How Does an HVAC System Work?

Most of us have a general idea of what an HVAC system does – it keeps our homes cool in the summer, warm in the winter, and comfortable all year round. Whether it’s blasting cold air in July or pumping out heat in January, it’s easy to take that comfort for granted.

But do you actually know how your HVAC system works? Understanding just a bit about how it all operates can help you recognize when something’s not quite right, make sense of what your technician is saying, and even guide you when it’s time for a repair or replacement. Let’s break it down in a way that makes sense.

Central HVAC: The Most Common Setup

Most homes in the U.S. use a central HVAC system, which means there’s a centralized system pushing air through a series of ducts and vents. It’s meant to serve an entire building or home. Depending on your climate, your home might have:

• A furnace + air conditioner (common in colder areas)
• A heat pump + air handler (popular in milder winter climates)
• A heat pump + furnace (also known as a dual-fuel system, for efficient year-round comfort)

Each of these setups works a bit differently, but they all share the same basic goal: maintain a comfortable indoor temperature.

Furnaces

Let’s start with furnaces. Furnaces heat air and distribute it through your home using a blower and a series of ducts. There are a few types of furnaces – gas, electric, and oil – but they all operate on the same basic principle. Here’s how each works.

• Gas Furnace: A gas furnace, the most popular type of furnace in the U.S., works by mixing natural gas with oxygen in the combustion chamber, where it’s ignited to create heat. This heat is then transferred to the heat exchanger, which in turn heats the air that is circulated by a blower fan.
• Oil Furnace: An oil furnace works similarly to a gas furnace but uses heating oil instead of natural gas as its fuel source. The oil is stored in a tank – usually located outside the home – and is pumped into the combustion chamber, where it’s mixed with air and ignited to produce heat. The heat warms the heat exchanger, and the blower distributes the heated air throughout the home.
• Electric Furnace: Electric furnaces use electric heating elements to generate heat directly. Air passes over these elements, warming it before a blower distributes it through the home’s ductwork.

Furnaces are especially common in northern states where winters are long and cold. They’re great at producing quick, powerful heat and are often paired with central air conditioners to handle cooling in the summer.

Central Air Conditioners

Central air conditioners do the opposite of furnaces; they remove heat from your home and push cool air in. A central AC system has two main parts: an indoor air handling unit and an outdoor condenser unit.

An air conditioner works by removing heat and moisture from the air inside your home to keep it cool and comfortable. It starts when warm indoor air is pulled into the system and blown across evaporator coils filled with cold refrigerant. As the refrigerant absorbs heat from the air, it also pulls moisture from it, helping to balance your home’s humidity levels.

The warmed refrigerant flows to the outdoor unit. There, the compressor pressurizes the refrigerant and sends it to the condenser coil, where the absorbed heat is released into the outside air. Once cooled, the refrigerant cycles back to the evaporator coils indoors, and the process starts all over again.

Central AC is often bundled with a furnace in one cohesive system that shares ductwork and a thermostat. It’s the most familiar setup in homes across the U.S.

Heat Pumps

A heat pump is a versatile system that can heat and cool your home using the same technology as an air conditioner, but in reverse when needed.

In cooling mode, a heat pump works exactly like an air conditioner. But in the winter, it reverses the process:

1. The outdoor unit pulls heat from the air (yes, even when it’s cold outside).
2. That heat is transferred indoors using refrigerant.
3. Warm air is distributed through your ducts by an air handler or furnace.

Because they move heat rather than generate it, heat pumps are very efficient, especially in milder climates. In colder regions, they’re often paired with a furnace that kicks in when temperatures drop too low (typically below freezing), creating a dual-fuel system.

What’s an Air Handler?

If you don’t have a furnace, your heat pump will be connected to an air handler. This is basically the indoor half of the heat pump system.

A heat pump air handler contains a blower fan that circulates air through your home and an evaporator coil that either absorbs or releases heat depending on the season. It works with the heat pump to move heated or cooled air through your ductwork.

Unlike a furnace, it doesn’t generate heat through combustion; it relies on the refrigerant system to do the heavy lifting.

Ductless Mini-Splits

Not every home has ductwork, and that’s where ductless mini-splits come in.

A ductless mini-split system has one outdoor condenser unit and one or more indoor air handlers mounted on the walls of your rooms. The two units are connected by a conduit, which houses the refrigerant tubing, power cable, and condensate drain.

Each unit operates independently, allowing for zoned temperature control, which is perfect for homes with additions, garages, or rooms that always seem to run hot or cold.

Mini-splits use heat pump technology, meaning they can provide both heating and cooling. They’re efficient and an ideal choice for homes where ductwork isn’t practical.

Boilers

Boilers are a type of heating system that are more common in older homes or colder climates like the Northeast. Instead of pushing air, boilers heat water and send it through a network of pipes to radiators or baseboards.

They can be powered by gas, oil, or electricity and are known for providing comfortable, steady heat without blowing dry air. Boilers are typically very durable and can last for decades with proper maintenance. However, because they don’t provide cooling, many homes with boilers use window units or mini-splits to stay cool in the summer.

Geothermal Systems

Geothermal systems take HVAC to another level. These systems use underground pipes to pull heat from the earth in the winter and dump heat back into the ground in the summer.

While installing geothermal is expensive upfront, the energy savings are significant, especially if you plan to stay in your home long term. It’s one of the most efficient, sustainable options out there, and it can provide both heating and cooling with minimal environmental impact.

Geothermal systems are paired with a specialized type of heat pump, which works with an air handler to move air throughout the home. Because ground temperatures stay relatively constant year-round, geothermal systems operate more efficiently than traditional air-source heat pumps.

Thermostats

Last but certainly not least, let’s talk about thermostats – the brains of your HVAC system. A thermostat monitors the temperature in your home and tells your heating and cooling equipment when to turn on or off to maintain your desired comfort level. When the indoor temperature drifts above or below your setting, the thermostat sends a signal to your HVAC system to kick into action.

There are several types of thermostats, ranging from basic manual models to programmable and smart thermostats. Manual thermostats require you to adjust the temperature by hand, while programmable ones let you set schedules for different times of day. Smart thermostats take it a step further by learning your habits and allowing remote control through your phone or smart home system. They can also learn your temperature preferences over time and make automatic adjustments.

Common HVAC Setups at a Glance

Here’s a quick look at the most common HVAC combinations you’ll see in homes:

Why This All Matters

Understanding how your HVAC system works isn’t just good trivia; it can help you:

• Spot issues early, like airflow problems or odd noises
• Know when it’s time to call a pro (and explain what’s going on)
• Make better decisions when replacing or upgrading your system
• Improve energy efficiency and comfort in your home

Whether you’re battling uneven cooling in July or your furnace is on the fritz in January, having a clear picture of how things work behind the scenes can save you money and headaches in the long run.