to the evaporator coil.

The blower pulls cold indoor air and moves it across the coil. The cold air absorbs heat from the refrigerant and that is how it is heated before being supplied to the house.

2. Blower Motor

The air handler’s blower is the one responsible for moving air between the indoor spaces, the heat exchangers as well as the supply and return air ducts.  Depending on the available technology when the air handler was made, there are 3 types of blower motors:

  • Single-speed
  • Multi-speed
  • Variable-speed

As their name implies, single-speed air handler blower motors have a single fixed speed. When the thermostat calls for heating or cooling, they operate at 100% until proper heating is achieved then they will turn off.

The disadvantage of single-speed blower motors is that they are not energy-efficient. They operate at 100% even when such is not necessary.

And that is why multi-speed blower motors were invented. Multi-speed blower motors can operate at 100% or slower speeds as the heating and cooling needs maybe.

When there is a high demand for heating or cooling, the motor will operate at full speed (100%). However, when needed to adjust humidity levels or minimally increase or reduce temperature, the motor will run at a slower speed.

You can say that a variable-speed blower motor is an improved version of the multi-speed motor. The motor precisely varies the speed of the fan to make sure that only the required heat or air is delivered.

As you can tell, variable-speed motors are very energy-efficient but there is already a new kid on the block. Electrically commutated motors (ECMs) are a three-phase DC motors developed by GE for even higher efficiency.

3. Electric Heat Strips

If your air handler is connected to an air conditioner, it means that there will be no furnace (gas furnace) in the house. Heating will therefore be in the form of electric resistance heating.

This electric strips are heated until they turn red so that when the blower moves cold air across them, the air will absorb heat from the strips and get heated in the process.

Although heat pumps can extract heat from the outside air even during winter, they will be unable to extract 100% of the heat needed when temperatures dip below 35/40 degrees. And that is where the resistance heating come on. Check out this post for more information on that.

Auxiliary heating will kick in on your thermostat. This heating will stay on until the temperatures outside improves.

4. Air Filter

There are lots of suspended particles in your house like dust, lint, pollen among others. An air handler is designed to remove them before heating or cooling and later supplying the cooled air or heat.

When the blower pulls air from the house, it is first passed through a filter where all these impurities are removed. As a result, the air supplied in the house is not only fresh but clean.

5. Supply and Return Air Plenum

An HVAC system has a supply and return air plenum at the air handler/furnace connections. Basically, a plenum in HVAC systems is a box that connects the ductwork to the air handler.

A supply air plenum connects the air handler to the supply air ducts which supplied cooled air or heat to the house, while a return air plenum connects the air handler to the return air ducts which brings in hot or cold air to the air handler.

How an Air Handler Works


As I had mentioned earlier, an air handler can either be connected to a heat pump or an air conditioner. When used to cool the house, an air conditioner and an AC work exactly the same way.

Both ACs and heat pumps have outside units known as condenser units which also look exactly the same.  The 3 main components of central air and heat pumps are:

  • Compressor
  • Condenser coil
  • Evaporator coil

The compressor and condenser coil forms the outside unit while the evaporator coil is located inside the house (in the air handler). Let us now look at how a heat pump works when the house is being cooled or heated.

1. During Cooling/Air Conditioning

When you are using your HVAC system for cooling, a refrigerant (Freon, Puron etc.) enters the evaporator coil inside the house as a very cold liquid. At that point, the air handler’s blower pulls hot indoor air from the house via the return air ducts.

The hot air is blown over the cold coil. As you would expect, the cold refrigerant absorbs heat from the air and that is how cooling happens. The cooled air is then supplied to the house through the supply air ducts.

Apart from pulling and releasing air from the house, an air handler is also responsible for ventilation. Ventilation is basically the process of replacing stale indoor air with fresh outdoor air.

Therefore, not all of the return air from the house enters the air handler. The ductwork branches off and some of the air is exhausted out of the house.

To bring in fresh air from outside, the air handler is also connected to a vent that supplies it with outdoor air.

Therefore, an air handler receives air from both inside and outside the house and after mixes it before cooling it and supplying it to the house.

Note: The reason the air handler does not draw 100% of air from outside is that the incoming air is way hotter than the return air and it would therefore be too expensive to heat it. The system therefore has to mix the indoor air (slightly cooler) and outdoor air to save on cooling costs.

2. During Heating

Although I said that an air handler can be connected to an air conditioner, it is rarely done. Most air conditioners are paired up with furnaces.

Why is that though? Why is it better to connect an air handler to a heat pump but not an air handler?

The reason air conditioners are normally not used with air handlers is because air handler’s use electricity for heating (electrical resistance heating). It is way more expensive to heat a house using electricity than a natural gas-powered furnace.

Unless the cost of electricity falls dramatically and that of natural gas keeps going up, air conditioners will continue being paired with gas furnaces.

Heat pumps however work very well air handlers. As I mentioned earlier, a heat pump will in most of the time extract heat from the outside air. Electrical resistance heating will only be for auxiliary heating.

When used for heating, a reversing valve reverses the direction of flow of the refrigerant. As such, the refrigerant at temperatures lower than the outside air enters the condenser coil outside the house.

Due to the difference in temperature, the refrigerant absorbs heat from the outside air and afterwards evaporates and enters the compressor. The compressor then compresses it increasing its pressure and temperature.

A high pressure superheated refrigerant gas enters the evaporator coil inside the air handler unit. The blower fan then brings in cold air from the house and passes it through the coil where the air absorbs heat from the refrigerant.

When the air is heated, it is supplied back to the house and that is how a heat pump heats a house.

Just like in heating, the air handler will also receive fresh air from outside for ventilation purposes. Since the air will be too cold, it will be mixed with some of the return air from the house to lower heating costs.

What is the Difference between an Air Handler and an AC?

I choose to tackle this concern towards the end to give you a chance to first understand what an air handler is and how it works. 

To start with, when people talk about air conditioners in relation with air handlers, they usually mean central air conditioners. However, as you already know there are other types of air conditioners like:

  • Ductless air conditioners
  • Window air conditioners
  • Portable air conditioners

Only central air conditioners and heat pumps work with air handlers since they are the only ducted types of air conditioners. A special type of air conditioners that also have air handlers is the packaged units.

In packaged units, all the components of the air conditioners including the air handler are contained in one big metallic cabinet and installed outside the house. The unit is then directly connected to the supply and return air ducts.

So, what is the difference between an air handler and an AC?

As we have seen, the function of an air handler is to move air between the cooling and heating coils, and the indoor spaces of a house as well as ventilation. Importantly, the air handler is located inside the house and paired with a heat pump or air conditioner.

On the other hand, the function of an air conditioner is to cool the indoor air by removing heat (using a refrigerant) and releasing it outside. An AC has an inside and outside unit and can be paired with a furnace or an air handler.

Air Handler vs Furnace

A furnace can also be considered to be an air handler. It receives cold air from both inside and outside the house and after heating it, its fan pushes it to the ductwork to be supplied throughout the house.

The main difference between air handlers and furnaces however is that air handlers are 100% electric while furnaces are powered by natural gas, propane or oil.

Secondly, a furnace has a heating coil only while an air handler contains a cooling and heating coil. A furnace is only connected to the air conditioner while an air handler can be connected to either a heat pump or an AC.

An air handler is always running (cooling and heating applications) while a furnace is usually off when the air conditioner is being used.

Wrap Up

And basically that is everything in as air handlers are concerned. I hope that this guide was helpful and that you enjoyed reading it.