You rely on your HVAC system to keep your home nice and warm in the winter and pleasantly cool in the summer. But there’s more to how an HVAC system works than “makes air hot” and “makes air cold.” 

In this guide, we will cover the intricacies of how an HVAC system works, including the process furnaces use for heating air and the process central air conditioners use for cooling it. 

What Is an HVAC System?

An HVAC system is a collection of indoor and outdoor components that work together to control temperature, maintain indoor air quality, and ensure proper ventilation in your home. The specific configurations of HVAC systems can vary based on where and when they were installed. However, most HVAC systems have a common set of components: a furnace, an air conditioner, a thermostat, ductwork, and vents or registers. 

How Does HVAC Work? 

A home HVAC system works by taking in air and either heating or cooling it and then distributing that air throughout your home. The target temperature is set by a thermostat, which tells the furnace or the AC unit the current temperature in the home, which then dictates whether or not more warmed or cooled air is produced and sent through the ductwork. 

A furnace and air conditioner are the main components of an HVAC system. Here’s how they work: 

How Does a Furnace Work? 

A furnace operates by employing various heat sources to raise the temperature of air, which is then distributed throughout a building via ductwork. The process is simple and it works like this:

Combustion process 

Most residential furnaces use natural gas as the primary fuel source. When the thermostat detects that the indoor temperature has dropped below the set point, it signals the furnace to start. The gas valve opens, allowing a controlled flow of natural gas into the combustion chamber.

Ignition and burners

Once the gas is flowing, an ignition system (such as a pilot light or electronic igniter) ignites the gas-air mixture. This produces a controlled flame within the combustion chamber. The burners are designed to evenly distribute the flame, ensuring efficient and consistent heating.

Heat exchanger

The combustion process generates a significant amount of heat, which is captured and transferred to the air passing through the heat exchanger. The heat exchanger is a component made up of a series of metal tubes or coils, and as the combustion gasses flow over the exchanger’s surface, heat is transferred to the metal, heating it up.

Blower fan

A blower fan is positioned after the heat exchanger. Once the heat exchanger reaches the desired temperature, the blower fan starts. It draws in air from the surrounding area and forces it over the hot heat exchanger. The air absorbs the heat from the exchanger’s surface, becoming heated in the process.

Air distribution

The now-heated air is directed into the ductwork of the HVAC system. Ducts carry the heated air to various rooms and spaces within your home. The warm air is released through registers or vents, warming the space to the desired temperature as set by the thermostat.

Flue and exhaust

The combustion process produces byproducts, including carbon dioxide and water vapor, along with small amounts of other gasses. These byproducts are safely expelled from the combustion chamber through a flue or exhaust pipe, which leads to the outside of the building. Proper ventilation is required to ensure that these gasses do not enter the indoor air, which is one of the main reasons why we recommend you keep up with regular maintenance. 

How Does an Central Air Conditioning Work?

Central air conditioners use a more complicated method for cooling air than furnaces use for heating it. The process of cooling air is called a closed-loop refrigeration cycle, which is used to transfer heat from indoors to outdoors. A closed-loop refrigeration cycle works like this:

Evaporator coil and indoor unit

The process begins with the evaporator coil, typically located inside the air handler or furnace. Warm indoor air is drawn into the air handler, where it passes over the cold evaporator coil. The refrigerant inside the coil evaporates, absorbing heat from the surrounding air. In addition to physically cooling the air, moisture from the air is condensed, lowering humidity levels and therefore lowering perceived warmth.

Compressor

The refrigerant vapor, now carrying the absorbed heat, is then compressed by the compressor. Compression increases the temperature and pressure of the refrigerant, converting it into a high-pressure, high-temperature gas.

Condenser coil and outdoor unit

The high-pressure, hot refrigerant gas is sent to the condenser coil, located in the outdoor unit of the air conditioner. As outdoor air flows over the condenser coil, the heat from the refrigerant is released. This causes the refrigerant to condense back into a liquid.

Expansion valve

The high-pressure liquid refrigerant flows through the expansion valve, which reduces its pressure and causes it to expand. This expansion results in a drop in temperature, turning the refrigerant back into a low-pressure, low-temperature vapor.

Evaporator coil (again)

The now-cooled and expanded refrigerant vapor returns to the indoor evaporator coil to repeat the cycle. As warm indoor air passes over the cold coil, heat is once again absorbed from the air, and the process continues. 

Air distribution

While the air conditioner itself is primarily responsible for cooling the air, the blower fan of the air handler or furnace is used to circulate the cooled air throughout the ductwork and into the rooms of your home.

Trust the HVAC Experts

Now that you know how HVAC systems work, you may have a better appreciation for how complex heating and cooling can be. That’s why we always recommend trusting a professional HVAC technician for HVAC maintenance, repair, or installing a new system. And when the time comes, you can give us a call to set up an appointment.