If you are considering a central air conditioner, you may be asking if you need a return air in every room? So, I did some research on this topic.
It turns out that every room should have a return vent so that the air conditioner is able to sense the mixed return air temperature and adjust the supply air temperature accordingly to prevent overcooling or undercooling at certain rooms.
Many people struggle with mostly overcooling at certain rooms after they've installed their central air conditioner. They often have a hard time figuring out what went wrong and sometimes, the installer just can't help them.
I encountered similar issues during my work with a few of the bungalow house central air conditioning systems. Most of the time, it is the return air that is causing the trouble. Trust me, it is very costly to modify and add return air ducts if you screw it up.
What is the Purpose of Air Conditioner Return Air?
Return air is a must to draw the air in a room back to the air conditioner so that the air conditioner knows what is the current room temperature and adjust its supply air temperature accordingly. At the same time, maintaining balanced air pressure in the room.
As you can see from the above diagram, return air from several rooms ultimately join together before entering the air handling unit or AHU. A return air temperature sensor (sometimes located inside the AHU) senses the return air temperature and adjusts the supply air temperature.
Note: An air handling unit or AHU is a common type of central air conditioner that typically use for large cooling applications such as large houses, shopping malls, and hotels. Shopping malls and hotels require much more cooling than typical large houses. Therefore, multiple AHUs are often used in these mega-sized buildings.
The Problems with No Return Air in Every Room
Some house centralizes the air conditioner return air at one room instead of every room that it serves. Although this way can save some upfront cost from the reduction of the return air duct, it poses a risk of overcooling, undercooling, high energy usage, and other problems.
Some Rooms are Too Cold, Some Rooms are Not Enough Cold
When you centralized return air at one room, the air conditioner supply air temperature is solely based on that one room with return air. If another room is sufficiently cooled while the room with the return air is not, the air conditioner will keep blasting cold air to all rooms. Consequently, some rooms will be either too cold or too hot depending on the condition of the room with the return air.
As you can see in diagram 1, this is an example of a bad return air installation. On the first floor, only the living room has the return air and on the second floor, only bedroom 1 has the return air.
We know that air conditioners adjust room temperature based on the return air temperature, this means that both air conditioners in the above diagram only captured the room temperature of a single room. Thus, the other rooms that they served will be either too cold or not enough cold depending on the condition of the room with return air.
For example, the first-floor air conditioner (indicated as D in diagram 1) is set at 25°C. If the living room temperature is currently at 25°C, the air conditioner is able to sense 25°C from its return air. Then, the air conditioner will reduce its compressor speed and thus, reducing the cooling. If the kitchen is at 27°C now, the air conditioner will still reduce the cooling because it doesn't know that kitchen require more cooling.
Another example, the first-floor air conditioner (indicated as D in diagram 1) is still set at 25°C. If the living room temperature is currently at 27°C because people keep opening the door, the air conditioner also senses a temperature of 27°C from the return air. Hence, the air conditioner will full blast on cooling. Imagine during this period, you are sleeping in bedroom 4 with a comfortable room temperature of 24°C. The air conditioner doesn't know that and full blast cooling to your room. Consequently, you'll feel too cold in bedroom 4.
Now you may have understood that the room temperature of the kitchen and bedroom 4 is controlled by the condition in the living room. The huge difference in room temperature makes sleeping in bedroom 4 almost impossible. At the same time, overcooling is a huge waste of energy.
To avoid all the problems mentioned above, let's try to install a return air in every room as follow:
From diagram 2, you can see that each room has a return air. All return air go back to their respective air conditioner.
Since air conditioners adjust supply air temperature based on the return air temperature, each room contributed their room temperature via the return air. Then, the mixed return air temperature tells the air conditioner what is the average room temperature now. Hence, the air conditioner is able to adjust a more suitable supply air temperature for all rooms.
Let's look at one example, again, the first-floor air conditioner (indicated as D in diagram 1) is set at 25°C. If the living room temperature is currently at 27°C, the kitchen is at 25°C and bedroom 4 is at 24°C, the mixed return air temperature can be somewhere around 25°C. Although the air conditioner will still provide cooling, this time it is at a percentage of its full capacity. Thus, bedroom 4 will still feel slightly colder but it is much better than the previous example.
One of the downsides of the central air conditioner is the temperature of each room is very hard to be equal and precise. Extensive air balancing work needs to done to make the temperature differences in each room as little as possible.
Nevertheless, good air balancing works couple with return air in every room makes a central air conditioner much better in terms of temperature control and energy efficiency.
In summary, central air conditioners control room temperature based on the return air temperature. Air conditioners can't tell if a room is sufficiently cooled or not without a return air.
The Room Without a Return Air will have a Positive Pressure
It is not hard to get a grasp if a room only has a supply air and not having a return air, the room is in a positive pressure state. What happens if a room is constantly in a positive pressure state?
A room in a positive pressure state meaning that the room air pressure is slightly higher than the atmospheric pressure. In the long run, you may feel uncomfortable staying in the room without knowing why.
Besides, higher pressure means that the cold air is more likely to push outward from the room. Cold air constantly slips through door and window gaps. In the long run, you've wasted more energy than you realize. Furthermore, when air slipping through the gaps, it may create a whistling noise that irritates you.
In addition, if your room door doesn't have a hydraulic door closer, positive pressure will make the door close automatically. In severe cases, the door may slam close at a great speed, potentially hurting yourself, your kids, and your pets. What's more problematic is, it happens all the time as long as your air conditioner keeps throwing in cold air and create a positive pressure in your room.
Therefore, having a return air in every room is very important to keep the air pressure balanced. One in, one out. Equal pressure means you can stay comfortably in your room.
Centralized Return Air Create Significant Noise
The central air conditioner handles a great amount of airflow. A 4-5HP air conditioner can produce about 1200-1500 CFM of airflow. The airflow is distributed throughout many supply air diffusers and thus, the noise is barely audible. However, if your return air is combined or centralized, it can create significant noise that disturbs you on a daily basis.
If we refer back to above diagram 1, there are two central air conditioners in the house with each about 4-5HP, meaning each centralized return airflow is about 1200-1500 CFM. Next, we go on to look for the noise data from a louvre grille supplier.
As shown above, with 1200-1500 CFM requirement, the noise criterion (NC) is about NC25-NC26. By the way, you can see the grille size is 30″x18″ and 36″x18″ respectively (remember that you'll need some space for the big grille too).
So, what does the NC means? How does it compares? Check out the below table.
NC25-NC26 is equivalent to an approximately overall sound pressure level of about 30dBA. This 30dBA is the noise level produced by the return air grille itself. Adding the general noise of your living room, you may be looking at around 50dBA or more, constantly! In another word, your living room will sound like an open office or hotel corridor every time you turn on your air conditioner.
Say if we distribute the return air to each room, we are looking at an airflow of about 400-500 CFM per grille. Referring back to the noise data from the grille supplier, I'm looking at a noise criterion of <NC15. At below NC15, it is almost inaudible from a distance, assuming your return air grille is installed in the ceiling which most people do.
Therefore, one of the bonus you can get from having a return air in every room is quietness. Worth considering.
If you ever wonder if a flex duct or flexible duct can be used for return air, I strongly recommend you to check out my article on flex duct with airflow tips included. It is a shorter article highly focusing on flex duct application for return air.
My Past Experience with Air Conditioner Return Air
Throughout my career, I came across a few designs and problems after the commissioning of the air conditioning system. Some of the problems are related to insufficient return air or wrong return air location. Below I show you a portion of my previous work to highlight the importance of return air in air conditioning.
As you may have noticed, the return air is significantly more than the supply air. Partially is because this design utilized the ceiling space as a plenum to gather all the return air, another consideration is to achieve low noise.
Of course, with such a design, we are required to do extensive air balancing work to ensure each supply and return air grilles have similar airflow. Thankfully, not many of this type of design involved in the entire project.
My objective of showing you my previous work is for you to know that when it comes to the central air conditioners and commercial applications, designers take serious consideration of the return air. Your application might be smaller and less complex but, it is worth reconsider after seeing so much on return air.
List of Problems that I Commonly Face due to Return Air
Sometimes, problems with the air conditioning system can involve multiple causes. For instance, when we had identified that the problem is due to low airflow, it could be an issue with the return air or the filter. Hence, most of the time, we have to troubleshoot our way until we find the root cause.
Here are some of the common issue with return air from my past experience just to share with you:
- Not enough return air causing positive pressure – door slamming all the time
- Too much return air causing negative pressure – door very hard to open
- Too much return air near to the air conditioner – a huge amount of return air focus on the nearest few grilles, need to do air balancing again
- Return air ducts and grilles not sized properly causing low airflow – have to add more return air or else, more problem will come with low airflow
The key takeaway here is the importance of return air in the air conditioning system. The biggest problem with improper return air is too cold or not enough cold in certain rooms. Other issues such as positive pressure and noise may look like minor problems but, you'll be dealing with them on a daily basis.
Again, my recommendation here is to have a return air in every room so that the air conditioner is able to sense the mixed return air temperature and adjust the supply air temperature accordingly to prevent overcooling or undercooling at certain rooms.
However, sometimes, you can take shortcuts with return air to cut costs but, you'll need to have a constructive discussion with the installers to prevent the above-mentioned problems from occurring.
In air conditioning, return air may be one of the most confusing and troublesome topics for many people. Nevertheless, after some reading, I'm sure you understand more about it and you can make a better decision now.
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