HVAC stands for Heating, Ventilating, and Air Conditioning, and HVAC systems are, effectively, everything from your air conditioner at home to the large systems used in industrial complexes and apartment blocks. A good HVAC system aims to provide thermal control and indoor comfort, and one that is designed using the principles of thermodynamics, fluid mechanics, and heat transfer.

The big air conditioner boxes that you might see on top of apartment blocks or offices are examples of (the visible part of) HVAC systems. They’re typically deployed in large industrial buildings, skyscrapers, apartment blocks, and large interior environments. They’re also an essential component of environments where there are health regulations requiring that temperature and humidity be kept at certain levels, using air taken from outside.

But heating and cooling systems you use in your home are also HVAC systems. They may take a different form, but many of the fundamental principles determining how they operate, as well as their efficiency, crosses over from the smallest of personal devices right through to the biggest commercial installations.

Different types of HVAC systems

For most people, heating and cooling will account for as much as half of the energy they use. With this in mind, it’s important to choose a HVAC system that will meet your comfort needs, without drawing on excess power and escalating your cost of living.

  • People living in hot or cold climates might go for a single stage system, designed to produce just heating or cooling. These tend to be inexpensive, but are also rather inefficient and will typically be working at capacity even when it’s not required.
  • More advanced models will offer variable fan speeds to cut down on power use, however they remain inefficient when compared to multi-stage systems, and are thus more expensive to run over the long term.
  • Zoned systems, on the other hand, have been designed to heat or cool individual parts of your home. This is done by designing zone valves and dampers inside the vents and ductwork that selectively block the flow of air. For people with larger properties this is of immeasurable value as it prevents the system from heating or cooling areas in the home that are not in use.
  • HVAC systems can also be built to offer humidity control, and both humidifiers and dehumidifiers can be added as options to heating and cooling systems. People that live in very dry environments or the tropics find these additions to the system essential. With that being said, some people prefer to install separate humidifier or dehumidifier systems, so that they can manage the humidity of their environment without also having to turn on the air conditioner.

Heating systems

Heating systems can take a couple of different forms. Some are furnaces that burn material to provide heated air through the ductwork, while another popular choice is boilers that heat water for steam radiators, or forced-water systems with baseboard radiators, electric heat, and heat pumps. A furnace will generally operate on natural gas or propane, while a boiler will use gas or oil to heat the water.

Another option is a radiant floor, also known as a hydronic heating system. These use piping under a floor, and are made up of flexible tubes that are filled with water or a glycol solution. These can heat any kind of floor, including concrete, and are an efficient method of providing warmth in a home. They can even be retrofitted into wooden flooring, though they need to be carefully installed in sheathing for wooden floors.

Cooling systems

Air conditioners come in many forms, from the massive boxes designed to cool an entire house to a portable window-mounted box that can be pulled out and used in cooler climates to handle short summers.

Many air conditioners can even be installed by the owner, with ductless mini split systems a popular choice. Installation is still a major project, as the interior and exterior elements of the system need to be properly connected, but they are relatively inexpensive to buy and run.

For dryer climates, evaporative coolers are a popular choice. They draw outside air into the system, passing it through water-saturated pads, which cool and moisten the air before pushing it into the living space and displacing the hot air.

How HVAC systems work

The following diagram illustrates the basic function of a HVAC system:

  • The building automation device (computer or wall-mounted gadgets usually) can be used to determine whether to heat or cool the space, and to what temperature
  • The system then either heats or cools the coils inside
  • When air brought in from the outside is pushed through these coils, it is either heated or cooled before being pushed into the living space
  • At the same time, air will be displaced from the room back into the system.

Smaller air conditioning systems may operate in the reverse, however, where they collect heat from within a space and push it out in order to cool a room down. We can see these kinds of systems in places like cars. Here’s how these types of HVAC systems work:

This diagram reveals how a system will collect heat from within a space, soak it up like a sponge, and push it back out into the air.

There are five key parts in this system that allow it to operate:

  • Expansion valveThis device creates a restriction in the liquid line of a system, which in turn creates a pressure drop.
  • EvaporatorThis device is what removes heat from the space by exchanging it for a boiling refrigerant.
  • CompressorThink of this component as the system’s heartbeat - it creates the energy and thrust to move refrigerant around the system.
  • CondenserThis is the device that pushes the heat that has built up inside the refrigerant into the outside air.
  • Receiver drierIn order to preserve the air quality, this storage area for excess refrigerant contains a drying agent and a filter that removes contaminants from the system.

The different types of
HVAC operation

The six main types of HVAC systems all offer slight variations to the operations outlined above.

Heat pump

Heat pumps are an efficient system that extracts heat from a cold space (such as the outdoors during winter), and then warms and releases it into a room so as to control the temperature inside. When used for heating, heat pumps use the same refrigeration-type cycle that is used in an air conditioner, but rather than release the air outside as a cooling system would do, it pushes air in the opposite direction (i.e. back into the room to be heated).

Heat pumps can also be used for cooling a room, reversing the flow of air to again expel the heated air that is brought into the system. Their real strength, however, benefits those who are in need of heating, as heat pumps can be up to four times as efficient in their use of power than more traditional heater systems.

Rooftop unit

Rooftop units are also often known as air handlers, and as their name suggests, they’re a large HVAC system that is placed on a rooftop in order to moderate the temperature of a large space. Inside the big boxes that you see on top of office or apartment buildings are a blower, heating and cooling elements, filter racks, and chambers and dampers.

These boxes typically connect with a ductwork ventilation system, that will then distribute the air through the building before returning it to the box to either discharge or return air back into the system (depending on the model).

Water source heat pump

For those who are interested in sustainable cooling and/or heating, the water source heat pump - or broadly, any geothermal heat pump - is the way to go.

Water source heat pumps are relatively uncommon as they require proximity to a body of water; geothermal heat pumps, however, are rapidly escalating in popularity. Regardless of whether it’s a system drawing on water or the ground, these pumps offer both heating and cooling systems that transfer heat into or out of the ground by taking advantage of the more moderate temperatures of the earth to boost the efficiency of the system.

Taking this system one step further, however, would involve drilling down to create a bore near the HVAC system. The cooled water underneath the earth could then be drawn on by the system to provide drinking water and feed an open-loop heat pump. This would take the heat from the water and use it to raise the heat in a home’s water system, providing heating and hot water. Excess grey water can then be used for irrigation for the garden.

Packaged HVAC

Packaged air conditioners look a little like the rooftop units, but are designed for smaller domestic use. Where window and mini split air conditioners are good for small room cooling of up to around five tonnes, central air conditioning systems are designed for loads in excess of 20 tonnes. For that reason, the packaged air conditioner has been designed to accommodate the needs of anyone who fits between those two frames.

There are two types of packaged air conditioners:

  • Packaged air conditioners with water cooled condensers are, as the name suggests, air conditioners in which the condenser is cooled by water. Water needs to be supplied constantly so as to keep these air conditioners in working order. These air conditioners are generally installed inside buildings.
  • Packaged air conditioners with air cooled condensers, meanwhile, are cooled by the atmospheric air and are therefore outdoor units. These devices have a fan that sucks in air before blowing it onto the condenser coil, much like in the larger rooftop units. These are the more popular of the two types of packaged air conditioners, as they don’t need constant maintenance to ensure a smooth flow of water.

Split system HVAC

The term ‘split system’ simply refers to an air conditioning unit where key components are separated and deployed in different places. They come in two forms - mini split (also called a ‘ductless system’) and a central system, as illustrated above.

Central systems are ducted systems that are designed specifically around cooling the space, and are able to offer multi-zone temperature control capability through the use of air-louver-control boxes. This is good for spaces that are used sporadically, and they can be ‘switched off’ when nobody is using the space so as to save on costs.

In these units, the heat-exchanger is placed inside the central furnace/AC unit of the forced air heating system, which is then used to distribute chilled air through the space.

Ductless system

The smallest of all systems, the ductless or mini split air conditioner is designed for small deployments such as a single large room, or multiple small rooms. They require minimal wall space, and the compressor and heat exchanger unit can be located further away from the main building, allowing greater flexibility in use.

This is the main type of air conditioner you’ll find on the market, as it’s explicitly designed for home use and fits into the consumer mass-consumption model. These systems are easy to install, even as a home project, and the internal unit is aesthetically pleasing as it forms part of the furniture. The downside to ductless systems is that they can cost more to operate than central systems. However, as with other split systems, these are the only option for customers looking to retrofit existing buildings, as they don’t require the installation of ducts.