HVAC System 101: What It Is, How It Works

What is an HVAC System?

HVAC stands for heating, ventilation and air conditioning. An HVAC system refers to the equipment and processes that work together to heat, cool and regulate the air quality in commercial and residential properties. The system moves heat in or out of the house and also cleans the indoor air.

HVAC systems comprises of the equipment, technology and processes that cool and heat a house as well as regulate the quality of the indoor air. An HVAC system therefore facilitates heat exchange between the outside and inside air while also bringing in and cleaning fresh air, and expelling stale air.


Most people can’t tell the difference between air conditioning and HVAC and even tend to use the 2 terms interchangeably. But what is the difference between air conditioning and HVAC?

HVAC stands for heating, ventilation and cooling. Air conditioning is therefore only covers 1 of the 3 functions of an HVAC system. An AC will only cool your home while an HVAC system will cool, heat and regulate the quality of air inside the house (ventilation).

The purpose of an HVAC system is to cool the house during the summer and heat it during winter (or any other time when air conditioning and heating may be needed).  HVAC systems also regulate and maintain the quality of the indoor air by bringing in fresh air, lowering humidity, filtering out impurities and expelling stale air.

I have also had folks ask me “What is the difference between AHU and HVAC”? So, are they the same thing or what’s the difference between the two?

HVAC is short for heating, ventilation and air conditioning. An air handling unit (AHU) is just one part of an HVAC system (ventilation). An AHU receives indoor and outdoor air where the air is filtered to remove impurities, the humidity and temperature are controlled before being released to the house.

The following are the basic components of an HVAC system:

  • Evaporator coil
  • Condenser coil
  • Compressor
  • Thermostat
  • Refrigerant
  • Furnace
  • Blower motor
  • Air filter
  • Ductwork
  • Exhaust fans
  • Air outlet
  • Air intake
  • Return air system
  • Humidifiers/dehumidifier

How Does and HVAC System Work


AS I have mentioned above, an HVAC has 3 main functions. These are:

  • Heating
  • Ventilation
  • Air conditioning/cooling

Let us now separately look at how HVAC systems achieve these functions:

1. How an HVAC System Works in Air Conditioning/Cooling

Air conditioning is basically the process of cooling a room or a house by removing heat from the indoor air. It involves the transfer of heat from the indoor spaces to the air outside the house.

In air conditioning, air is not transferred between the inside and outside of the house. It is the heat in the air that is extracted and released to the outside air.

Air conditioning is made possible by a chemical known as a refrigerant. A refrigerant (most people call it Freon although Freon has now been phased out) is a fluid with a low boiling point and high latent heat of vaporization.

Refrigerants can easily vaporize due to their low boiling points and when they do so, they absorb lots of heat from the surrounding, thanks to the high latent heat of vaporization.

There are many types of air conditioners but all of them work the same way. The principle behind how they operate is the same.

To cool the whole house, most homeowners go with either central air (ducted) or ductless air conditioners. Packaged units are another alternative. For small spaces, a portable or window air conditioner is preferred.

For the purpose of explanation in this post, I will use central air conditioners as a reference.

An air conditioner has 4 main components:

  • Evaporator coil
  • Condenser coil
  • Compressor
  • Expansion valve

Evaporator Coil


An evaporator coil is also known as the indoor AC unit. It is basically a copper tube coiled round a frame (like an A-frame) where heat exchange between the indoor air and the refrigerant takes place.

The refrigerant enters the evaporator coil as a very cold liquid. At that time, the indoor air would be hot making heat transfer between the 2 mediums possible.

A fan pulls warm air from the house (through return air ducts) and passes it through filters to remove impurities that may be present. The air is then forced to pass across the evaporator coil.

Heat in the indoor air is absorbed by the refrigerant and that is how the indoor air is cooled. The cooled air is then circulated back to the house through supply air ducts.

After absorbing heat from the indoor air, the refrigerant will evaporate (turn from liquid to gas). This refrigerant gas is then ejected out of the evaporator coil.


From the evaporator coil, the refrigerant gas enters the compressor. A compressor is basically a pump which compresses the gas and in the process increase its pressure.

But why is a compressor necessary? Why do we need to compress the refrigerant?

The function of the outside unit (which contains the compressor and condenser coil) is to release the gas absorbed by the refrigerant so that it (the refrigerant) can go back inside the house for more cooling.

The problem however is that for the refrigerant to release the heat to the surrounding, the temperature of the outside air has to be lower than that of the refrigerant. Since that is usually during summer, the outside air would be hot as well making heat transfer impossible.

The good thing is that when you compress a gas, it is not only its pressure that increases but its temperature as well.


After compression of the refrigerant, it enters the condenser coil as a high-pressure high-temperature gas. Due to the temperature differential between the refrigerant and the surrounding air, heat transfer can now take place.

A fan installed at the top of the condenser unit blows cooler air over the coil which helps to increase the rate of heat exchange between the refrigerant and the surrounding.

The air absorbs heat from the refrigerant and takes it away with it. By the time the refrigerant is exiting out of the condenser coil, it will have lost so much heat that it will have turned back to its liquid state (condensation)

Expansion Valve

An expansion valve is a small valve located just before the inlet of the evaporator coil. It has 2 functions.

Firstly, the valve controls the amount of refrigerant entering the evaporator. It ensures that only enough refrigerant enters the coil.

When too much refrigerant enters the coil, not all of it will vaporize meaning that some of it in liquid form will enter the compressor. Since a compressor should only compress gases, liquids inside it will damage it.

Secondly, the valve reduces the pressure of the refrigerant before it enters the refrigerant. You see, although the refrigerant is now a liquid, it is not as hot as it needs to be.

By lowering its pressure, its temperature will also automatically be reduced. The refrigerant then enters the evaporator coil and the cycle goes on and on.

2. How an HVAC System Works in Heating a House

There are 2 ways to heat a house in an HVAC system:

  • Using a heat pump
  • Using a furnace

Heat Pump

A heat pump looks exactly like a central air conditioner and even works like one to cool the house during the hot months of summer.

However, unlike an air conditioner which is paired with a furnace for heating purposes, a heat pump cools and heats a house. It is a great HVAC systems for areas that receive moderate winters for reasons I will explain later.

Credit: Open University

A heat pump is able to heat the house by extracting heat from the surrounding air. Even when it is evidently cold outside, the air still contains enough thermal energy which the heat pump can extract and use it to heat the house.

Heat pumps are able to heat the house because of the presence of a device known as the reversing valve. The reversing valve reverses the direction of flow of the refrigerant so that instead of flowing from the evaporator to the compressor and lastly to the condenser, the refrigerant now flows from the evaporator to the condenser and then to the compressor.

But how does reversing the direction of flow of the refrigerant help in extraction of heat from the outside air.

When a heat pump is used to heat the house, the condenser coil works like an evaporator coil while the evaporator coil works like a condenser.

To start the process, the refrigerant flowing from the evaporator to the condenser passes through an expansion valve where its pressure and temperature are reduced to way below that of the surrounding air.

By so doing, a temperature differential will have been established between the refrigerant and the surrounding air.

As the refrigerant goes through the condenser coil, it absorbs heat from the surrounding air. The condenser fan makes that process even quicker.

After absorbing enough heat from the air, the refrigerant vaporizes and enters the compressor as a low pressure gas.

Inside the compressor, the refrigerant is compressed to increase its pressure. As is the case when you compress a gas, its temperature also increases.

The refrigerant therefore leaves the compressor and enters the evaporator coil as a high-pressure superheated gas. Inside the house, the evaporator fan will pull cold air from the house and pass it across the coil.

Due to the difference in temperature, the indoor air will absorb heat from the refrigerant. And that is how a heat pump heats a house.

After losing heat to the indoor air, the refrigerant will condense back to the liquid state and the cycle will be repeated over and over again until the house is properly heated.

Heat pumps will however not extract 100% of your heat needs when the temperatures outside falls below 35/40 degrees. For that reason, heat pumps have a secondary source of heat, usually in the form of electrical resistance heating.

When the secondary heat source kicks in, you will see the auxiliary heat displayed on your thermostat. That is why this units are not a good fit for areas that receive extreme winters.

Running the heat pump on auxiliary heat is more expensive than running a natural gas-run furnace in many states in the United States.

The secondary heat source is also important should the heat pump get damaged or fail completely. In that case, you would need to manually activate emergency heating on your thermostat.



Houses that have standard air conditioners use a furnace to heat the house during the months of winter. The furnace runs on natural gas, propane or even oil.

The main components in a furnace are

  • Thermostat
  • Gas valves
  • Burners
  • Heat exchanger
  • Blower
  • Ductwork

When the temperature inside the house drops below the set level, the thermostat sends a signal to the furnace to come on. It starts by pulling indoor air via the return ducts.

Before the air is heated, it is first passed through filters where all the suspended impurities are removed.

Heating starts when an electronic ignition starts the burner component inside the combustion chamber. The flames from the burner component then heat a metal heat exchanger.

The heat generated circulates through the looped tubes of the heat exchanger. As the air moves across the tubes, it absorbs heat from the heat exchanger and that is how it gets heated.

To move the heated air to where it is needed, a blower pushes it out of the furnace and into the supply ducts. From the supply ducts the heated air is delivered to every corner of the house.

3. How an HVAC System Works in Ventilation


Cooling and heating a house is not enough. You need to also ventilate it. But what exactly is ventilation?

In simple terms, ventilation is a process where clean outdoor air is purposely supplied to a space while also removing stale air from the same space. Ventilation can be natural or mechanical.

Natural ventilation for example is when you open doors or windows to a room and allow exchange of air between the inside and the outside. On the other handle, mechanical ventilation is where equipment are used to bring in fresh air and remove the stale air from a space.

There are lots of activities that take place inside a house that makes the air inside stale. This activities including breathing, cleaning, showering among others.

After such activities, the indoor air will have suspended impurities, high humidity and also have low levels of oxygen. That is why ventilation is vital.

Apart from improving the quality of indoor air, ventilation also preserves the house and the equipment/appliances inside. Without proper ventilation, mold is very likely to grow and the low air quality can even make you ill.

While opening doors and windows is very effective, it is very expensive to cool or heat a house whose doors or windows are open. The HVAC system will be forced to work hard to keep cooling the incoming hot or cold air.

For that reason, ventilation is incorporated in heating and cooling systems to form an HVAC system

In mechanical ventilation, clean outdoor air is pushed inside the house through ducts and released to the different areas of the house. Stale air is then pulled from the house and exhausted through a stack.

To reduce heating costs, the air can also be cleaned and recirculated. You may also needed other ventilation types like PVC vents or chimney flues to remove excess heat from an HVAC’s system furnace.

If ventilation is crucial for your, a central air HVAC system would be the best to install. Ventilation in ductless HVAC systems is not as good as that in central air systems and you may need a technician to help you with an alternative ventilation method.

Check out this post for more differences between central and ductless HVAC systems

 HVAC Zoning

Zoning in HVAC refers to the practice of dividing the different areas of a house into zones based on their cooling and heating needs.

Most homeowners deal with cases where some parts of their houses are hot than other “hot spots” and other that are cooler than other “cold spots”. If you have a storied house, you will always find that the upper floors are always warmer than those below.

Zoning in HVAC is designed to deal with those problem. Apart from that, zoning can help increase your comfort levels since you can heat or cool different areas of the house independent of the others.

For instance you can set the system such that the living areas of the house are colder than say the bedrooms.

To achieve zoning in HVAC, each of the intended zone is fitted with a thermostat while the ductwork to that zone is also fitted with a moderating damper.

Once a thermostat cools for cooling or heating, the damper to that zone opens while all the others close. That allows all the cool air or heat from the air handler to be directed to that specific zone until it is sufficiently air conditioned.

Once all zones are taken care of, the system turns off. As a result, zoning in HVAC also helps to improve the efficiency of the system.

Apart from the above scenario which happens in central air, ductless air conditioning and heating is another way to create zones in an HVAC system.

In a ductless HVAC system, each room has its own inside unit but all are connected to the same outside unit. Each inside unit can be set to its own temperature thereby creating built-in zoning.

Another type of zoning in HVAC systems is to have 2 separate set of HVAC systems. For more information on HVAC zoning systems check out this post.


And basically that is everything about how HVAC systems work. I hope you enjoyed reading this guide.