Types of HVAC Systems: How Each One Works (with Animated Diagrams)

From a single bedroom to a 50-storey tower, every building is cooled by one of a handful of HVAC system types. They all move heat from inside to outside, but they do it in very different ways, and choosing the wrong one costs money for the life of the building. This guide explains the main types of HVAC systems one by one, with animated diagrams that show exactly how refrigerant, water and air move through each system.

Contents

1. The cycle behind every AC
2. Window AC
3. Split AC
4. Ducted split AC
5. Packaged and rooftop units
6. VRF / VRV systems
7. Chilled water systems
8. Heat pumps
9. Evaporative cooling
10. District cooling
11. Air distribution: CAV, VAV and DOAS
12. Comparison table
13. How to choose the right system
14. FAQ

HVAC system types at a glance

System	Typical cooling capacity	Best suited for	What carries the heat to the room
Window AC	0.75 to 2 TR (2.6 to 7 kW)	Single rooms, low budget	Refrigerant, all in one box
Split AC	0.75 to 3 TR (2.6 to 10.5 kW) per indoor unit	Bedrooms, offices, small shops	Refrigerant
Ducted split	2 to 20 TR (7 to 70 kW)	Villas, apartments, small offices	Refrigerant to one unit, then air through ducts
Packaged / rooftop unit	3 to 50 TR (10.5 to 175 kW) per unit	Single-storey retail, schools, warehouses	Air through ducts
VRF / VRV	Up to about 60 HP (170 kW) per outdoor system	Offices, hotels, multi-zone buildings	Refrigerant to every indoor unit
Chilled water	50 TR to several thousand TR	Large buildings, malls, hospitals	Chilled water
Heat pump	1 to 20 TR in most homes and small buildings	Heating and cooling from one system	Refrigerant or water
Evaporative cooling	Sized by airflow	Hot, dry climates, warehouses	Air cooled by evaporating water
District cooling	Thousands of TR per plant	City districts, master-planned developments	Chilled water from a central plant

The one cycle behind every AC

Almost every system in this guide (except evaporative cooling) uses the same vapor-compression refrigeration cycle. The refrigerant goes round a closed loop through four main parts:

1. Compressor: squeezes low-pressure refrigerant gas into hot, high-pressure gas.
2. Condenser: a coil where the hot gas gives its heat to outdoor air (or cooling tower water) and condenses into a liquid.
3. Expansion device: a valve or capillary tube that drops the pressure, so the liquid becomes very cold.
4. Evaporator: a coil where the cold refrigerant absorbs heat from the room air (or from water in a chiller) and boils back into a gas, ready for the compressor again.

What makes one system different from another is where these four parts sit, and whether the cold is carried to the rooms by refrigerant, by water or by air. The cooling load that every system must meet comes from a HVAC load calculation.

1. Window AC

A window AC is the simplest air conditioner: all four parts of the refrigeration cycle are in one box that fits through a window or a hole in the wall. An insulated partition divides the box into a room side and an outdoor side.

How it works

1. The blower on the room side pulls warm room air through a filter and across the evaporator coil.
2. Cold refrigerant in the evaporator absorbs the heat and moisture from the air, and the cooled air is blown back into the room.
3. The compressor on the outdoor side pumps the warmed refrigerant gas to high pressure.
4. The outdoor fan blows outside air across the condenser coil, which releases the heat outdoors.
5. A capillary tube drops the refrigerant pressure before it returns to the evaporator. Condensate drains to the outdoor side.

Where it is used

Single rooms, older buildings without space for split units, staff accommodation and small site offices. Typical sizes are 0.75 to 2 TR (2.6 to 7 kW). For room sizes, see the AC tonnage chart.

Advantages	Limitations
Lowest purchase cost Simple installation, no refrigerant piping Easy to replace	Noisy, because the compressor is in the wall next to the room Blocks part of a window and needs a wall opening Lower efficiency than modern inverter splits Limited to one room

2. Split AC

A split AC separates the noisy, hot parts from the room. The indoor unit holds the evaporator coil and a quiet fan; the outdoor unit holds the compressor, condenser and fan. Two copper pipes and a control cable connect them through a small hole in the wall.

How it works

1. The indoor fan draws warm room air across the evaporator coil, where cold refrigerant absorbs the heat and removes moisture.
2. The refrigerant, now a cool low-pressure gas, flows through the larger suction pipe to the compressor in the outdoor unit.
3. The compressor raises it to a hot, high-pressure gas, and the condenser coil and fan release the heat outside.
4. The expansion valve drops the pressure so the refrigerant becomes very cold, and it flows back to the indoor coil through the smaller liquid pipe.
5. Condensate from the indoor coil drains away through a separate drain pipe.

In the diagram the expansion valve is drawn on the line between the units for clarity. In most modern inverter splits it is an electronic expansion valve inside the outdoor unit.

Types of split AC

• Wall-mounted (high wall): the most common type for bedrooms and offices.
• Ceiling cassette: fits into a false ceiling and blows air in four directions; good for shops and open offices.
• Floor-standing: for larger rooms, majlis and halls.
• Multi-split: one outdoor unit serving two to five indoor units.

Inverter models vary compressor speed to match the load, which saves energy and holds temperature more steadily. Typical sizes are 0.75 to 3 TR (2.6 to 10.5 kW) per indoor unit; use the AC Tonnage Calculator to size one. Line sizes and length limits are covered in Refrigerant Pipe Sizing for VRF and Split Systems, and you can download the split AC installation detail DWG from CADBIMHub.

Advantages	Limitations
Quiet indoors, because the compressor is outside Higher efficiency, especially inverter models Flexible indoor unit types Only a small wall opening needed	One indoor unit per room (or a multi-split) Outdoor units on every facade can look cluttered Refrigerant piping must be installed and tested on site Limited fresh air; needs separate ventilation

3. Ducted split AC

A ducted split works like a split AC, but the indoor unit is a concealed fan coil hidden in the ceiling void. Instead of blowing air straight into one room, it pushes cool air through supply ducts to diffusers in several rooms, and draws room air back through a return grille.

How it works

1. Room air returns to the indoor unit through a return grille, either directly or through door undercuts and transfer grilles.
2. The indoor fan pushes the air across the cooling coil, connected by refrigerant pipes to the outdoor unit.
3. Cool air travels through the supply ducts and leaves through ceiling diffusers or linear grilles in each room.
4. The outdoor unit compresses the refrigerant and rejects the heat outside, exactly as in a split AC.

Where it is used

Villas, apartments, small offices and clinics where a clean ceiling with no visible units is wanted. Typical sizes are 2 to 20 TR (7 to 70 kW). Because the fan must push air through ducts, ducted units are selected by airflow and external static pressure; see External Static Pressure (ESP) Calculation. Size the ducts with the Online Ductulator or the Duct Size Chart PDF.

Advantages	Limitations
Nothing visible in the rooms except diffusers One unit can serve a whole apartment or floor zone Fresh air can be added through the return side Quiet, because the unit is above the ceiling	Needs ceiling void height for the unit and ducts One thermostat usually controls several rooms Duct design and balancing matter for comfort Access panels needed for filter cleaning and maintenance

4. Packaged and rooftop units

A packaged unit puts the whole system (compressor, condenser, evaporator coil, filters and supply fan) into one factory-built cabinet, usually on the roof. Ducts carry the cooled air down through the roof into the building and bring the return air back.

How it works

1. Return air from the building and outdoor air from the intake hood mix inside the unit.
2. The mixed air passes through filters and across the evaporator coil, and the supply fan pushes it down the supply duct to the diffusers.
3. In the same cabinet, the compressor and condenser coil reject the heat to the outdoor air.
4. A return grille and duct bring the room air back up to the unit to start again.

Where it is used

Single-storey and low-rise buildings with a flat roof: retail stores, supermarkets, schools, warehouses and restaurants. Units range from about 3 to 50 TR (10.5 to 175 kW), with larger models available. Vertical packaged units are a variant for indoor plant rooms. For coil selection principles, see Cooling Coil Selection; for fan energy, see Fan Laws and VFD Energy Savings.

Advantages	Limitations
Everything factory-assembled and tested in one cabinet No plant room or refrigerant piping inside the building Easy to add fresh air and economizer control Simple to replace at end of life	Needs a strong, accessible roof Long duct runs on multi-storey buildings Exposed to sun, dust and weather Less efficient than large central systems

5. VRF / VRV systems

A VRF (variable refrigerant flow) system is a large, smart version of a multi-split. One outdoor unit, or a group of outdoor modules, feeds dozens of indoor units through a single pair of refrigerant pipes. VRV (variable refrigerant volume) is Daikin’s trade name for the same idea.

How it works

1. Each indoor unit has its own electronic expansion valve and controller, so every room sets its own temperature or switches off.
2. The outdoor unit’s inverter compressor speeds up or slows down to match the total load of all the indoor units that are running.
3. Liquid refrigerant travels up or down the building in the liquid pipe and is split to each floor and indoor unit by branch joints (refnet joints).
4. After absorbing heat in the indoor coils, the refrigerant returns as gas through the larger gas pipe to the outdoor unit, which rejects the heat outside.

Heat pump vs heat recovery VRF

A heat pump VRF can cool or heat, but all indoor units on one system must be in the same mode. A heat recovery VRF adds a third pipe or branch selector boxes so some rooms can heat while others cool, moving heat from one zone to another. This is useful in buildings with sunny and shaded facades or with server rooms next to offices.

Where it is used

Offices, hotels, hospitals, schools and apartment buildings where many rooms need separate control but there is no space for a central plant. Outdoor modules combine up to about 60 HP (around 170 kW) per system, and one system can serve dozens of indoor units, depending on the manufacturer. Piping lengths and height differences are limited; see Refrigerant Pipe Sizing for VRF and Split Systems. VRF indoor units recirculate room air, so fresh air needs a separate system (see DOAS below).

Advantages	Limitations
Individual control of every room High part-load efficiency from inverter compressors No plant room, chillers or water pipes Heat recovery models can heat and cool at the same time	Large amount of refrigerant in occupied buildings, so leak detection may be required Strict pipe length and height limits Needs a separate fresh air system Repairs usually need the manufacturer’s trained technicians

6. Chilled water systems

Large buildings rarely send refrigerant to every room. Instead, a central chiller cools water to around 6 to 7°C, and pumps circulate this chilled water through pipes to air handling units (AHUs) and fan coil units (FCUs) all over the building. The refrigeration cycle stays inside the chiller in the plant room.

How it works

1. The chiller’s evaporator cools the return water from about 12 to 13°C down to about 6 to 7°C.
2. Chilled water pumps send it up the risers to AHUs and FCUs, where it flows through the cooling coils and absorbs heat from the air.
3. The warmer water returns to the chiller to be cooled again, in a closed loop.
4. On the other side of the chiller, the condenser rejects the heat. In a water-cooled chiller, condenser water carries it to a cooling tower on the roof; in an air-cooled chiller, fans blow outdoor air over the condenser coils directly.

Air-cooled vs water-cooled chillers

	Air-cooled chiller	Water-cooled chiller
Where it sits	Roof or ground, outdoors	Plant room, with cooling towers outside
Typical efficiency at full load	About 1.0 to 1.3 kW per TR	About 0.5 to 0.7 kW per TR (centrifugal)
Water use	None for heat rejection	Make-up water for cooling towers
Typical size	Up to about 500 TR per chiller	From about 200 TR to several thousand TR
Best for	Small to medium buildings, water-scarce sites	Large buildings and campuses

Where it is used

Malls, hospitals, airports, hotels and high-rise offices, typically from about 50 TR to several thousand TR. The design work behind a chilled water system is covered in detail in our guides: Chilled Water Pipe Sizing, Chilled Water Pump Head Calculation, Expansion Tank Sizing, Cooling Tower Sizing and Cooling Coil Selection. For drawings, see the HVAC Chilled Water System DWG.

Advantages	Limitations
Highest efficiency for large loads No refrigerant in occupied areas Long service life of central plant Easy to add thermal storage or connect to district cooling	High first cost and large plant room More complex controls, water treatment and maintenance Pumping energy for long pipe runs Water-cooled plants consume make-up water

7. Heat pumps

A heat pump is an air conditioner that can run in reverse. In summer it cools like an AC; in winter a reversing valve swaps the jobs of the two coils, so the system pulls heat from the outdoor air and releases it inside. Because it moves heat instead of making it, a heat pump typically delivers 3 to 4 kW of heat for every 1 kW of electricity (a COP of 3 to 4).

How it works in heating mode

1. The compressor raises the refrigerant to a hot, high-pressure gas.
2. The reversing (4-way) valve sends this hot gas to the indoor coil instead of the outdoor coil.
3. The indoor coil acts as the condenser: the indoor fan blows room air over it and warms the room.
4. The refrigerant, now a warm liquid, passes through the expansion valve and becomes very cold.
5. The outdoor coil acts as the evaporator, absorbing heat from the outdoor air even when it is cold, and the cycle repeats.

In cold, humid weather frost builds up on the outdoor coil. The unit briefly switches to cooling mode to melt it, which is called a defrost cycle.

Types of heat pump

• Air-source: takes heat from outdoor air. Available as splits, packaged units and VRF systems.
• Air-to-water: heats or cools water for underfloor heating, radiators, fan coils or domestic hot water.
• Water-source: exchanges heat with a water loop, lake or well.
• Ground-source (geothermal): exchanges heat with the ground through buried pipes, giving very high efficiency because ground temperature is stable all year.

Where it is used

Homes, apartments and small commercial buildings that need both heating and cooling, especially where gas is not available or electricity is low-carbon. Many split ACs sold in mild-winter regions are actually heat pump models with a heating mode.

Advantages	Limitations
One system for heating and cooling Very efficient heating compared with electric resistance heaters No combustion, no flue, no fuel storage Works with solar power for low-carbon buildings	Heating output falls as outdoor temperature drops Defrost cycles in cold, humid weather Ground-source systems have high installation cost May need backup heating in very cold climates

8. Evaporative cooling

Evaporative cooling uses no refrigerant and no compressor. Water evaporating from wet pads absorbs heat from the air passing through them, the same effect you feel when you step out of a pool. It works very well in hot, dry climates and uses a fraction of the electricity of a refrigerated AC.

How it works

1. A small pump lifts water from the sump to a distributor at the top of the pads.
2. The water trickles down and soaks the pads, and the excess drains back to the sump.
3. The fan pulls hot, dry outdoor air through the wet pads. Some of the water evaporates, taking heat from the air.
4. The cooled, more humid air is ducted into the building, and relief openings let the same amount of air out.

How cool can it get?

An evaporative cooler can only cool air toward its wet-bulb temperature, never below it. A good direct cooler reaches 70 to 90% of the difference between dry-bulb and wet-bulb:

Supply temperature ≈ T_db − Effectiveness × (T_db − T_wb)

On a 45°C dry-bulb, 22°C wet-bulb day with 80% effectiveness: 45 − 0.8 × 23 ≈ 27°C. On a humid coastal day the wet-bulb is much higher, and the cooling effect almost disappears. Check conditions with the Psychrometric Calculator, and see Psychrometric Processes in HVAC for the theory.

Direct, indirect and two-stage

• Direct: air passes straight through the wet pads, so it is cooled and humidified.
• Indirect: a heat exchanger cools the supply air without adding moisture, using a separate wetted air stream.
• Two-stage (indirect-direct): an indirect stage pre-cools the air, then a direct stage cools it further, reaching lower temperatures with less added humidity.

Where it is used

Warehouses, factories, workshops, greenhouses, poultry and dairy farms, and homes in hot, dry inland regions. It is a poor fit for humid coastal cities and for spaces that need close humidity control.

Advantages	Limitations
Very low running cost, often a quarter or less of refrigerated AC No refrigerant Delivers 100% fresh air Simple, low-cost equipment	Only works well in dry climates Adds humidity to the space Uses water continuously Needs pad cleaning and water treatment to control scale and bacteria

9. District cooling

District cooling takes the chilled water system one step further: instead of every building having its own chillers, one large central plant makes chilled water for a whole district and sends it through insulated underground pipes to many buildings. Each building simply connects to the network, much like connecting to the city water supply.

How it works

1. Large, high-efficiency chillers in the central plant cool water to about 4 to 5°C. Many plants also store chilled water or ice in thermal energy storage (TES) tanks, made at night when electricity demand is low.
2. Distribution pumps send the chilled water through underground supply pipes to every connected building.
3. In each building, an energy transfer station (ETS) with plate heat exchangers cools the building’s own chilled water loop, keeping the two water systems separate.
4. The building’s pumps then circulate its chilled water to AHUs and FCUs, exactly as in a normal chilled water system.
5. The warmed water, at about 13 to 14°C, returns through the network to the plant to be cooled again.

Where it is used

Dense city districts, airports, universities and master-planned developments, especially in hot climates with high, steady cooling demand such as the Gulf region. Plants range from a few thousand to tens of thousands of TR. A large temperature difference between supply and return is critical, because it sets how much cooling each pipe can carry. Inside the building, the design follows the chilled water system principles above.

Advantages	Limitations
No chillers or cooling towers on each building Very high efficiency and reliability at large scale Frees roof and plant room space Thermal storage shifts power use to off-peak hours	Only available where a network exists Connection and capacity charges set by the provider Building depends on an outside supplier Poor return water temperature (low ΔT) causes penalties and capacity problems

10. Air distribution: CAV, VAV and DOAS

The systems above decide how cold is made. Equally important is how the cooled air, and the fresh outdoor air every building needs, reaches each room. Three approaches cover most buildings.

CAV (constant air volume)

The fan always moves the same amount of air, and the supply air temperature is varied to match the load. It is simple and robust, and suits single large spaces such as halls, auditoriums and supermarkets. Because the fan never slows down, it wastes energy at part load, and one thermostat cannot satisfy rooms with different loads.

VAV (variable air volume)

Each zone has a VAV box with a damper and its own thermostat. The box reduces airflow when its zone needs less cooling, and the AHU fan slows down through a VFD. This gives room-by-room control and large fan energy savings, which is why VAV is the standard for central-AHU office buildings. See VAV Box Sizing and Selection and Fan Laws and VFD Energy Savings.

DOAS (dedicated outdoor air system)

A DOAS, often called a fresh air handling unit (FAHU) in the Gulf, treats 100% outdoor air and delivers it to each room for ventilation, while separate room units (fan coils, VRF indoor units or chilled beams) handle the heat load. An energy recovery wheel uses the cool exhaust air to pre-cool the incoming fresh air, cutting the ventilation load. DOAS is the usual partner for VRF and fan coil systems, and it gives much better humidity control in hot, humid climates. Ventilation rates come from ASHRAE 62.1; see the ASHRAE Ventilation Calculation XLS.

Comparison table: all HVAC system types

System	Typical capacity	First cost	Running efficiency	Space needed	Room-by-room control	Typical buildings
Window AC	0.75 to 2 TR	Low	Low	Wall opening	One room	Single rooms, site offices
Split AC	0.75 to 3 TR per unit	Low	Medium to high (inverter)	Small	Yes	Homes, small offices, shops
Ducted split	2 to 20 TR	Medium	Medium	Ceiling void	Per zone	Villas, apartments, clinics
Packaged / rooftop	3 to 50 TR	Medium	Medium	Roof	Per unit or zone	Low-rise retail, schools, warehouses
VRF / VRV	Up to about 60 HP per system	Medium to high	High at part load	Outdoor units only	Yes, every room	Offices, hotels, apartments
Chilled water	50 TR to thousands of TR	High	Highest for large plants	Plant room and towers	Yes, with FCUs or VAV	Malls, hospitals, high-rise
Heat pump	1 to 20 TR in most homes	Low to medium	High, heating COP 3 to 4	Small	Yes	Homes needing heating and cooling
Evaporative cooling	Sized by airflow	Low	Very high in dry climates	Small	Limited	Warehouses, workshops, dry regions
District cooling	Thousands of TR per plant	Connection fee	High at district scale	ETS room only	Depends on building system	Master-planned districts

How to choose the right HVAC system

Start with the size of the building and its cooling load, then narrow down by space, budget, control needs and climate. This flowchart gives a quick first answer:

1. Calculate the cooling load with a proper HVAC load calculation, or for a quick estimate the AC Tonnage Calculator.
2. Check the space: roof area, plant room, ceiling void and facade space for outdoor units.
3. Decide on control: one zone, a few zones, or every room separately.
4. Consider climate: heating needs, humidity and whether evaporative cooling is viable.
5. Compare life-cycle cost, not just first cost: energy, maintenance and replacement over 15 to 25 years.

Conclusion

All the types of HVAC systems in this guide move heat from inside a building to outside; the difference is where the equipment sits and whether refrigerant, water or air carries the cold. Small buildings are usually best served by split or ducted split units, medium buildings by VRF or packaged units, and large buildings by chilled water or district cooling, with a DOAS supplying fresh air wherever room units are used. Once you have chosen the system, the detailed design guides on MEPBase take you through each step, from fan static pressure to pump head and refrigerant pipe sizing.

Frequently Asked Questions

What are the main types of HVAC systems?

The main types are window AC, split AC, ducted split, packaged or rooftop units, VRF/VRV systems, chilled water systems, heat pumps, evaporative coolers and district cooling. Air is then delivered to rooms by CAV, VAV or DOAS air distribution.

Which type of HVAC system is the most energy efficient?

It depends on the building. For large buildings, water-cooled chilled water plants and district cooling are the most efficient. For small and medium buildings, inverter split and VRF systems perform best at part load. In hot, dry climates, evaporative cooling uses the least energy.

What is the difference between VRF and a chilled water system?

A VRF system sends refrigerant directly from outdoor units to each indoor unit, with no plant room. A chilled water system cools water in a central chiller and pumps it to AHUs and fan coils. VRF suits medium multi-zone buildings; chilled water suits large buildings.

Which HVAC system is best for a house?

Split or multi-split units suit homes with a few rooms. A ducted split is better when you want hidden units serving the whole house. Where winter heating is needed, choose heat pump versions of either.

What is the difference between a split AC and a ducted split?

A split AC blows cool air directly into one room from a wall or ceiling unit. A ducted split hides the indoor unit above the ceiling and sends cool air through ducts to several rooms.

Why do buildings need a DOAS or fresh air unit?

Split, VRF and fan coil units mostly recirculate room air. A DOAS brings in treated outdoor air to meet ventilation requirements and removes humidity, often using an energy recovery wheel to save energy.