Heating, ventilation, and air conditioning (HVAC) systems make use of ducts and vents to cycle air around your house. These connect rooms within the home and allow the system to heat or cool the interior 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 vents.
Can an air return be too big? No, an air return cannot be too big, except in extreme cases where a closed room is temporarily under negative air pressure. Return vents maintain air pressure, filter out debris, and are critical to the efficient operation of any HVAC system. The system moves a finite amount of air so that it will remain the same regardless of the number of air returns or how big they are.
In this article, we will explain how air returns function in HVAC systems and how important it is that supply and return are balanced. We’ll look at how best to install air returns and some of the challenges they can present.
We’ll also suggest some solutions to ensure that you can manage your system as efficiently as possible.
How Do HVAC Systems Work?
HVAC technicians design HVAC systems to ventilate buildings. They bring outdoor air inside, circulate it, and then return it to the outside.
This ventilation happens naturally via air movement through windows and doors, or it can happen mechanically, as is the case with HVAC systems.
Mechanical ventilation is more controllable and responsive. It aims to condition the air in an indoor space by either heating or cooling it to increase comfort for the occupants.
There are many different types of HVAC systems, but they all share the following basic elements:
- Air intake from outside
- A system to handle air (heating, cooling, filters, etc.)
- Distribution of air
- Exhaust of air
Generally, the system draws in outside air through a filter, and the air is then heated or cooled as it circulates through the building’s distribution system of ducts and vents.
Most buildings then include a return air system whereby it returns conditioned air to the air-handling unit through one of the following:
- Ducted returns
- Plenum returns
Ducted returns utilize air collected from each room through return air devices (usually vents) in the ceiling, floor, or walls that are directly connected to the building’s ductwork and feed into the air handling unit.
With Plenum returns, the air is collected from several spaces through devices (again, usually vents) that empty into the plenum — the space between the structural ceiling and the suspended ceiling or under a raised floor — and then returned to the system through ducts (source).
History and Development of HVAC Systems
The origin of HVAC systems goes back thousands of years to rudimentary heating and cooling systems devised by the Greeks and Romans and the early uses of chimneys in Europe.
However, it was only during the early twentieth century that the principles of contemporary systems were explored and tested.
The first air conditioner was devised in 1902, using fans to control humidity by circulating coolant in a printing plant in New York. This first machine established the principles of an HVAC system, which seeks to control the following five factors:
- Air circulation
- Indoor air quality
These early designs in air conditioning were advanced in the early 1900s, with air conditioning systems becoming more common in public spaces during the 1920s.
The first window-ledge air conditioning units were developed in the 1930s, making it more accessible for domestic use.
Demand continued to increase during the 1940s, and the first peaking power plant was built to service the high demand for electricity to run air conditioning during the summer months. By the mid-1950s over a million units were sold annually.
During the 1970s, central air became common in the US, and there was much research focused on increasing efficiency.
This research has continued to the present day, and customers now have multiple options to choose from when installing an HVAC system (source).
Role of Ducts & Vents in HVAC Systems
Ducts and vents play an essential role in the functioning of HVAC systems. Supply ducts, which are connected to supply vents, blow conditioned air into your home while return ducts, connected to return vents, pull used air out of your home.
In this way, they function together to ensure air is continually circulating through an area.
Supply vents expel heated or cooled air into the rooms of your home, and you can feel them blowing the air out. Return vents do the opposite, and you can feel them sucking air from the room.
Supply vents usually have slats that allow you to direct airflow, while return vents don’t have slats and are generally larger.
Both vents generally have a grill in front of them. Return vents usually have a filter behind them, which acts to prevent dirt, dust, and other contaminants from entering the HVAC system and causing damage.
Purpose of Return Air Vents
Return air vents work by sucking air into the HVAC system. They ensure that the system conditions air from inside the house — not from outside — and that air pressure indoors remains equalized.
The chief roles of return vents are as follows:
- Maintain air pressure
- Maintain air quality
- Reduce energy bill
Maintaining Air Pressure
Return vents maintain pressure inside a home by pulling conditioned air and pumping it back into the system, giving it a place to escape.
Because an HVAC system essentially functions as a recirculating pump, this function is essential to ensuring pressure stays under control.
If there isn’t enough space for the air to escape, the pressure inside the home will build up, and the environment will be uncomfortable.
Maintaining Air Quality
Return vents maintain air quality by filtering out debris. Dust particles and pet dander collect in the air of your home — you can visibly see dust dancing in beams of light.
The return’s filter will collect this dirt and ensure the circulation of clean air through the system.
It is, therefore, vital to ensure that filters are maintained and kept clean so that your air quality is optimal. Experts recommend filters be cleaned every two weeks and possibly more often in very dusty or polluted environments.
Saving on Energy
If your ducts are properly installed and don’t have leaks, they can contribute dramatically to energy savings. Allowing conditioned air to escape through leaking ducts means the system won’t run efficiently and will be more costly (source).
The ideal size of vents varies depending on the room’s requirements for heating or cooling, as well as the room’s size. If vents are too small, the system may have to force air through, and this can result in a disturbing sound.
Typically, supply vents are located in each room of the house. Ideally, there should be an equal number of return vents, but this is often not the case, and many homes have only one or two return vents.
In two-story houses, there is usually one on each floor. Because there are often fewer return vents, they tend to be sized larger than supply vents.
A typical supply vent is 4 inches by 10 to 12 inches, and a standard return vent would be 16 inches by 20 inches or larger.
Positioning Return Vents
Return entries, if there aren’t any in every room, are generally placed in central locations, especially places where cold air flows naturally, such as hallways and passages.
If you’re having return vents placed in each room, then they should be on the opposite side of the room from the supply vents to allow air the opportunity to circulate before returning to the system.
If you have them placed too close to the supply vents, then conditioned air could return directly back into the system without being able to rotate through the room first.
Installers can place return ductwork wherever convenient and, unlike supply air, does not need to be in a specific position in the room. One can also choose not to have return air in spaces such as kitchens and bathrooms.
Homes built with a single central return duct sometimes present issues with insufficient return air. Especially when bedroom doors are closed, there is not enough return air available, and this forces the system to bring air from wherever it can find it.
This alternative air can be from outdoors — especially if there are leaks in the ductwork — or from the attic or crawl spaces.
If there are too few return air vents, or if they are too small for the area, the system will struggle to circulate the air effectively, and it will have to run longer and use more power, resulting in greater inefficiencies in the system.
Covering Return Vents
The design of return vents means they should not be covered, and doing this will affect the operation of your HVAC system.
When vents are closed, the pressure in the room is affected because the system will continue to send the same amount of air according to its initial design.
Homeowners often think that covering them won’t make any difference because they aren’t sending warmed or cooled air into the room. Besides damaging your system, covering return vents could result in any of the following issues listed below.
Low airflow can impact comfort levels in the room as the system will struggle to maintain a constant temperature.
Increased Energy Use
The system will have to work harder and consume more energy to overcome insufficient airflow.
The same amount of air will be forced through fewer ducts, making it harder for the system to maintain temperature and distribute air. Over time, this will increase the cost of electricity.
Increased Duct Leakage
The increased pressure caused by blocking vents could cause a duct to either burst or leak. Although there are often minor leaks in ductwork from poor connections, this could result in a sizable leak that will compromise your system.
Ductwork needs to be maintained regularly to ensure they are functioning correctly.
Carbon Monoxide Leaks
A rare complication that can result from too much pressure is causing the heat exchanger to crack and to release carbon monoxide into your home.
The release of these gases is life-threatening, making it a must to have a carbon monoxide detector installed in your home and never cover vents.
In winter, condensation can result if surface temperatures drop. This condensation leads to the growth of mold, which can leave an unpleasant smell and is not healthy for your environment (source)
Can a Return Be Too Big?
In theory, it is infrequent for an air return to be too big for a system. Returns don’t actively suction air but rather act as an exit point for higher pressure air to return to the system.
The design of the supply system is to pump out a specific amount of air, and the returns will, therefore, channel that same amount of air through whatever means are available.
If there are too many returns, or if they are bigger than necessary, the air will naturally divide between them but will still be the same finite amount of air.
With doors closed, a single room could temporarily be put under negative pressure if there is too much return air, but this would right itself once someone opens the doors.
Can a Return Be Too Small?
When there is insufficient return air, the pressure inside will build. This pressure will cause heated or cooled air to escape wherever it can and results in an inefficient and expensive system.
This is often the case in homes built with a single central return duct.
Especially when bedroom doors are closed, the air inside the closed rooms will become positive relative to the rest of the house and will push through any available openings. The opposite effect happens in the rest of the house, and the pressure becomes negative.
This negative pressure can cause several issues, such as drawing in unpleasant air, the infiltration of unconditioned air, and back-drafting.
Unpleasant Air from Outside
If air gets pulled in from outside, it could bring fumes from the garage or stagnant air from crawl spaces or attics.
The system may be forced to pull unconditioned air from outside if it can’t pull sufficient air from the conditioned spaces.
If the negative pressure reaches an area with a natural gas water heater, air may come down the flue instead of exhaust gases releasing up the flue. This negative pressure could result in carbon monoxide coming down the flue and poisoning inside air (source).
Issues that Affect Air Return
Various issues could cause your air returns to function less than optimally. We have already discussed what happens if they are undersized but other issues that could affect them include a dirty air filter, a dirty grill, or leaking ducts.
Dirty Air Filter
Your return vent usually has a filter inside the grill that is designed to keep the HVAC system clean. It traps dirt, pollen, dander, and other airborne contaminants.
If the filter is soiled, it will naturally not be able to take in as much air, and this will impact the effective functioning of your system.
Filters should ideally be checked monthly and should be cleaned regularly and changed when necessary. It is important to clean them correctly, as outlined in the following steps:
- Undo the screw or clip holding the frame and swing it open
- Slide out the filter
- Remove all lint from the filter (vacuuming works best)
- Wash the filter thoroughly in warm water
- Allow it to dry out completely
- Replace filter in frame and fasten it back in place
The grill receives the dirty air before it gets to the filter. As a result, it can very quickly become soiled and can also restrict airflow.
You should regularly clean the grill, which you can do with a vacuum cleaner. You can clean any thick grime with a degreaser.
If there are significant leaks in your return air ducts, the air they carry from the rooms will be compromised.
These leaks can create areas that are too hot or too cold. You should have your ducts regularly serviced to ensure the optimal functioning of your system (source).
When considering a heating or cooling system, it would be easy to think that the delivery of warmed or cooled air would be the most important part of the process.
However, it is equally important to consider the role of the return air of the system and the crucial role it plays in the effective functioning of the system.
Air returns maintain circulation and balance the system. They ensure there is plenty of clean air returning to the system to be reconditioned and sent out again.
Crucially, they need to be big enough to perform this function and return sufficient air to ensure that the system keeps your home temperature comfortable.