The main parts of an HVAC system are the thermostat, the furnace or heat pump, the evaporator coil, the condenser, the compressor, the blower, the ductwork, and the air filter. Each part has a job, and the system only works well when every part does its job.
Additionally, if you are buying a home, or just want to understand the equipment in your own home, knowing these parts makes an inspection report much easier to read.
At Edifice Inspections, our ASHI-certified inspectors look at this equipment in homes around Peachtree Corners and Greater Atlanta. Below, you will find what each part does, how the parts work together, and what can go wrong.
What Is an HVAC System?
An HVAC system is the equipment that heats, cools, and moves air through your home. HVAC stands for heating, ventilation, and air conditioning. Many people use the word to mean only the air conditioner or only the furnace. In reality, a full system includes both, plus the parts that control, move, and clean the air.
Most houses use a central, forced-air system. One set of equipment makes warm or cool air, and a network of ducts carries it to every room. The table gives a quick look at each part. The sections after it explain them one at a time.
| Part | What it does | Where you will find it |
|---|---|---|
| Thermostat | Tells the system when to turn on and off | On a wall inside the home |
| Furnace | Makes heat with gas, propane, or electric elements | Basement, closet, attic, or garage |
| Heat exchanger | Passes heat to the air without mixing with exhaust | Inside the furnace |
| Evaporator coil | Pulls heat and moisture out of indoor air | On or in the furnace or air handler |
| Compressor | Pumps refrigerant through the system | Outdoor unit |
| Condenser coil and fan | Releases heat outside | Outdoor unit |
| Refrigerant lines | Carry refrigerant between the indoor and outdoor parts | Between the two units |
| Blower | Pushes air through the ducts | Inside the furnace or air handler |
| Ductwork | Carries air to and from every room | Walls, attic, and crawl space |
| Air filter | Catches dust and debris | Near the blower or at a return vent |
| Vents and registers | Deliver and return air in each room | Floors, walls, and ceilings |
| Condensate drain line | Carries away water the coil removes from the air | Near the indoor unit |
Control and Air-Moving Parts
The thermostat, blower, ductwork, filter, and vents control and move the air. These parts work all year, in both heating and cooling season, so they see more wear than most people expect.
Thermostat
The thermostat is the control center. It reads the temperature in your home and tells the system when to run. When the room drifts away from the temperature you set, the thermostat sends a signal to the furnace or air conditioner.
A smart thermostat does the same job and adds schedules and phone controls. A thermostat placed in direct sun, or near a kitchen, can read the wrong temperature and make the system run at the wrong times.
Blower and Air Handler
The blower is a large fan that pushes air through the ducts. In a home with a furnace, the blower sits inside the furnace. In a home with a heat pump or electric heat, the blower sits in a cabinet called an air handler, which also holds the indoor coil.
Without airflow, warm or cool air never reaches the rooms. A weak or noisy blower is a common reason some rooms feel hotter or colder than others.
Ductwork
Ducts are the metal or flexible tubes that carry air through the house. Supply ducts bring heated or cooled air to each room. Return ducts pull air back to the system. Ducts often run through attics, crawl spaces, and walls, so gaps and loose joints can waste air you already paid to heat or cool.
Sealed, insulated ducts keep that air where it belongs.
Air Filter
The air filter catches dust, pet hair, and other particles before they reach the blower and coil. A clogged filter blocks airflow, which makes the whole system work harder.
Many standard filters need a change every one to three months, and the packaging will tell you its own schedule, though how often to change a furnace filter can depend on your home, pets, and the type of filter you use.
Vents and Registers
Registers are the covers where air comes out of the ducts into a room. Return grilles are the larger openings where air goes back in. Furniture, rugs, and curtains that block them cut airflow and can leave a room uncomfortable.
Heating Parts of an HVAC System
The furnace makes the heat, and the heat exchanger moves that heat into your air. A heat pump can do the same job without burning fuel. Here are the parts that matter most.
Furnace
A furnace creates heat. A gas furnace burns natural gas or propane in burners. An electric furnace warms the air with heating elements. A gas furnace also has an igniter, a gas valve, and a vent pipe, called a flue, that carries exhaust gases outside.
Heat Exchanger
The heat exchanger is a metal chamber inside a gas furnace. The burner flames heat the metal, and air from the blower flows across the outside of it. The two streams never mix. A crack in the heat exchanger can let combustion gases, including carbon monoxide, reach the air in your home.
That is one reason inspectors look closely at gas furnaces, and why every home needs working carbon monoxide alarms. If an alarm has you worried, it helps to know how to reset a carbon monoxide alarm and what to check before you do.
Heat Pump in Heating Mode
A heat pump is both a heater and an air conditioner in one outdoor unit. In winter, it pulls heat from the outside air and moves it indoors. In summer, it runs in reverse and moves heat out. A part called the reversing valve switches the direction. Many heat pumps also have electric backup heat strips that turn on during the coldest weather.
Cooling Parts of an HVAC System
An air conditioner does not make cold air. It moves heat from inside your home to the outdoors using a liquid called refrigerant. Five parts make that happen.
Evaporator Coil
The evaporator coil sits indoors, on top of or inside the furnace or air handler. Cold refrigerant flows through the coil while the blower pushes warm indoor air across it. The coil absorbs the heat and also pulls moisture out of the air. A dirty coil cannot absorb heat well, and that’s one reason clean coils matter during a home inspection.
Compressor
The compressor is the pump in the outdoor unit. It squeezes the refrigerant, so it becomes hot and moves it around the loop. When the compressor fails, the system stops cooling, and it is often one of the most costly parts to replace.
Condenser Coil and Fan
The condenser coil is the large, finned part of the outdoor unit. Hot refrigerant flows through it, and a fan pulls outdoor air across the fins. That air carries the heat away. Plants, leaves, and grass clippings packed against the unit block airflow, so keep the area around it clear.
Refrigerant Lines
Two copper lines connect the outdoor unit to the indoor coil. One line carries refrigerant to the coil, and the other carries it back. The larger line is usually wrapped in foam insulation. Torn or missing insulation is easy to spot and worth fixing.
Condensate Drain Line
As the evaporator coil cools the air, water forms on it. That water drips into a pan and flows out through the condensate drain line. Atlanta summers are hot and humid, so your system makes a lot of water, and a clogged line can back up, leak, or shut the system off.
That’s why water dripping from an AC is worth checking out before it turns into a bigger problem.
How the Parts of an HVAC System Work Together
The parts work as a team, and the thermostat starts the process. Here is what happens in each season.
In heating mode:
- The thermostat senses the room is too cool and calls for heat.
- The furnace ignites and heats the heat exchanger.
- The blower pushes air across the heat exchanger and into the supply ducts.
- Warm air leaves through the registers.
- Cooler air returns through the return ducts and passes through the filter to start the loop again.
In cooling mode:
- The thermostat senses the room is too warm and calls for cooling.
- The compressor and condenser fan start in the outdoor unit.
- Refrigerant carries heat from the indoor coil to the outdoor coil.
- The blower pushes indoor air across the cold evaporator coil.
- Cool, drier air flows through the ducts, and the extra water drains away.
Types of HVAC Systems
The parts stay mostly the same from one system to the next, but the layout changes. These are the four setups you will see most often.
- Split system: The compressor and condenser sit outside. The evaporator coil and furnace or air handler sit inside.
- Packaged system: All the main parts live in one outdoor cabinet, often on a slab or roof. Ducts connect it to the house.
- Heat pump: One system that heats and cools, with an outdoor unit and an indoor air handler.
- Ductless mini-split: An outdoor unit connects to one or more wall-mounted indoor units. There are no ducts.
If you replace part of a split system, the U.S. Department of Energy advises matching the new outdoor unit with a new evaporator coil, because an old coil can lower efficiency and shorten the life of the new equipment. You can read more in its guide to central air conditioning.
HVAC Parts a Home Inspector Looks At
A home inspector runs the heating and cooling with normal controls and checks the parts they can see and reach. Inspectors do not take equipment apart, and they are not a replacement for an HVAC technician. They do notice the problems that show up on the surface.
During a home inspection, an inspector may look at:
- The age and rating label on the furnace, heat pump, and outdoor unit
- Whether the system turns on and responds to the thermostat
- The air filter and how clean it is
- The outdoor unit’s condition and the space around it
- The foam insulation on the refrigerant lines
- The condensate drain line and pan, and any water stains nearby
- Rust, scorch marks, or damage around the furnace
- The flue pipe on a gas furnace
- Air coming out of the registers
Age matters too. ENERGY STAR suggests thinking about replacing a heat pump or air conditioner that is more than 10 years old, and a furnace or boiler that is more than 15 years old. Their replacement guidance explains how to decide. If you are buying a home, an older system is something to plan for in your budget.
Heating and cooling equipment is one of the highest costs in a house, so it pays to know what you are buying. Our home inspection covers the HVAC system along with the electrical and plumbing systems.
Related Questions to Explore
What are the main parts of an HVAC system?
The main parts are the thermostat, furnace or heat pump, heat exchanger, evaporator coil, compressor, condenser, refrigerant lines, blower, ductwork, air filter, and vents. Together, they heat, cool, and move air. The table near the top of this post lists what each part does and where it sits.
What are the parts of a central air conditioner?
A central air conditioner has an outdoor unit with a compressor, condenser coil, and fan. Indoors, it has an evaporator coil, a blower, and a condensate drain line. Refrigerant lines connect the two sides, and ducts carry the cooled air to each room.
How does an HVAC system work?
The thermostat calls for heating or cooling. In heating mode, the furnace or heat pump warms the air, and the blower sends it through the ducts. In cooling mode, refrigerant moves heat from the indoor coil to the outdoor unit. Return ducts bring air back, and the filter cleans it on the way.
What is the difference between HVAC, a furnace, and an air conditioner?
HVAC is the whole system for heating, cooling, and moving air. A furnace is only the heating part, and an air conditioner is only the cooling part. A heat pump can handle both jobs in a single unit.
What does a home inspector check on an HVAC system?
An inspector tests the system with normal controls and looks at the visible parts, such as the filter, outdoor unit, refrigerant line insulation, drain line, and furnace area. They note age, damage, and signs of trouble. If something looks wrong, they may recommend a licensed HVAC contractor take a closer look. See our home inspection page for what is included.
When to Call a Professional
Some HVAC care is simple. You can change the filter, keep plants away from the outdoor unit, and keep registers clear. Opening the furnace cabinet, handling refrigerant, or repairing electrical parts is work for a licensed HVAC contractor.
Call a professional if you notice any of these:
- The system does not heat or cool at all
- A burning smell or loud banging, squealing, or grinding sound
- Water around the indoor unit or ice on the refrigerant lines
- The system turns on and off again and again
- Some rooms stay much hotter or colder than others
- Energy bills climb with no other explanation
Leave the home right away and call your gas company or 911 from outside if you smell gas. Evacuate everyone outside and call 911 the moment a carbon monoxide alarm sounds. Have a licensed HVAC contractor evaluate the system before closing if a home inspection report flags an issue.
A home inspection is the best place to start, because it shows you the condition of the whole house. Edifice Inspections serves Peachtree Corners, Greater Atlanta, Newnan, and Fairburn.
Conclusion
Knowing the parts of an HVAC system helps you ask better questions, read an inspection report with confidence, and spot trouble early. The key points to remember:
- The thermostat controls the system, and the blower, ducts, and filter move the air.
- The furnace or heat pump makes heat, and the compressor, coils, and refrigerant lines handle cooling.
- Age, airflow, and water leaks are the most common signs that a system needs attention.
Want a clear look at the HVAC system in a home you are buying? Schedule your home inspection online today.