How Does Your HVAC Duct System Work?

hvac duct in a room

An HVAC duct system is responsible for keeping your indoor air clean and comfortable. It both circulates the air to improve your indoor air quality and transports heated and cooled air to keep the home at a comfortable temperature. Here’s an explanation of how your HVAC duct system works so you can better understand your home.

The basics of an HVAC duct system are as follows; your indoor air is sucked into the return ducts by the blower fan, cycled through the air conditioner or furnace, and sent out of the supply ducts.

What is an HVAC Duct System?

An HVAC duct system isn’t as complicated as you might think. It’s a network of ducts typically made of galvanized steel or aluminum. The ductwork is often unobtrusive, concealed in walls, ceilings, basements, and attics. 

These ducts take in air from the different rooms of your home, the air travels through the return ducts, is cycled through and conditioned by the air conditioning system or furnace, and the conditioned air goes through the supply ducts back into the rooms.

HVAC Duct Systems: Step-by-Step

closeup of air duct on a new ceiling
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An HVAC ductwork system’s process can be divided into three basic steps; intake, conditioning, and return. Each step is outlined below:

Step 1: Intake 

First, there’s the intake step. Your HVAC’s return air vents suck in the indoor air, which then travels through the intake portion of your ductwork, passes through the air filter, and reaches the blower fan. The blower then sends the air into your system’s conditioning or heating unit. 

Step 2: Conditioning 

Once the air reaches the air conditioner or furnace, the conditioning step starts. It’s either heated or cooled, depending on how you have your thermostat set. 

To cool air, the system circulates it through an evaporator system, which uses coils and liquid refrigerant to remove heat. The heat is then dispersed outdoors.

If you have a combined heating and cooling system, however, a furnace or heat pump warms the air instead.

Step 3: Supply 

Lastly, you have the supply step. This step is possibly the simplest out of the three. Once the air has been properly conditioned, it’s simply blown through the supply ducts and back out into your home for the process to repeat itself.

The Parts of an HVAC Duct System

Each step of the HVAC duct system’s process involves a lot of different parts. Every part is vital for keeping the indoor air quality up and the temperature at a comfortable level. Here’s a list of the different parts of an HVAC system and a short explanation of their function:

  • Return Ducts: Air ducts that take in air from your living space and deliver it to your air conditioner or heating system.
  • Supply Ducts: Air ducts that deliver conditioned air from your air conditioner or heating system to your living space.
  • Grilles: A cover for the vents that regulates the airflow.
  • Registers: Types of grilles with dampeners that allow you to control the airflow.
  • Plenum: A box that attaches to the AC unit and is responsible for air distribution. There are typically two plenums in a system; a return plenum and a supply plenum.
  • Air Handler: The HVAC unit itself. This is the part that heats and cools the air, then circulates it throughout the system.
  • Filter: A filter catches contaminants in the air so that only relatively clean air circulates through the rest of the system.
  • Flue: A pipe that vents harmful gases into the outside air.
  • Blower: The blower fan is a part in the plenum that pushes air through the system.
  • Evaporator: Evaporator coils absorb heat from the air through liquid refrigerant to cool the air.
  • Dampers: Valves that regulate the flow of air inside the ducts.

HVAC Duct System Problems

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Of course, like most anything else, an HVAC system doesn’t always work as it should. When there’s a problem with your home’s HVAC duct system, there will be certain signs. Being able to recognize them will help you know when your system needs service, which will improve your home’s indoor air quality and possibly save you money. Here are a few signs that something’s not right with your HVAC duct system:

  • Weak airflow
  • Strange noises
  • Odd smells
  • Humidity in the home
  • Pockets of warm/cool air
  • Leaks
  • Frequent/short cycles
  • High energy costs/utility bills

FAQ About HVAC Duct Systems

Why is the airflow from my vents weak?

If the airflow from your vents is weak, there are a few possible reasons. Here’s a short list of possible causes for poor airflow:

  • Clogged AC fans
  • Loose fan belt
  • Stuck wheels
  • Broken motor
  • Dirty air filter

However, there’s no way to diagnose weak airflow without a close examination. You should contact a professional HVAC technician so that they can diagnose the exact issue and come up with a plan for repairs.

Should I get my air duct system cleaned?

You can. It’s not an absolute requirement, but air duct cleaning does have its benefits; efficiency, safety, and pest control among them. If you’re doing any major home renovations, it’s a good idea to get the air ducts cleaned once they’re done since they can deposit a lot of dust and debris into the duct system.

Is it okay for my HVAC to run all day in hot weather?

Yes. If you live in areas with high temperatures, your air conditioning system can likely run continuously without safety concerns. Most modern systems are designed to run all day, especially ones in areas known for their hot climate. However, you will likely see a greatly increased energy bill, so be careful when running your AC constantly.

Find an HVAC Expert

An HVAC duct system might seem complicated, but once you know the basics of how it works, it becomes a lot simpler. You don’t need to be a professional HVAC technician to understand how your air ducts and ventilation system function.

If your HVAC system needs service, contact your local HVAC professionals. They can help you with HVAC maintenance, repairs, or installation to help keep your indoor air comfortable.

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Austin Geiger

Austin Geiger is a writer who's passionate about home care. He enjoys writing about home maintenance practices, as well as projects to turn an outdoor space into a backyard paradise.