How Do HVAC Systems Work?

Hvac Systems

There are a lot of moving parts for HVAC techs to consider on every job they take on. To keep things running smoothly, it’s crucial that they have a solid understanding and knowledge of each major system and its complexities.

From mechanical components to specialized chemicals, we’ll answer the basic question; how do HVAC systems work?

Components of an HVAC System

There are a lot of pieces to the puzzle when it comes to HVAC. This is why it’s important for technicians to have an understanding of each component and how it contributes to the overall HVAC system.


To understand condensers, you must first understand the process of cooling. It starts with a gaseous refrigerant that is compressed until it becomes a liquid, which is then evaporated and allowed to condense so it can turn into a liquid again. You can think of it as a cycle of compression and expansion, or evaporation and condensation, as the end result is the same.

The condenser, then, is the device that allows the gas to return to a liquid state again, extracting heat and transferring it to outside air. In the case of a refrigerator, automotive AC, or most residential AC units, the condenser consists of loops and loops of tubing along with fins that help increase surface area to transfer heat out of the unit, with a fan that blows the heat out as exhaust.

Evaporator Coil

When we’re thinking about an HVAC system as a cycle of condensation and evaporation, the evaporator coil would be the counterpart of the condenser. It’s the first stop in this closed-loop system after the compressor, holding chilled liquid refrigerant from that component. The evaporator coil is located inside the blower compartment or air handler, where air moving past the unit speeds up evaporation.

The evaporation process isn’t just cooling though; it’s actually heat transfer, drawing heat from a home or building. The refrigerant becomes hotter and is then directed toward the outdoor condenser unit, where it can be expelled to the outside as exhaust.

Refrigerant Lines

Refrigerant lines are the tubing that carries the refrigerant through the system, from the compressor to the evaporator and condenser, then back to the compressor again. This process is controlled by valves located along the refrigerant lines.


The thermostat is an electrical device inside the home that senses ambient air temperature and then regulates HVAC functions to keep the home at a set temperature. It’s a closed-loop device that works to close the gap between desired temperature and actual temperature.

They’re generally designed with a thermistor, semiconductor, or bimetallic strip, to detect when ambient air reaches the desired temperature, and they then send an electrical signal to the HVAC system to control the unit.


The air that’s coming from the HVAC unit or furnace has to be distributed to the home somehow, and that’s where the vents come in. The blower forces air through a network of ducts, and the vents (or registers) are outlets for the ducts that allow air to circulate into the rooms.

How A Residential Air Conditioning System Works

With knowledge of each component you can now pull together each of the HVAC components and learn about how their place in the system contributes to the overall function of the HVAC cycle.

  • Liquid refrigerant transforms back into a gas again when it makes it to the evaporator, with a fan blowing past the evaporator to speed up the process
  • The fan loads up the liquid refrigerant with heat from inside air
  • The gas (or vapor, really) makes it to the outdoor condenser, where it releases its heat and turns into a liquid again
  • An outdoor fan blows out the heat from the condenser as exhaust
  • Now in the form of a cool liquid, the refrigerant makes it back into the compressor
  • The compressor forces the liquid through the system, completing the cycle

The Importance of Ventilation

Though not directly mentioned in the cycle above, proper ventilation is critical to a properly working air conditioning system. In fact, poor ventilation is one of the top causes of HVAC airflow problems . Simple blockages can lead to an iced-over system or a system that just shuts down completely.

The upside is that most of the time, it’s an easy fix. That’s why it’s important to be familiar with what to look for, including:

  • A dirty Filter
  • An obstructed duct or register
  • A leaking duct
  • A dirty secondary filter in the system
  • Failing blower, due to a bad switch or relay or bad motor

How A Furnace Works

The furnace is a component of an HVAC system, however, it’s considered the heart of central heating . In the furnace, the air is warmed using gas, induction, or electricity, and is then distributed throughout the building using ductwork. Some are paired with a boiler, which can serve as a water heater for the house or can circulate heat or hot water to heat the home.

Understanding the central heating system relies on having knowledge of these basic principles:

  • Natural gas or propane is ignited in a burner, or an electrical current heats up an element, just like in a toaster or electric stove
  • In the case of a gas or propane furnace, the flames heat up a metal heat exchanger
  • The heat exchanger (or electrical heating element) emanates heat to the outside air
  • The heated air is forced through the ductwork and registers via a blower
  • Cooler air in the home is drawn back into the system again through the return ducts and filter, completing the cycle

Combustion gases from the unit are then exhausted through the chimney or vents. Old-fashioned brick chimneys are almost a thing of the past, being replaced by modern, high-efficiency gas furnaces or heat pumps that need nothing more than a PVC pipe tube routed through the side of the house.

Make HVAC Learning Easy with SkillMill

HVAC systems are intricate, with many different components working together to perform both heating and cooling on any type of building. With that said, whether you’re new to the industry and just learning how HVAC works or are an experienced journeyman who is interested in continued learning, it’s important to solidify your knowledge in the field. For this, the SkillMill online HVAC training platform is a great resource.

SkillMill’s seasoned instructors teach courses using animation, quizzes, and 3D simulations to walk techs through all sorts of real-world scenarios. VR courses are also available and give techs hands-on experience in a safe environment. With SkillMill, you can advance your knowledge and skills anytime, anywhere.