HVAC Ductwork – The Complete Guide To Air Ducts In Your Home

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Having an efficient HVAC system in your home is critical for keeping the air clean and comfortable. But what many homeowners don’t realize is that the performance of their HVAC system depends largely on the condition of the ductwork. Poorly maintained, outdated, or inadequately designed duct systems can lead to higher energy costs, poor airflow, and even bad indoor air quality.

Fortunately, understanding how your home’s ductwork works and how it affects your indoor environment doesn’t have to be complicated. In this guide, we’ll explain everything you need to know about HVAC ducts—from types of materials used to installation tips and maintenance advice—to help ensure you get maximum performance from your heating and cooling system.

What Is Ductwork?

Ductwork is an essential part of a home’s heating, ventilation, and air conditioning (HVAC) system. It consists of pipes or ducts that transport heated or cooled air to and from the central heating and cooling unit throughout the house. This allows you to control temperature and humidity levels in different areas of your home.

What are the different types of ducts used in HVAC systems?

picture of a residential ductwork system

As mentioned, ductwork includes pipes or ducts to transport heated or cooled air. But not all HVAC duct systems are created equal.

Here are the different types of ducts you can use in HVAC systems, their benefits and drawbacks, installation tips, and maintenance advice—everything you need to know about making sure your home has an efficient HVAC system.

Air Handler

residential air handler

While your home’s air handler isn’t technically part of your ductwork, it is important to consider it when looking at your HVAC system’s ductwork. An air handler is the unit that houses the blower, evaporator coil, filters, burner, and other components of your HVAC system. All of the ductwork in your home connects back to the air handler.

That is why it is critical to ensure the air handler is properly sized for your house. For example, if the air handler is undersized, it may not be able to apply enough pressure and airflow inside your ductwork. This could lead to poor heating and cooling in your home.

Supply Air Ducts

Supply air ducts are the pipes that transport heated or cooled air from the central heating and cooling unit to the various rooms in your home. These ducts come in various sizes and materials, including metal, rigid fiberglass, flexible plastic, and insulated foam.

Metal is typically used for the supply of air ducts because it is durable, easy to install, and cost-effective. However, metal ducts can be prone to corrosion over time.

Fiberglass is also a common choice because it is lightweight and resistant to corrosion, but it may not be able to handle the airflow required for larger HVAC systems. Insulated ducts are ideal for preventing heat loss in the winter and heat gain in the summer, but they are more costly and may not be suitable for all HVAC systems.

Most newer homes have a combination of rigid, flexible, square, and round ducting.

Rigid Ducts

rigid air ducts

Rigid ducts are a type of metal or fiberglass ductwork that are typical in HVAC systems. These ducts are more durable and can handle higher pressures, making them an ideal choice for homes with large air handlers. Rigid ducts are also easier to install than other types of ductwork because they come in pre-cut sections that are designed to fit together with minimal modifications.

Note: The downside of rigid ducts is that they can be prone to condensation and mold growth because the metal or fiberglass material does not allow for insulation. This makes rigid ducts more suitable in climates with milder temperatures. Most HVAC professionals will use rigid ducts for plenum and the main trunk of the ductwork.

Flexible Ducts

flexible air duct

Flexible ducts are a type of air duct that is made from a flexible material, such as plastic or insulated foam. They come in long sections that can be easily bent and cut to fit into tight areas, such as attics and crawl spaces or in areas with tight turns or corners.

Unlike rigid ducts, flexible ducts come with insulation, which can help reduce heat loss or gain and helps control sound transmission. They are also easier to install than rigid ducts because they don’t require any special tools or cutting equipment.

However, flexible ducts have a lower pressure rating than rigid ducts, so they are not suitable for use with larger air handlers. That said, flexible ducts are compatible with large air handlers if they are installed far enough downstream from the air handler where the pressure is lower. They tend to have shorter lifespans and are more prone to leakage over time than rigid ducts.

Round Ducts

Round ducts are a type of metal or fiberglass ductwork ideal for short runs and tight turns. They come in rigid and flexible variations. Rigid round ducts come in pre-cut sections that can easily fit into small spaces, making them an ideal choice for retrofits and renovations. Flexible round ducts are typically made from plastic or insulated foam and can be easily bent to fit in tight spaces as well.

Round ducts have become increasingly popular for HVAC systems because of their versatility. They are also easy to install, which is why they are often used in residential applications. However, round ducts tend to be less efficient than rectangular ducts because of the increased friction caused by the bends and curves, which reduces airflow and decreases your HVAC system’s efficiency.

Square Ducts

Square ducts are typically used for larger HVAC systems. They only come in rigid styles and are more common in residential applications because of their durability and cost-effectiveness. Square ducts are also easier to install than other types of ductwork because they fit well between floor joists and wall studs.

Square ducts are more efficient than round ducts because the airflow is not hindered by bends or curves, which allows for greater air volume and velocity. However, square ducts tend to cost more than other types of ductwork because the metal materials cost more.

Return Air Ducts

return air duct

Return air ducts are the pipes that transport warm or cool air from the various rooms in your home back to the central heating and cooling unit. Return air ducts usually connect to the grilles/registers near the ceiling and suck air from the room back to the air handler.

These ducts can also be made of metal, fiberglass, or plastic and come in a variety of shapes and sizes. Return air ducts are usually insulated to help reduce energy loss while transferring warm or cool air.

Note: Return air ducts should not be confused with supply ducts, which move warm or cool air from the central HVAC unit into the rooms.

Registers and Grilles

hvac air register or grille

Registers and grilles are the parts of your HVAC system that control air flow into and out of the ducts. You will find them in your home’s ceilings, walls, or floors to direct forced warm or cool air into a room, while grilles are used to return air back to the central unit.

Typical Air Flow in an Air Duct System

typical duct airflow

The typical airflow in an air duct system is from the central unit to the rooms via supply ducts and then back to the central unit via return air ducts.

The temperature and speed of the airflow are regulated by the air handler’s blower along with registers and dampers, which you can adjust to be more or less open/closed according to your desired climate control settings.

HVAC Duct Design Rules and Problems

hvac duct design

Common ductwork mistakes include improper sizing of the ducts, not using the correct type of ducts for the application, and poor duct installation. Improperly sized ducts may result in weak airflow from the vents or insufficient heating and cooling. Additionally, using the wrong type of duct may cause efficiency issues as some are better suited for certain applications than others.

Here are the common design rules and issues.

Proper Air Handler Sizing

First off, if the air handler is sized too small, your ducts will never be able to transport enough air to keep your house warm in the winter and cool in the summer. If it is oversized, the air handler will cycle too quickly (known as short cycling), resulting in higher energy costs and uneven heating and cooling in your home.

Although this isn’t a ductwork problem, many homeowners with improperly-sized air handlers, furnaces, or central AC units, may incorrectly suspect issues with their ductwork because of the low airflow.

Proper Duct Sizing and Type

When installing ducts, it is important to make sure that the size of the duct is proportional to the size of the air handler. If your ducts are too small, they will not be able to transport enough air and your home won’t get properly heated or cooled. If a duct is too long, rooms at the end of the run won’t get adequate airflow. For longer runs, wider ductwork is required to ensure airflow to rooms far from the air handler.

Additionally, if you choose a type of ducting unsuitable for the application, you may end up with an inefficient system. For example, rigid metal ducts are better suited at transporting large amounts of airflow than flexible plastic or fabric ducts because they do not sag or collapse easily and can’t handle higher pressure and airflow like their non-metal counterparts.

You can use an HVAC Duct Calculator to help calculate which ducts suit your blower.

Minimal Turns and Bends

Turns and bends leads to friction and pressure loss, resulting in poor heating and cooling in your home. Whenever possible, use straight runs of ducts and avoid turns and bends to ensure that your system is running as efficiently as possible. Of course, some turns and bends are required to run ducts to every room in your home. An experienced HVAC contractor will be able to properly design the system so that there are minimal turns and bends without compromising airflow.

Proper Installation

Ductwork should be securely fastened to the walls or ceiling joists using straps and screws. If not done properly, sections of ducts can sag or come apart over time resulting in air leaks which will further decrease the efficiency of your system. Additionally, all joints should be sealed with mastic or foil tape to prevent air leakage and ensure that the airflow is properly directed to the rooms in your home.

Ultimately, having a well-designed and properly installed ductwork system is essential for the efficient heating and cooling of your home. With good design rules, an experienced HVAC contractor can ensure that your system runs efficiently and without problems.

Correct Number of Supply and Return Registers

It’s important to have the correct number of supply and return registers in each room. While it is possible to oversize supply or return registers, it is not recommended as it can lead to short cycling of your system and inefficient airflow.

Additionally, having too few registers means air cannot be properly distributed throughout the home. Again, HVAC designers can recommend the appropriate number of supply and return registers for each room in your home.

Minimal Ducts Coming Off Plenum

The plenum is the chamber at the end of your air handler where ducts are connected. When connecting ducts to your plenum, it’s important to minimize the number of ducts coming from it.

During operation, each additional branch reduces the amount of airflow that can be delivered to each room in your house, resulting in inefficient heating and cooling. An experienced HVAC designer can minimize the number of duct branches coming off from the plenum without compromising airflow in the home.

If your home is larger and has many rooms, your HVAC designer will have to design the HVAC system with many ducts branching from the plenum. But to ensure each zone in your home receives adequate airflow, they will install balancing dampers on the ducts that cut off airflow entirely to one or more areas in your home.

For example, the main floor and upstairs ductwork may each have a damper. With this setup, just the main floor zone can be heated or cooled for one hour with the upstairs damper closed. Then, the upstairs floor’s damper can open (and the main floor one can close) to allow the air handler to provide warm or cool air in that zone.

Proper Takeoff Placement

When designing ductwork, HVAC designers must ensure that they place the takeoff correctly. The takeoff should be installed as close to the plenum as possible without compromising airflow in other rooms. Installing takeoffs too far away from the plenum can lead to a pressure drop and reduce the efficiency of your system.

Additionally, installing the takeoff directly on top of the plenum can lead to one area in your home getting too much airflow and others not getting enough.

How is HVAC ductwork sized and designed?

Professional HVAC designers will use variety of tools to ensure that your system is properly sized and designed.

They will take into account the: 

  • Size of your home
  • Number of rooms and floors
  • Type of HVAC system you have
  • Local climate, 
  • Amount of shade your home gets 
  • Local building codes

If you’re getting a new furnace, central AC, and/or heat pump, the HVAC designer will use a Manual J calculation to determine the size of your system. This calculation takes into account the square footage, local climate, and other factors to determine how much cooling and heating capacity you need.

They will also use specialized software to calculate the air velocity, air pressure, and volume of air that needs to be delivered for each room in your home. Additionally, they will use specialized tools to measure the airflow coming out of each register in the home. This will help them make sure that no part of your house is under or oversupplied with heated or cooled air.

Common Home Duct Problems

HVAC technician checking HVAC ductwork

Here are some of the most common duct problems that can occur in your home’s ductwork:

1. Leaking Ducts

This is one of the biggest problems with ductwork and can lead to energy loss. Leaks in the ducts can be caused by poor installation or joints that have not been properly sealed with mastic sealant or aluminum foil tape. This can be particularly problematic in unconditioned areas, such as attics and crawlspaces.

To fix this issue, you’ll need to find the source of the leak and seal it up with mastic sealant or aluminum foil tape. But if there are large holes or gaps between two duct sections, it is best to hire an HVAC contractor to replace the entire section of ductwork.

2. Long and Winding Ductwork

Ductwork that has too many turns and is longer than it should be can lead to decreased airflow in your home. This means rooms in your home may not be getting enough heated or cooled air, resulting in uncomfortable temperatures.

To resolve this issue, you’ll need to hire an HVAC contractor to inspect the ducts and reroute them to make them shorter and less winding. This will help ensure that your system is running efficiently and that all the rooms in your home are getting enough heated or cooled air. Alternatively, the contractor may recommend a larger air handler. There’s no a one-size-fits-all solution for this issue.

5. Improper Takeoff Placement

The takeoff, which is the point where air enters the duct system from the plenum, needs to be installed properly for your system to run efficiently. If not placed properly, it can lead to inconsistent airflow in your home and cause certain rooms to be too hot or cold.

If the takeoff is not in an ideal location, you can call an HVAC contractor to relocate it.

6. Unsupported Ductwork Sections

Without adequate support, sections of ductwork can sag and create restrictions that impede airflow. This problem is especially common in attics and basements where the ducts are not properly supported by joists or rafters.

To fix this issue, you’ll need to hire an HVAC contractor to add extra supports to the sagging sections of ductwork. This will help ensure that the ducts remain properly supported and improve airflow throughout your home.

7. Blocked Airflow from Obstructions

Blocked airflow is another common issue with ductwork. Obstructions in the ventilation system, such as insulation, can block airflow and cause rooms to be too hot or cold.

To fix this problem, you’ll need to remove any obstructions and ensure your ventilation system is free of blockages. If necessary, you can also call an HVAC contractor to inspect and clear any blockages in the ductwork.

8. Too Many Bends in the Ductwork

Bends in the ductwork can reduce airflow, resulting in rooms that are too hot or cold. This is especially common with older homes, as many of them were not designed for efficient HVAC systems.

To fix this issue, you’ll need to hire an HVAC contractor to inspect the ducts and reroute them to minimize the number of bends. This will help ensure that your system is running efficiently and that all the rooms in your home are getting enough heated or cooled air.

9. Not Enough Supply or Return Registers per Room

Inadequate supply or return registers can lead to rooms in your home being too hot or cold. This is because the system may not be able to deliver enough air to multiple rooms, or it may not be able to draw enough air back into the return register for efficient operation.

Adding additional supply and return registers is the best way to fix this issue. However, this type of project can be expensive and disruptive to your household as your installer may have to remove drywall. Another solution is to install ductless mini-splits, which allow you to control the temperature in each room independently. You can install these without tearing open walls.

How can I tell if my HVAC ducts are leaking or damaged?

Leaking or damaged ductwork can lead to inadequate air flow, higher energy bills, and potential health hazards. To determine if your HVAC ducts are leaking or damaged, you’ll need to hire a professional HVAC company to inspect and test the system. They will use specialized tools such as an airflow meter and pressure gauge to measure the amount of air coming out of each register and check for leaks. They can also use a smoke machine to test the ducts for any leaky spots.

If you suspect that your HVAC ducts are leaking or damaged, it’s best to hire an HVAC professional to inspect and repair them as soon as possible to ensure your system is working properly and efficiently.

HVAC Ductwork Installation

technician installing HVAC ductwork

Ductwork installation is best left to professionals as it can be a complex and time-consuming process. The installation team will need to measure the existing ductwork, fabricate new sections as needed, and properly secure all components of the system.

The professional HVAC installer will also have to consider any local building codes that may apply to your project in order to ensure that your system is installed correctly and safely.

If you have any questions or concerns about HVAC ductwork installation, it’s best to hire a professional HVAC company that can answer all your questions and takes care of the job for you.

Can I install Ductwork Myself?

Yes, you can install ductwork yourself, but you should hire an HVAC expert unless it is a small job (or you have HVAC experience). This ensures that all building codes are followed and the system is installed correctly.

Can you put ductwork in an old house?

Yes, you can put ductwork in an old house. However, you will likely need to knock out plaster or drywall in every room to add the ductwork. Additionally, you will need to make sure the existing structure is sturdy enough to accommodate the additional weight and stress of the new ducts. It’s best to consult with a professional HVAC installer if you are planning on adding new ductwork to an old home.

How To Maintain Your Home Ductwork System

Maintaining your ductwork is essential to having an efficient and reliable HVAC system. Here are some tips for keeping your home ductwork system in good shape.

Replace Filter Regularly

Change the air filter regularly to keep your system running efficiently and reduce strain on the motor. The HVAC filter prevents dust, hair, dirt, etc., from getting into the system and clogging it up.

You should replace your HVAC filter every 1-12 months, depending on the filter type, your home’s condition, and local climate. For example, homes with lots of shedding pets or in areas with wildfires should replace their filters more often.

Keep Registers and Grilles Clear

Ensure that all registers and grilles are clear of any obstructions such as furniture, rugs, toys, clothes, etc. This helps ensure proper airflow and keeps the system working efficiently.

Routinely Look for Damage, Blockages, and Leaks

Check for any signs of damage or wear and tear, such as rust, cracks, or broken seals. If any are found, you will need to contact a professional HVAC technician for repair or replacement. You should also look for blockages and leaks during your inspections. Blockages can restrict airflow, while leaks can lead to energy loss and higher utility bills.

Clean Ducts

Most HVAC professionals recommend cleaning your ductwork every 2-5 years. This will help keep your ducts free of dust and debris, which can reduce energy costs and improve indoor air quality.

HVAC contractors or duct cleaning service companies have special vacuum cleaners that can snake through your ductwork and remove dust, hair, and other buildups.

Keep Vents Open

Make sure all vents are open and unobstructed to allow for proper airflow. Almost all HVAC systems are designed to run with all the vents open, so closing some can strain the system and lead to inefficiency.

Setup Annual Service and Maintenance

Have your HVAC system serviced annually to ensure it runs properly and efficiently. During service, a technician will check the system for any signs of damage, leaks, blockages, or other issues. They can also provide maintenance and repairs as needed.

Frequently Asked Questions

Here are some common questions about ductwork.

Is it worth it to replace ductwork?

Yes, replacing old and inefficient ductwork is worth it, as it can help improve the efficiency of your HVAC system. It can also reduce energy costs and improve indoor air quality.

Is it better to undersize or oversize ductwork?

It is better to undersize ductwork, as it allows for more efficient airflow and helps reduce energy loss. Oversized ducts will cause low pressure and airflow, which can drastically reduce efficiency and increase utility bills.

What are the benefits of sealing and insulating HVAC ducts?

Sealing and insulating HVAC ducts can help reduce energy loss, improve indoor air quality, and make your home more comfortable. It helps prevent unwanted air leakage and temperature fluctuations in different rooms of the house. This can also save you money on energy bills over time.

How much does it cost to install or repair HVAC ductwork?

The cost of installing or repairing HVAC ductwork depends on the size and complexity of the job. Generally, you can expect to pay around $500-$3,500 for a basic installation or repair job. However, for more complex projects, costs may be significantly higher. It is best to consult a few professional HVAC contractors in your area for a proper cost estimate.

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Jonathon Jachura
Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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