HVAC is a huge industry. When I first started working in the HVAC industry, I was overwhelmed by the complexity of the HVAC system. Hence, I look back on everything that I’ve learned about HVAC and decided to post the basics of HVAC for beginners out there.
The acronym HVAC (heating, ventilation and air conditioning) is more commonly used in certain regions of the world where heating is needed such as the United States. In countries like Malaysia, we use ACMV (air conditioning and mechanical ventilation) since heating is not needed.
Regardless, both are governed by the same principles. If you are still a student, I suggest you dive deeper into the following subjects:
- Refrigeration Cycle
- Psychrometric Chart
- Second Law of Thermodynamics
- Refrigerant Properties
The refrigeration cycle can help you to understand how air conditioners and heat pumps work using the 4 major components which are the compressor, the condenser coil, the expansion valve and the evaporator coil.
The psychrometric chart can help you to understand what is dew point and why condensation happens. Besides, you’ll understand what is relative humidity. These are the most common problems in HVAC.
The second law of thermodynamics and refrigerant properties helps you to understand why heat pumps are able to extract heat from cold air.
Back then, I did not know that I was going to work in the HVAC industry. Hence, I did not pay enough attention to subjects that were closely related to HVAC. Fortunately, I am still able to learn on my own through google and real-life experience.
Why Do We Use HVAC Systems?
When it comes to HVAC as a whole, I like to see only the air conditioning part and the mechanical ventilation part. Heating was a separate topic back then when people are using gas furnaces.
Nowadays, heat pumps are replacing gas furnaces and they work just like air conditioners. Thus, I think there are not enough differences between heat pumps and air conditioners to make us worth separating them into two different topics.
Air conditioning is all about “conditioning the air” for humans. We use air conditioners to create a comfortable and healthy environment for ourselves. Back then, people used to find shades or live in caves. Now, we use machines to do the same thing but better.
Air conditioners serve three purposes; a) temperature control, b) humidity control, and c) air quality control. Most people only think about temperature control when it comes to air conditioners. However, humidity control and air quality control are equally important.
a) Temperature Control
Most people know that air conditioners keep them cool or warm (if it’s a heat pump). In most places, air conditioners help people to stay comfortable and thus, improve productivity. However, in certain places, air conditioners and heat pumps save lives.
According to a 20-year study done by a team of researchers from Monash University in Australia and Shandong University in China, 4.6 million people died due to extreme cold while 0.48 million people died due to extreme heat [source].
What’s more worrying is that the ongoing climate change will raise the death toll, especially on heat-related deaths. Hence, the need for air conditioners (cooling) is increasing every year.
b) Humidity Control
In order for humans to feel comfortable, the air must be conditioned to a relative humidity of around 50%. At that level of humidity, people generally feel comfortable on top of a temperature around 25°C [source].
Therefore, air conditioners were designed to remove moisture in the air through condensation. However, oversized air conditioners couldn’t reduce the humidity level and they are causing a lot of mold growth issues. Thus, calculating the correct capacity for air conditioners is very important.
c) Air Quality Control
Compare to temperature control and humidity control, air quality control is not prioritized in air conditioning for most applications. However, cleanrooms for the manufacture of semiconductors and hospital ICUs required air conditioners that are excellent in all three controls.
Ventilation can have many forms including natural ventilation. It is called mechanical ventilation because we use machines to force the air into and out from desired spaces.
As far as I know, mechanical ventilation controls the level of four substances; a) carbon dioxide, b) carbon monoxide, c) fire smoke, and d) temperature. Conventionally, ventilation is all about maintaining oxygen level but there is more to that.
a) Carbon Dioxide Level Control
A ventilation system will always be found in a toilet that has no window. In many countries, it is a safety requirement to have a ventilation system in a windowless room.
Outdoor air has an average CO2 level of 400ppm. In occupied spaces, the typical CO2 level is about 400-1000ppm with good air change. When the CO2 level gets above 1000ppm, it’s considered poor air. At 40,000ppm, it’s deadly [source].
A good mechanical ventilation system keeps the air healthy and prevents people from having drowsiness, sleepiness and headache.
b) Carbon Monoxide Level Control
Mechanical ventilation is used in basement carparks and confined carparks to control carbon monoxide (CO) levels that are emitted from cars’ exhausts.
Most people will not experience any symptoms from prolonged exposure to CO levels of approximately 1 to 70 ppm but some heart patients might experience an increase in chest pain. At sustained CO concentrations above 150 to 200 ppm, disorientation, unconsciousness, and death are possible [source].
c) Fire Smoke Control
Almost all buildings (especially high-rise buildings) have mechanical ventilation systems to retain the effectiveness of escape routes and save lives. Mainly, the smoke spill system (or smoke extraction system) and the staircase/lift lobby pressurization system.
Fire smoke is lethal. If a fire started in a basement carpark, the fire smoke has nowhere to go and it’ll quickly spread across the entire floor. Due to the thick smoke, people will not able to see exit signs, making the escape extremely difficult.
Hence, the smoke spill system ensures that fire smoke is quickly exhausted out from the basement carpark to clear a path for the people to escape as quickly as possible. At the same time, the pressurization system keeps the fire smoke away from the staircase and lift lobby.
d) Reduce Temperature
Mechanical ventilation is used in equipment rooms to prevent machines such as electrical transformers and fire pumps from overheating and tripping. These machines often don’t need cooling but they generate enough heat to trip themselves if the room is not ventilated.
So, the above is what HVAC is all about. However, these are basic requirements and purposes of HVAC. Today, people working in the HVAC industry are more than just making sure that the basic requirements of HVAC are met.
HVAC is a significant contributor to global warming and climate change. Hence, the entire HVAC industry is researching, developing, producing and implementing new technologies to help reduce the harmfulness of HVAC to the environment.
“Nowadays, we are not asking how air conditioners work anymore. Instead, we ask ourselves how to improve the efficiency of the air conditioning system.
Hence, my advice to HVAC beginners is to focus on energy efficiency whether it is individual products or the HVAC system as a whole. If you know how to design and implement high-efficient HVAC systems, you’ll be more valuable as opposed to just following the footstep of HVAC seniors.
From what I see today, the energy efficiency improvement on individual products is minor. The opportunity lies in the HVAC system with the combination and integration of multiple high-efficiency products.
Most Common Type of Air Conditioning System
Basically, there are two most common types of air conditioning systems used in buildings around the world. For residential buildings such as landed homes and apartments, the split system is the most common one. For commercial buildings such as office towers and hotels, it is the chilled water system.
There are many different types of air conditioning systems developed and available in the market. It can be confusing for beginners. So, I categorized all of them into 12 different types. You can see all of them in one of my previous posts where I rank them by efficiency [read post].
Residential: Split System
As of 2016, split systems accounted for 77% of the air conditioning system used in the world [source]. Split systems can be further broken into single-split and multi-split.
Split systems offer the highest energy efficiency while remaining affordable and practical. Packaged systems such as window air conditioners are more affordable but they are not as efficient and occupy the window.
In my opinion, split systems are simple, reliable and perform very well. A split system consists of one indoor unit and one outdoor unit. When used for cooling, the indoor unit and the outdoor unit are called the evaporator and the condenser respectively. Vice versa when used for heating.
Split System Major Components
Generally, a split system has the following major components:
- Scroll compressor
- Electronic expansion valve
- Finned-tube heat exchanger
- Axial and centrifugal fan
Compressors have 4 types; a) scroll, b) screw, c) centrifugal, and d) reciprocating. Almost all split systems use the scroll type of compressor.
Expansion valves have 2 types; a) thermal, and b) electronic. New split systems are using the electronic type of expansion valve because it is more accurate and efficient.
As far as I know, heat exchangers have 3 types; a) finned-tube, b) brazed-plate, and c) shell & tube. Split systems use the finned-tube type of heat exchanger.
Besides, heat exchangers can be coated with a layer of hydrophilic for corrosion-resistant. Coated finned-tube heat exchangers are blue in color as opposed to the natural silver color of aluminium fins.
Many people are not familiar with inverter despite it is widely used in split systems. Basically, the inverter enables the control of the speed of compressors. Thus, it gives split systems higher efficiency and precision temperature control.
However, some people may come across “DC air conditioners” which essentially referred to inverter air conditioners most of the time. Sometimes, DC air conditioners also mean solar air conditioners. You can check out my post where I explained why people called the inverter as DC air conditioners [read post].
Most split systems are ductless. Hence, ductwork is not required and they are left with only the pipework. Meanwhile, wiring work is sort of like a universal work, it’s not much of a difference in HVAC.
Split System Pipework Components
The pipework of split systems involved the following components:
- Copper tube
- PVC pipe
- Closed-cell insulation
- PVC cable
- R410A refrigerant
- Temperature sensor
The indoor unit and the outdoor unit of a split system are connected by a set of closed-cell insulated copper tubes. The condensate water produced by split systems is discharged via a closed-cell insulated PVC pipe.
In split systems, the most problematic component is the PVC drain pipe for condensate water. Previously, we had so many water leakages due to improper drain pipe installation. I recommend you check out my post on condensate drain pipe basics to avoid the mistakes that I did [read post].
Currently, the R410A refrigerant is flowing in most split systems. Newer split systems are adopting the use of R32 refrigerant which is a more environmental-friendly refrigerant. If you wish to know why, check out my post where I also compared R32 to other refrigerants [read post].
Furthermore, several temperature sensors are used in the split system for temperature control and damage protection. Split systems are powered by basic PVC cables.
While there are many materials for the refrigerant pipe (copper tube) and insulation, split systems are all about simplicity and practicality. Hence, the materials used are best-suited for split systems.
If you are interested to know why copper is the holy grail for air conditioners, you may jump to one of my posts where I explained how copper beats other materials when it comes to air conditioning [read post].
Other residential air conditioning systems:
- Multi split system – One outdoor unit connecting up to 5 indoor units. Learn more from my post where I unpack the principle of capacity sharing [read post].
- VRF system – One outdoor unit connecting up to 64 indoor units. Learn more from my post where I explained the entire VRF system working principle including the air-cooled and water-cooled type [read post].
- Packaged system – Affordable but low efficiency units such as window and portable air conditioners.
Commercial: Chilled Water System
Chilled water systems are complicated. Many engineers spend most of their working hours on the design and installation of chilled water systems. Some people consider that if you know the chilled water system very well, you are an HVAC expert.
A chilled water system can be further broken into two types; a) air-cooled and b) water-cooled. Water-cooled chilled water systems are more complicated than air-cooled chilled water systems and they are more efficient too.
Chilled Water System Major Components
Generally, a water-cooled chilled water system consists of the following major components:
- Water-cooled centrifugal chiller
- Counterflow cooling tower
- Make-up water tank
- End-suction pump
- Air-side equipment
- Non chemical water treatment
A water-cooled chiller can have 3 types of compressors which I’ve mentioned earlier except for the scroll type. Most of the time, water-cooled chillers use a centrifugal compressor. Hence, they are named centrifugal chillers.
Nowadays, centrifugal chillers have a magnetic bearing. These chillers are oil-free and have zero friction and thus, they are more efficient. More and more buildings started to use magnetic bearing centrifugal chillers now.
Cooling towers generally have 2 types; a) counterflow and b) crossflow. Most cooling towers are counterflow and they can be seen on the roof of commercial buildings. Crossflow cooling towers offer better accessibility but they occupy a larger space.
Since cooling towers operate using the principle of evaporative cooling, the water evaporated at the cooling tower is refilled by the make-up water tank via gravity flow. Make-up water tanks are just basic water tanks assembled by multiple steel panels. Their construction is very similar to the plumbing cold water tank.
Chilled water pumps and condenser water pumps have 2 types; a) end-suction, and b) horizontal split case. End-suction pumps are more common than horizontal split case pumps in HVAC. Horizontal split case pumps are used for larger water flow. They are more expensive and occupied more space.
Air-side equipment is referring to air handling units (AHUs) and fan coil units (FCUs). They are called “air-side” because they only handle the cold air supply and return while the other equipment handles the chilled water supply and return.
AHUs are physically bigger and their capacity is larger. They are often installed in dedicated mechanical rooms. On the other hand, FCUs are smaller units that are mostly installed above the ceiling. FCUs have different types such as wall-mounted type, ceiling cassette types and ceiling concealed ducted type.
Normal water is used for chilled water systems but it is treated either chemically or non-chemical to maintain good water quality and prevent corrosion. Nowadays, many chilled water systems preferred to use non-chemical water treatment to eliminate the storage and handling of harmful chemicals.
The distribution network of the chilled water system can break into two parts; a) pipework, and b) ductwork. Pipework is responsible for the flow of chilled water while ductwork is responsible for the distribution of cold air.
Chilled Water System Pipework Components
The pipework of chilled water systems involved the following components:
- Polyurethane insulated carbon steel pipe
- Galvanized carbon steel pipe
- Mechanical valve
- Motorized valve
- Measuring device
Water-cooled chillers are connected to air-side evaporators by polyurethane (PU) insulated carbon steel pipes for chilled water supply. So far, the most common type of pipe used for chilled water is carbon steel pipes.
However, there is new technology emerging to replace carbon steel pipes. Check out my post to find out more about it [read post].
All air-side evaporators are connected to end-suction chilled water pumps and then back to the water-cooled chillers for chilled water return. Together, they form a closed loop of chilled water flow.
Chilled water often flows at a temperature around 4°C to 7°C for supply and 9°C to 12°C for return. Hence, insulations on chilled water pipe is a must to prevent condensation.
Since one side of the water-cooled chillers absorbs heat, the other side must reject heat. Chilled water is for heat absorption (cooling) and condenser water is for heat rejection.
Hence, the same water-cooled chillers are also connected to end-suction condenser water pumps and then to counterflow cooling towers by galvanized carbon steel pipes for condenser water supply.
Then, the counterflow cooling towers return the condenser water back to the water-cooled chillers. Together, they form an open loop of condenser water flow.
Throughout the pipework, mechanical valves are installed and they are used to isolate water flow for maintenance purposes.
Besides, motorized valves are used to control water flow rate automatically based on the design flow rate.
Moreover, there are measuring devices such as BTU meters for energy monitoring and flow meters for water flow control.
Air-Side Evaporators Ductwork Components
Most air-side evaporators such as AHUs and FCUs use a network of ducts to distribute air throughout a building. Generally, ductwork involved the following components:
- Fiberglass insulated rigid duct
- Fiberglass insulated flexible duct
- 4-way supply air diffuser c/w radial damper
- Return air grille
- Air volume control damper
Rigid ducts are mostly insulated with fiberglass. However, polyethylene-insulated rigid ducts are equally common. Rigid ducts are made of sheet metal with different gauges. Gauges are referring to the thickness of the sheet metal according to SMACNA standards.
Flexible ducts have fiberglass insulated type and non-insulated type. Flexible ducts used for air conditioning are always insulated using fiberglass, not other materials. If you want to know the different types of insulation materials, check out my post where I explained the 5 types of insulation used in air conditioning [read post].
Supply air diffusers have many different types but the 4-way diffuser is the most common one. Other supply air diffusers are linear diffusers, round diffusers and jet diffusers (commonly found in airports).
Return air grilles don’t have as much variety as supply air diffusers. They are mostly just basic grilles with some are bar grilles. However, they can be significantly larger than supply air diffusers.
Air volume control dampers are installed in between ducts that have a handle for people to manually adjust air volume. They are more common in the ductwork of large air conditioners such as AHUs.
Other systems associated with commercial air conditioning systems:
- Variable air volume distribution system – Involved VAV boxes and actuators for precision airflow control.
- Pre-cooled fresh air system – Introducing air-conditioned and filtered fresh air.
- Heat wheel energy recovery system – Recover the energy from exhaust air systems.
Most Important Type of Mechanical Ventilation System
Many people thought that mechanical ventilation is just basic ventilation but there it is more than that. The smoke spill system and the pressurization system are the two systems often found in buildings that save lives during a fire.
Smoke Spill System
Any above-ground compartments that exceed 1,000 m2 of floor area always require a smoke spill system. For anything below the ground, the smoke spill system is required regardless of the floor area. The main purpose of the smoke spill system is to clear away fire smoke in order for the people in the affected area to escape.
I have a post explaining the smoke spill system requirement in Malaysia. If you are not from Malaysia, you can skip the first section and continue reading the rest of the post because other requirements are applicable around the world since they are based on international standards [read post].
A smoke spill system can break into two parts; a) smoke extraction, and b) fresh air replacement. We can’t just have the suction without introducing the outdoor air. Else, the suction will not be effective due to vacuum.
Generally, a smoke spill system consists of the following components:
- Smoke spill vane axial fan
- Fire-rated duct
- Fire-rated cable
- Remote start panel
As far as I know, smoke spill fans have 3 types; a) vane axial, b) axial, and c) cabinet. The most common one that I’ve seen is the vane axial type of fan. They have slight differences in terms of cost, performance and occupied space.
Smoke spill ducts must be fire-rated in order to stay in operation during a fire. Most of the time, a fire-rated duct is either coated with fire-rated material or using 1.2mm thick metal sheets or more.
Similarly, fire-rated cables are needed for smoke spill systems to ensure that the system will not stop working due to melted cables.
A remote start panel is used for smoke spill systems to activate the smoke spill fans manually and remotely in case the automatic signal fails.
Staircases that don’t have natural ventilation must have a pressurization system. The lift lobby used for firefighters must also have a pressurization system. Together, they are called staircase and lift lobby pressurization system.
The staircase pressurization system and the lift lobby pressurization system operate with the same principle which is to force outside air into the space to prevent fire smoke from entering. However, they have slight differences in terms of control.
A staircase is connected from the ground floor all the way up to the highest floor. Hence, a pressurization system is able to pressurize the entire staircase.
On the other hand, lift lobbies are not interconnected. So, they are pressurized using the sandwich system.
I strongly suggest you read my post on the staircase and lift lobby pressurization system where I outlined the working principle and the requirement to save lives [read post].
In terms of components, the pressurization system is very similar to the smoke spill system. It needs fire rating components to keep the system functioning under high temperatures as well.
However, the pressurization system has a few extra components:
- Differential pressure sensor
- Motorized fire damper
- Variable speed drive
Differential pressure sensors are used in the pressurization system to measure and control the pressure inside the staircase and lift lobby. It needs to maintain about 50Pa of pressure. Not too much or too little.
Motorized fire dampers are used to perform the sandwich pressurization system for lift lobbies. Both the damper and the actuator used need to be fire-rated as well.
Actuators in HVAC are underrated. They are extremely important. Learn why in my post where I outlined the 4 common types of actuators used in dampers and valves [read post].
New pressurization systems have variable speed drives (VSDs) to improve pressure control. Pressurization fans with VSDs are able to regulate their fan speed based on the pressure feedback from the differential pressure sensor.
List of Components Involved in HVAC
There are many other components used in HVAC. They may not be major components but they are necessary for the HVAC system. Together, they form a complete HVAC system.
- Spring isolator – Used for smoke spill and pressurization fans.
- Inertia block – Used for chillers and pumps.
- Canvas – Installed before and after a fan to absorb the movement.
- Flexible joint – Installed at pipes to absorb vibration.
Water Flow Control
- Ball valve – Isolate small water flow fast.
- Butterfly- Isolate large water flow fast.
- Gate valve – Isolate small water flow gradually.
- Globe valve – Isolate large water flow gradually.
- Pressure independent balancing control valve (PIBCV) – Control water flow automatically.
- Fiberglass flexible duct – Used to connect from rigid ducts to supply air diffusers.
- Diffuser – Used for supply air.
- Grille – Used for return air, small fresh air and exhaust air.
- Louvre – Weatherproof external grille for fresh air and exhaust air.
- Plenum box – Used to reduce the air velocity before exit/enter a diffuser/grille [learn more].
- Variable air volume (VAV) box – Automatically adjust supply airflow with the aid of an actuator.
- Volume control damper – Allow manual airflow adjustment.
- Non-return damper – Allow one direction airflow only.
- Ultraviolet (UV) lamp – Installed near the cooling coil of AHUs to prevent bacteria grow.
- Air filter – Installed before the cooling coil of air conditioners to block dust.
- Smoke hog – Used to filter oil in kitchen exhaust systems.
- Heat wheel – Used to recover some energy from the exhaust system.
- Energy recovery ventilator (ERV) – Used to recover some energy from the exhaust system.
- Programmable logic controller (PLC) – Used to control the HVAC system.
- Thermostat – Used to control the operation of air conditioners.
- BaCnet & Modbus – Communication protocols used in HVAC.
- Internal duct insulation – Used to reduce noise of air conditioners.
- Rockwool acoustic panel – Used to contain the noise inside a HVAC equipment room.
- Circuit breaker – Prevent electrical overload.
- Earth leakage protector – Prevent excessive earth leakages.
- Digital power meter – Power, voltage and current monitoring.
- Fire relay – Fire tripping and run signal.
- Starter – Used to start fans and pumps.
- Angle iron – Used to support ducts.
- Galvanized rod – Used to support ducts.
- Anchor – Used to secure the galvanized rod.
- Duct collar – Installed on rigid ducts for flexible duct connections.
- Steel band – Used to support flexible ducts.
- Cable lug – Used for cable connections.
- Conduit – Used for small cables.
- Trunking – Used for medium cables.
- Cable tray – Used for large cables.
- Y-strainer – Used to filter dirty in pipes.
- Sensor port – Used for the ease of sensor removals/insertions.
- Concrete plinth – Used as the base for large equipment to sit on.
- L-bracket – Simply wall mounting bracket for split outdoor units.
- Openings – Wall and slab openings for pipes, ducts and cables.
There is a lot more to air conditioning and mechanical ventilation, and HVAC as a whole. It may seem daunting at first but once you start to learn, it’s not that hard to understand. It’s important that you break down the system and components in HVAC to pick up quickly.
In the meanwhile, consider subscribe to my newsletter to get notified on new posts. I publish frequently and consistently on everything about HVAC from basics to technicals, as well as career-related topics. Getting into HVAC means there is a long journey ahead of you and I hope my blogs help you on this journey.