Should Return Ducts be Larger than Supply?

Should Return Ducts be Larger than Supply?

The network of ducts that exist behind your walls are important because they allow air to cycle to and from your heating and cooling system. These tubes connect to every room in your home, and their existence facilitates the movement of air. 

Supply ducts carry conditioned (heated or cooled) air into your rooms, while return ducts suck conditioned air back into the heating or cooling system through a system of vents.

Should return ducts be larger than supply? Yes, return ducts are usually larger than supply ducts to ensure that air within the home is balanced. There are generally more supply vents in the system creating the need for return vents to be larger. Ensuring that there is sufficient air supplied and returned is critical to maintaining an ambient environment.

In this article, we will explain how ducts and vents work in heating or air conditioning systems. We’ll look at the challenges they may present and suggest some solutions to ensure that you can manage your system as efficiently as possible.  

What are Ducts?

A duct is a tube or passageway that provides a conduit between rooms in a building. Heating, ventilation, and air conditioning (HVAC) systems use air ducts to deliver and remove air.

These ducts provide the pathway for an HVAC system to ensure a comfortable environment within your home.

Ducts are made from a range of materials and most commonly from sheet metal. Below are the most common materials used to make air ducts.

Galvanized Steel 

Galvanized steel is the most common material used to make ducts because of its high strength and durability. Because this steel has a zinc coating, it doesn’t rust or require painting. It expands and contracts as it heats and cools and is a very workable material.


Aluminum is the next most popular material because it is lightweight and resistant to corrosion and moisture. It is readily available and quick to install. 

Pre-Insulated Ductwork

Modern homes often have ductwork made from rigid insulation panels of polyurethane or fiberglass. These pre-insulated ducts are lightweight and quick to install. They can be modified on-site and allow for less air leakage.

Flex Material

Manufacturers can make ductwork from a flexible plastic shaped over a metal coil.

These are generally used to connect outlets to rigid ductwork and are easy and convenient to use, but technicians use them in short lengths because they tend to allow pressure loss.


Fabric ductwork is generally made of polyester and is lightweight. It is a lower-cost material and disperses air effectively (source)

Image by Tama66 via Pixabay

History of Ductwork

Modern ductwork owes its origins to the innovations of the Chinese and the Greeks in the 7th century BC with their use of flues and chimneys, which helped to keep buildings warm and eliminate smoke (source). 

Later, the Romans used vertical tubes underneath tiles to heat a room.

From about AD 1300, pipes were used to direct steam or hot water to heat buildings and was the primary method of heating buildings. This concept gave birth to the modern radiator that we are familiar with today.

Conversely, air conditioning developed in the early 1900s using water-filled ducts that sprayed a mist and cooled the air through evaporation.

Technology around both heating and cooling systems made rapid progress throughout the twentieth century.  Central air that can heat or cool, while delivered through the same ductwork, was only developed after World War II (source).

Today, technology has advanced to such a degree that HVAC systems are prevalent in homes across the US, offering energy-efficient systems that rely on advanced technology. 

Supply and Return Ducts

Supply Ducts & Vents

Supply ducts blow air into your home and connect to supply vents. These vents usually have slats (behind a grill), allowing you to direct airflow.

Supply vents in your home are easy to identify – hold a piece of paper or even your hand in front of a vent, and if air blows out, then this is a supply vent. 

When you run an air conditioner, you receive cool air from the supply vents, and when you run a heater, warm air is expelled from the supply vents.

Return Ducts & Vents

Return ducts pull air out of your home to deliver to your heating or cooling system and connect to return vents. Return vents are also located behind a grill, but they don’t have slats or louvers and are generally larger in size than supply vents. 

If you hold a piece of paper or your hand in front of them, you will feel a suction effect. 

Return vents also generally have a filter behind them, which prevents dirt, dust, or other contaminants from entering the heating or cooling system and causing damage. Conversely, supply vents don’t require a filter.

Where Should Vents be located? 

There are many mistakes that contractors can make when they install ductwork and vents in a home. For the most effective system, ventilation should be located in every room, and ideally in the right place.

There are no definitive rules for the specific place in a room they place vents, but there are some well-established practices for doing so.

Supply Vent Location

The ideal location for supply vents is on the floor or baseboards, and there should be at least one located in this position in each room.

Heat rises and, therefore, if the vents are in the ceiling, the heat will escape before it can warm the room. This will also help keep the room comfortable in summer. 

As a result, the ductwork should be located in the basement and crawlspace (for a second story). That way, the heat travels the shortest distance from the floor. This setup will ensure a more efficient and cost-effective system as well as good air circulation. 

Additional supply vents are ideally placed below a window. They will then create a barrier against the chill of a cold window (windows will always be cooler than the walls in cold weather).

The opposite will apply in summer, keeping the cool air flowing in front of hot glass.

Return Vent Location

Return vents are connected to separate ductwork, which draws the air in and returns it to the heating or cooling system. 

Installers should place these vents should on the opposite side of the room from the supply vents to prevent a situation where the air goes directly from the supply vent back into the system and to allow air time to rotate through the room.

Return ductwork can be wherever it is most convenient and doesn’t necessarily need to run through every room in the house. For example, one may choose not to have air from kitchens and bathrooms returned into the system (source)

Vent size

Vents will vary in size depending on the size of the room and the room’s heating or cooling requirements. If a vent is too small, this will force the system will to push air through at a faster rate, and this can create a disturbing noise. 

A reputable contractor should calculate the size and number of vents required in each room so that your system runs optimally.

Can You Cover a Vent?

Many people think that covering or closing vents in unused rooms will save energy. However, this is not the case and can result in various issues that will compromise the smooth operating of your system. The most common of these are detailed below.

System Damage

This could restrict the airflow within your system and cause coils to freeze — in the case of air conditioning — and damage your compressor.

Covering a vent causes pressure to build up within the system, which will interfere with the efficient operation of your unit and could even cause it to stop working altogether.

Burst or Leaking Ducts

The pressure caused by blocking vents could result in a duct bursting or leaking. There is usually some minor leakage already from poor connections in ductwork, but this could result in a significant leak. 

When this happens, your heated or cooled air will potentially escape outside, and your system won’t run efficiently.

It is important to maintain your ducts regularly by having them professionally serviced and never to close vents and compromise their functioning.

Lower Efficiency

Your system is generally designed to suit the size of your home and circulates the correct amount of air for that space. By closing one or more ducts, the same amount of air is then forced through fewer ducts, causing pressure to build within the system. 

Your system will then have to work much harder to maintain temperature and distribute air, with the result that it will not last as long or perform as efficiently.

Carbon Monoxide

There is a risk of the heat exchanger cracking when vents are closed and pressure within the system is too great. These cracks can release carbon monoxide into your home, which is a potentially deadly risk. 

It is important to have a carbon monoxide detector installed within your home and to keep air vents open to lower the risk of carbon monoxide poisoning.

Carbon monoxide is a colorless and odorless gas that is highly toxic and can be fatal. The effects of inhaling this gas on individuals will differ depending on their amount of exposure as well as their age and health. 

When you inhale too much carbon monoxide, your body replaces the oxygen in your red blood cells with the gas, which can lead to tissue damage or death.

Mold Growth

Closing a vent can result in condensation because the temperatures in that room will potentially be lower. Excess moisture causes mold and mildew, which cause an unpleasant smell and will require professional removal from your ducts.

Mold and mildew exposure is not healthy. It can cause irritation to eyes, nose, skin, and lungs and can exacerbate issues with allergies and asthma (source)

Can You Have too Much Return Air? 

Under most circumstances, it is not possible to have too many air returns. Your system can only move a finite amount of air, so that will remain the same regardless of how many return vents there are or how large.

If you add more returns, the system will just split the return through those returns, but the total amount of air will remain the same.

Can You Have too Little Return Air?

Homes that have a well-designed and efficient HVAC system and are airtight are generally comfortable and enjoy superior air quality.

Some systems are not optimally designed, especially if the contractor installing the system has skimped on airtightness or adapted an old HVAC system. 

A common issue is when homes have a central return duct. This generally results in more significant air leakage and low air quality. To save costs, many builders utilize just one return — typically in the upstairs hallway in two-story homes. 

When bedroom doors are closed, there is not sufficient return air, and the system brings air from wherever it can find it — usually, outdoor air or air from the attic, depending on how the system is set up. 

Too little return air is also an issue in homes where the return ducts or vents are too small for the size of the rooms, or there are too few vents.

The system will be unable to circulate all the air back into it, forcing it to run longer and increase the amount of power used.

beige bed with mattress
Image by Aw Creative via Unsplash

Ways to Address too Little Return Air

This is a relatively easy issue to fix, thereby increasing airflow, reducing infiltration, and experiencing less energy loss from your system.

Air must be able to return to the system and, therefore, it is essential to address any issues with your return vents and ducts.

Add More Return Vents

Adding more return vents is the best solution if it is possible. Here you can add vents to each bedroom that doesn’t already have one if you’re able to connect to a vent on the other side of the wall or add to existing ductwork.

Install a Transfer Vent

A transfer vent is a cutout in the wall, usually above a bedroom door, with a grille on each side. It opens an air passage into the room, allowing air to transfer freely. You could also place this at the bottom of the bedroom door.

Install Jumper Duct

A jumper duct is similar to a transfer vent applied to the ductwork.

Here, a duct is installed above a room that connects to the central return, allowing air to “jump” from the room to the central area. This is a quieter solution than transfer vents but is more costly (source).

Other Issues that Decrease Airflow

Even with adequately sized ducts, other issues can negatively affect airflow, such as those detailed below. 

Dirty Air Filter

There is a filter on your return vent to keep dirt out of the HVAC system. If this becomes dirty, it will restrict the amount of air it can take in. Filters need to be checked at least once a month and changed or cleaned when they become visibly soiled.

Leaking Air Ducts

When ducts have leaks, they cannot transport all the air to and from your rooms. Such leaks can create areas that are too hot or cold. You also need someone to service your ducts regularly to ensure the system is running optimally (source).

What Direction Should Air Vents Face?

It is advisable to keep air vents open. The system is designed to circulate a specific amount of air within the house, and this is compromised when vents are closed.

Return vents don’t have slats, so you can’t position them. Supply vents have louvers, and you can set them for your optimal comfort.

Cold weather 

You should set supply vents facing downwards when supplying heat because hot air rises and will, therefore, heat the room as it moves upwards.

Warm weather 

You should position supply vents facing upwards if they are delivering air conditioning to displace the hot air sitting higher in the room.

Generally, air vents are not directed at the center of the room because that can be uncomfortable. There are other options that you can consider if you want to redirect air or soften airflow. These include vent covers and vent deflectors.

Vent covers come in various designs. Wooden covers with thicker slats break up airflow more effectively than metal or plastic versions. Decorative covers with intricate patterns can also help to provide a less concentrated airflow. 

You can use vent deflectors if you want to direct airflow. Their main function is to:

  • Prevent drafts by deflecting airflow away from specific areas
  • Direct airflow away from plants, equipment, or drapes
  • Direct airflow away from walls and furniture, evening temperatures

If a piece of furniture or equipment is blocking a vent, then you can remedy this by using a vent extender that will guide air out from whatever is covering it and flow into the room. 

Final Thoughts 

Efficient and well-designed duct systems distribute air effectively throughout your home, keeping it at a comfortable temperature.

Supply and return flow must be balanced to maintain a neutral pressure and to allow the system to work according to its design

It’s essential to consult a professional to ensure there is sufficient supply and return and minimal leaking of your heated or cooled air.


I'm a Pharmacist and a passionate researcher into clean air and pure water for the home. I believe these 2 elements play a significant role in our health and overall wellbeing.

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