What is HVAC System? Meaning, Definition, Parts, Working, Applications

What is HVAC system?  A very common question. HVAC is a very common word now a day to make ourselves comfortable. The weather conditions are different based on the locations, seasons, etc. In a room, if there is heat-generating equipment, the temperature my very high even in the winter.

Now, how does HVAC  makes us comfortable as well as helps to control the temperature?

Let’s explore the details of the HVAC system!

What is HVAC system?

What does HVAC stand for?

HVAC is an abbreviation for heating, ventilation, and air conditioning. Here,

  • H – Heating
  • V – Ventilation
  • A – Air
  • C – Conditioning

HVAC includes:

  • Heating units, heat pump, furnace,
  • Ventilation
  • Comfort units, like split air conditioners, packaged air conditioners, chilled water system, etc
  • Filters, humidifiers, dehumidifiers,
  • Ductwork
  • Controls etc.

HVAC Meaning

HVAC is a system by which heating, or ventilation of air conditioning together or individually can be done.

This system can provide the required heating and cooling capacity to the residential and commercial buildings.

HVAC units are available in various capacities based on the applications.

HVAC Definition

HVAC is a system that is able to provide heating, or ventilation, or air conditioning based on the requirements.


Alfred Wolff made the first comfortable air conditioners and Willis Carrier first invented the first process air conditioners.

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Purpose of HVAC system


The use of heating or cooling to control environmental (room) space;

  • It helps to increase the temperature in the winter season
  • It helps to reduce the moisture content


The process of changing or replacing air in a space.  It is used to remove moisture, odors, heat, carbon dioxide, etc, and replace it with ‘fresh air’.  It is considered to be one of the most important factors for good indoor air quality. 

The design process would specify a number of ‘air changes’ within a room, depending on usage;

  • It filtrates and cleans the indoor air.
  • It includes oxygen in the air
  • It can restrict the dust ingression into the conditioned space.
  • It increases indoor air quality (IAQ)
  • Ventilation helps to replace or exchange air within a space and improve indoor air quality (IAQ).
  • It helps to reduce odors, smoke, heat, airborne bacteria, dust, carbon dioxide, and other gases

Air Conditioning

The supply air is ‘conditioned’ to control the temperature and humidity.   Humans like a room temperature of 21 – 24°C and relative humidity of 40 – 60%.  ‘Relative’ humidity is a measure of humidity compared to the maximum possible humidity.

  • It is used to move air between indoor and outdoor areas to meet the cooling or heating requirements.
  • It makes us comfortable in hot summer
  • It is able to make us comfortable in extreme winter too.
  • It reduces the humidity level when weather is high humid
  • It can reduce the humidity level down also, as required.

The main purpose in short,

The main purpose of the air conditioning system is as follows:

  • Heating,
  • Cooling
  • Humidifying and Dehumidifying
  • Control moisture content
  • Distribution of air
  • Filtration of air
  • Freshens air

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HVAC System Parts

There are so many components in the HVAC system, and it varies based on the type of system.

However, to understand a basic concept, a few components are listed:


  • Boilers
  • Hot water generator
  • Furnace
  • Heaters


  • Supply fan
  • Exhaust fans
  • Louvers

Air conditioning

  • Chillers
  • Chilled water pumps
  • Condenser water pumps
  • Cooling towers
  • Piping and valves
  • Air handling units (AHUs),
  • Sand trap louvers
  • Diffusers, grilles, dampers, etc.
  • Cooling coils
  • Heating coils
  • Fan Coil Units (FCUs)
  • Split air conditioners
  • Thermostats
  • HVAC Diffusers and grills
  • HVAC controls
  • Heat pump
  • Associated electricals and controls

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HVAC System Basics

Principle of HVAC

HVAC principle is based on three things

  • Thermodynamics – HVAC is related to the heat, work & energy, and their relationship.
  • Fluid Mechanics – In the water circuit, or refrigerant circuit, fluid mechanics are used to analyze.
  • Heat Transfer – Entire HVAC system is very closely related to the heat transfer. Heat is transferred from the conditioned space based on requirements.

Basic Idea

We will learn the basics of the HVAC system, briefly,


Heaters are necessary for many places to increase the temperature as required. In this process, steam or air or hot water or electric heating coils are used.

Heat can be transferred in three ways,

  • Convection
  • Conduction
  • Radiation


Ventilation is the process by which indoor contaminated or polluted air is replaced.

  • It removes odors, smoke, heat, dust, airborne bacteria, or carbon dioxide, and to replenish oxygen.
  • It is based on air changes per hour.
  • Nos of air change depending on the type of applications.
  • Methods for ventilating a building may be divided into forced and natural types.

Air conditioning

The air conditioning system is totally based on a refrigeration cycle consists of,

  • evaporator coil,
  • compressor
  • condensing coil,
  • expansion valve,
  • Refrigerant, which circulates within the closed circuits.

The basics of the refrigeration process, are as follows,

  • In the evaporator, refrigerant starts evaporating and it takes latent heat of evaporation from the adjacent area i.e. air.
  • A compressor is used after the evaporator. When the compressor starts, it creates a negative pressure and sucks the refrigerant vapor from the evaporator.
  • The compressor compresses the refrigerant vapor and increases the pressure and temperature
  • High pressure and high-temperature refrigerant vapor then go to the condenser and reduces the heat and become liquid.
  • The liquid refrigerant is then come to the evaporator again through an expansion valve.
  • In the expansion valve, the flow of refrigerant is controlled.

How Does an HVAC System Work?


Heating is one part of the HVAC system, In the cold area, as well as where humidity is very high, heaters are required to increase the temperature as well as to reduce the moisture level within acceptable limits.

Various kinds of heaters are used to generate heat in residential as well as commercial applications. The following can be used to produce heat,

  • Boilers,
  • Furnaces,
  • Heat pumps
  • Hydronic system
  • Solar system
  • Chimneys etc.
  • Electricity


The boiler is basically a pressure vessel. It has a combustion chamber to generate heat.

  • Fuel like coal, wood, waste oil, fuel oil, etc. are burned in a combustion chamber.
  • There is a working fluid normally water is used.
  • There is a piping system to transfer heat around the combustion chamber for heat transfer.
  • The heat from the combustion, heat the water.
  • In many boilers, heat is produced by electrical resistance elements.
  • Water becomes hot and used.
  • In many boilers, water is heated up to steam formation & steam is used.
  • Finally, water or steam released heat in the required space & return to the boiler through the piping system.

Boilers can be made of,

  • Cast-iron,
  • Carbon steel
  • Stainless steel
  • Aluminum
  • Copper etc.


In the furnace, there is a combustion chamber where fuel is burned.  Here, no secondary working fluid is required. Air is heated and it is transferred through a duct to the space to be conditioned.

  • The fan may be used to transfer air if required.
  • It is used in residential as well as commercial applications.
  • Natural gas, or oil or propane is used as fuel.
  • Electric elements can also be used.

The furnace consists of,

  • Casing
  • Heat exchangers;
  • Fuel or heat elements
  • Burners
  • Controls
  • Vent
  • Fan with motor
  • Filter etc

Heat pumps

Heat pumps, now a day widely used to have duel functions like furnace & air conditioners.

  • When heating is required, the heat pumps transfer heat from cool outside to conditioned space.
  • In this case, outside heat keeps the conditioned space warm.
  • When cooling is required, heat pumps transfer the heat from the conditioned space to outside.
  • Heat pumps are able to provide air conditioning requirements
  • Because they move heat rather than generate heat, heat pumps can provide equivalent space conditioning at as little as one-quarter of the cost of operating conventional heating or cooling appliances.

Heat pumps can be,

  • Air-to-air,
  • water source,

Air to air heat pumps is widely used in normal residential applications.

Hydronic system

In a hydronic system, water is heated or cooled and circulated to the areas to be conditioned through pipes.

  • The hydronic system circulates chilled water or hot water, based on requirements.
  • A hydronic system is used to heat swimming pools, floors, etc.
  • Boilers or other heating materials are used to heat the water.
  • It cools by circulating cold water to the conditioned space.
  • It normally uses water as a cooling or heating medium.
  • It can have both a cooling and heating water loop as well.
  • Water is circulated within a closed-loop piping system.
  • Piping is connected to terminal units or air conditioners.

Solar System

Solar heating is widely used now a day. Water is heated by the sun or solar energy and that energy helps for the heating of the space. There is various way, to harness solar energy & use to get heat energy.

  • There are photovoltaic panels to harness solar energy & use in heating
  • It helps for the central heating system also
  • It generates electricity which helps to heat


Chimneys are used to control the heat by controlling the draft to convey flue gas from a furnace or boiler to the outdoors.


Electricity is widely used to heat the heating elements & control the heating requirements.


Ventilation is one of the main parts of HVAC. It helps to provide fresh air into the conditioned space. It helps to limit a certain temperature range. Odors or fumes from an enclosed area can be removed by the ventilation system.

There are many different ventilating methods:

Natural ventilation

Natural ventilation is a primitive method and it is used for a long. In our home, we see small openings are given at a higher level. This is due to maintaining natural ventilation.

Now a day, there are many modern methods have been created to make proper natural ventilation systems.

Push type ventilation

In the supply ventilation system, fresh air is forced to provide into the room and maintain positive pressure.

Supply air fans are used and it may be non-ducted or ducted based on the capacity or requirements. This ventilation is also called as push ventilation.

Pull type ventilation

There are so many areas, where fumes or odors are produced like battery room or toilets or pantry etc. and it is required to be removed. In this case, exhaust fans are used to create negative pressure and remove it.

This ventilation is known as pull ventilation.

Push-pull ventilation

When supply and exhaust systems are required, we called the system a push-pull type ventilation system.

It can be done by providing supply fans for the supply system and exhaust fans for the exhaust system.

Air conditioning system

Air conditioning systems are used to cool the conditioned space. How air conditioning system works, is described in the simplest way to get a basic idea,

Air Handling Unit

This unit is used to handle air. It consists of a cooling coil, filters, moisture eliminators, etc. Refrigerant is circulated through this cooling coil (evaporator) and gets the heat from the air.

Refrigerant pickup heat

The air loses heat and becomes cool. On the other hand, refrigerant picks up the heat and gets heated. This refrigerant then passes through the compressor and condenser and comes back to the cooling coil through an expansion valve.

Cooling of air

Cold air is circulated to the rooms and cools. After some time, it takes the heat from the room and circulated back to the air handling unit through fan suction. This recirculation continues.


The air handling unit is having filters so that filtration is done and we get clean air.

Moisture eliminator 

The air handling unit is having a moisture eliminator, if the air is having moisture content, it separates from the air.


Ducting system is used to circulate the air, definitely it depends on the type of system and the air quantity.

Diffuser, grilles

Supply air is circulated through the duct at many points, these points are fitted with diffusers or grilles.

Condensing units

Refrigerant releases heat in the condensing units. In this unit, condenser coils are used and with the help of condenser fans, heat transfer happens between refrigerant and air.

Difference between HVAC and Air Conditioning

Don’t be confused between HVAC and air conditioning. Air conditioning is simply a part of HVAC.

Don’t think HVAC means big systems like a central system and air conditioning means a normal split system. Just remember, air conditioning is a part of HVAC.

Classification of HVAC Systems

There are so many types of HVAC systems, and the working principle of air conditioners are different. However, few of the standards system is explained for understanding;

Window air-conditioner

For small capacity and to meet normal residential cooling, window-type air conditioners are used.

  • It consists of an evaporator, compressor, condenser, expansion valve, filter, controls, etc. in a single unit.
  • It is placed in a room which has an exposed wall.
  • Evaporator portions will be inside the room
  • The compressor portion will be outside.

Split system 

Split AC means air conditioning divides into two parts, indoor and outdoor.

  • The indoor unit consists of an evaporator, expansion valve
  • The outdoor unit consists of a compressor, condenser fans, condensing coils
  • The indoor unit is placed inside the room
  • The outdoor unit is placed outside the room
  • The indoor and outdoor unit is connected by refrigerant piping.
  • In a normal room, split air conditioners are widely used.

This can be floor mounted, ceiling mounted or wall mounted.

Package AC

Packaged AC means all components are packed in a single unit. If you observe, you will see both indoor and outdoor units are placed in a single packaged.

  • It consists,
  • blower,
  • heating
  • cooling elements,
  • filter racks,
  • chambers

Normally, packaged air conditioners are used when medium capacities are required. Ductwork is connected to get the supply air into the conditioned space. To get return air from the room, a separate return duct is also provided.

Normally condenser of packaged units is cooled by air, but it can be cooled by water as well.

Central AC

Central AC is of two categories. DX type and chilled water type.

DX Central AC – In this case, the chiller is cooled by air.

Water-cooled chiller – In this case, condensers are cooled by water circuits that connect with cooling towers.

Why Does HVAC Maintenance Essential?

Installation of the HVAC system and running is not the end. There is one big term is associated with maintenance. During design, it is always considered the best and efficient system so that the HVAC system will have many benefits.

However, in case, if there is any lack of maintenance then there will be huge problems for the end customers.

Let’s see the benefits of proper maintenance and why is it required.

Low Efficiency

Without maintenance, whatever efficient system you consider, will not give satisfactory results and the efficiency of the system will reduce.

Proper maintenance helps the system be trouble-free and efficiency will be optimum.

Operating cost

If the efficiency of the system reduces, then the individual equipment must work more to produce the same quantity of work and take more power. Hence, it increases power consumption which gives an increase in operating costs or electricity bills.

Proper maintenance will help to increase efficiency which will help to maintain optimal operating cost.

Health of HVAC system

The HVAC system includes different types of rotating equipment and it is essential to check the health of all equipment based on the manufacturer’s recommendations.

Regular maintenance helps to maintain the good health of the entire HVAC system.

Equipment Life

During HVAC design, the Manufacturer designs the system considering the project requirements, it may vary to 20 years or 25 years, or 30 years.  Now, without maintenance, it cannot be achieved.

Hence, proper maintenance only can help the HVAC equipment as well as the system to live longer.

Indoor Air Quality

Outside air consists of a lot of dust, other foreign pollutants. It depends on the place. There are so many filters used in the HVAC system to filtrate the outside air which increase the indoor air quality or IAQ.

Now, when the system is running, the filter needs to be checked else outside untreated air will be entered into the building which affects the IAQs.

  • If anyone has asthma and has taken dust in their breathe will have health issue.
  • If anyone has lungs issue, dusty air can harm.
  • Without filters, unwanted bacteria can enter the buildings, which creates health problems.  

Reduce the Repair Cost

Regular maintenance can reduce the repair cost. In case of any problem, in any machine, it can be repaired instantly with the help of regular maintenance by the maintenance team.

However, if the maintenance is not done at that time and it runs for a longer period then the equipment will be more problematic and repair costs will be more.

Many times, repairing may not be sufficient and new parts need to be installed.

Health & Safety

Safety is one of the most vital aspects of HVAC maintenance. Routine maintenance can eliminate the probability of fire or any accident.

In case of obstruction in exhaust air path of condensing unit, condensing unit cannot be worked efficiently, and it needs to work more to give the required cooling effects. Hence, the compressor needs to work more, and gradually its temperature is increased.

Continuous increase in temperature results in fire or accident. Hence, routine maintenance will help to identify if any problems and avert any kinds of accidents.

Sudden Failure

Sudden failure may hamper normal life. Suppose you live in a place where the outside temperature is 50 deg. C. Machine is running and all set.

However, due to some reason, suddenly the machine stopped, or it is not giving required cooling. What will you do then?

It is always better to have regular maintenance than sudden failure.

Season Changes

The place where summer and winter temperature are high, then in the winter condition, AC may not work, and it became ideal for a couple of months.

Keeping the unit ideal may impact the health of the HVAC system and it is mandatory to have maintenance at the starting of hot weather.

Advantage of HVAC system

The advantage of an HVAC system simply is a lot in human comfort as well as all industries.

  • Due to HVAC only, we feel comfortable.
  • When the weather is very hot, we use air conditioning.
  • When the weather is very cool we use heaters.
  • We are able to breathe fresh air.
  • There are so many equipment that generate heat, HVAC is used to cool the same for proper functioning.
  • In medical, space, etc. HVAC is widely used.
  • Due to this, precise air filtration is possible which requires various research as well as critical medical applications.

Disadvantage of HVAC system

There are a few disadvantages as well,

  • The main disadvantage is the environmental impacts.  The refrigerant used in HVAC systems has Ozone Depletion Potential and Global Warming Potentials. These beaks the ozone layer which is not recommended. Now a day, environment-friendly refrigerants like R410a, R134a, R407c, etc. are used and these also harms our environment at some extent. Recently R1234yf is developed which is the best environment friendly refrigerant.
  • Continuous heat release to the atmosphere, making trouble for nature.
  • In the case of a centralized system, if the air is contaminated by chance, the same will be circulated which creates a problem.
  • Split type air conditioners can not give fresh air, hence, for log time headache may be encountered.
  • The HVAC system is costly and it is not feasible for poor people.
  • Operating costs especially energy costs are very high.
  • It requires periodic maintenance.

HVAC standards

There are so many codes and standards available for the HVAC design, construction, system design, equipment design, operation & maintenance, quality, safety, etc. Few are listed,

  • ISHRAE (India)
  • ACCA,
  • Uniform Mechanical Code,
  • International Mechanical Code,
  • AMCA
  • AHRI
  • DW
  • National standards country wise

Reference High Rated Course

HVAC Fundamentals and BMS Controls

A Practical Introduction to the BACnet Protocol

HVAC Psychrometry, Air Handling System and Duct Selection

Refrigeration & Air conditioning – Design and calculation

Become an HVAC PRO The Ultimate HVAC Guide


In this lesson, we have learned the basics of the HVAC system. Hope, it is clear, any doubt please let us know.

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Check out few interesting articles,

What is HVAC

How does air conditioner work

Basics of Air Handling Units

Cooling Towers

Global Warming Potential

Ozone Depletion


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