Air balancing is not uncommon for HVAC project engineers. Besides, homeowners may also need to do air balancing once in a few years to maintain thermal comfort. So, what is HVAC air balancing and how do you balance the air?
HVAC air balancing is a commissioning procedure that involves the adjustment of dampers in the ductwork and the fine-tuning of the airflow at each diffuser and grille to ensure that all diffusers and grilles have equal and balanced airflow.
Many people said that HVAC air balancing is an artwork. I tend to agree with it because it requires trial and error as well as the experience of the adjuster. However, difficulty can be reduced when proper procedures are followed.
Purpose of Air Balancing in HVAC
Air balancing is essential for the health of the HVAC system. Without air balancing work, the newly installed HVAC system will not perform as intended.
Not all HVAC systems required air balancing. Air balancing is only applicable to HVAC systems that use ductwork. For instance, ducted air conditioning, toilet exhaust system and smoke extraction system.
Air Balancing for Centralized Air Conditioning System
Homes that are using a centralized air conditioning system often require to do air balancing once every few years or when necessary.
Centralized air conditioning systems rely on ducts, diffusers and grilles to distribute air across the entire house. If the diffusers in the living room are imbalanced and supply too much airflow, the living room will be too cold.
Similarly, if the diffusers in one of the bedrooms are not supplying enough airflow, the bedroom will not be cool sufficiently.
Central air conditioners often use one thermostat to control their cooling capacity and fan speed. If the thermostat is located in the living room where the airflow is high, the central air conditioner will not ramp up its cooling power for the bedroom that is suffering from low airflow because it detects sufficient cooling in the living room.
Below is a diagram illustrating the imbalanced air problem in HVAC:
When the airflow of the centralized air conditioning system is balanced, the thermostat located in one of the rooms can better represent the average temperature of the entire house.
If one of the rooms is not enough cold, it means that the entire house is not enough cold as well. Thus, the central air conditioner will increase its cooling power accordingly.
Air Balancing for Smoke Extraction System
During a fire, the HVAC smoke extraction system will operate to discharge smoke to the outside. There could be 30-50 smoke exhaust grilles in the system. If their airflow is not balanced, the HVAC system may not able to extract smoke effectively in certain regions.
Air Balancing for Pressurization System
The HVAC pressurization system needs to ensure that sufficient pressure is maintained inside the stairwell and lift lobby area in case of a fire so that smoke won't enter the exit path.
The pressurization system could have more than 30 pressurization grilles. If their airflow is not balanced, smoke may find its way to the stairwell and lift lobby area, hindering the escape process.
HVAC Air Balancing Tools
When it comes to the tools used for HVAC air balancing work, I like to break it down into two parts; a) HVAC accessories and b) airflow measurement devices.
The first part is about the HVAC system itself which is the accessories for airflow control. Airflow accessories such as volume control dampers, opposed blade dampers and radial fan blade dampers must be present in order for the air balancing work to happen.
It is impossible to do air balancing without a radial damper or an opposed blade damper installed at each diffuser and grille. However, if an air volume control damper is installed at the duct branch, up to a certain level of air balancing can be done but it won't be perfect.
The air volume damper at a duct branch only allows you to adjust the airflow of that particular duct branch. The first diffuser of the duct branch will supply more airflow than the last diffuser of the duct branch.
So, diffuser dampers are essential if you want to have an air-balanced HVAC system.
Airflow Measurement Devices
The second part is the instruments used to measure the airflow of the diffuser and grille. Normally, we use a flow hood to capture the airflow of diffusers. If you are not sure what is a flow hood, follow the link below to see photos, prices, brands and specifications of a flow hood on Amazon:
Flow hoods are expensive. Though the brand does make a difference, flow hoods are generally expensive airflow measurement devices. So, either you request from your company (for project engineers) or consider hiring a contractor to help you (for homeowners).
If you are unable to get a flow hood, you may consider using an anemometer that has a display with a small fan attached to it. Anemometers are used to measure air velocity rather than air volume or airflow in CFM or m3/hr.
However, you can multiply the air velocity by the duct area to obtain the air volume. But, it is not the proper way of doing the air balancing work. Nevertheless, at least, it is one of the solutions to get your HVAC system back to working properly. For project engineers, using an anemometer to do air balancing is mostly not acceptable.
Other than airflow devices, proper layout drawings with the desired airflow indicated must be prepared alongside pens, calculators, ladders and one helper.
HVAC Air Balancing Procedures
Once you got your tools and helper ready, it's time to conduct the air balancing work. As far as I know, there are no standard procedures for HVAC air balancing. However, the following is my experience and suggestion:
1. Get a helper and teach your helper how to measure the airflow correctly
The most important skill required to do the air balancing work efficiently is the ability to judge how much you need to close the damper of each diffuser. Most of the time, you need to make the call rather than leave it to your helper.
So, assign your helper to do the airflow measurement work. You probably need to teach your helper how to use the flow hood first. Brief your helper and test a few rounds to make sure your helper is measuring the airflow correctly.
It is very important that your helper is measuring the airflow correctly at the beginning. The last thing you want is an inaccurate measurement that causes you to redo everything.
Of course, to ensure that your helper knows how to measure the airflow correctly, you need to learn it by yourself first. Fortunately, it is not difficult to learn how to use a flow hood correctly.
Hence, you'll do the recording, find the patterns and tell your helper how much the damper needs to be closed. Keep in mind that the airflow measurement work is exhausting. So, try not to ask your helper to close and open the same damper too frequently.
2. Measure the airflow of each diffuser
Skip this step if you're dealing with new HVAC units. You do not need to measure the airflow of each diffuser if you are commissioning a new HVAC unit.
But, if you have recently replaced the fan motor or you want to re-balance the airflow of your HVAC system, you'll need to measure the airflow of existing diffusers. No damper adjustment is needed at this stage. Just measure and record the airflow.
Needless to say, you need to switch on your HVAC unit in order to measure the airflow. If your HVAC unit have multiple fan speeds, select your preferred fan speed or use the higher fan speed.
For me, I don't measure the total airflow of the HVAC unit using a pitot tube manometer at the supply air duct because the total airflow will not be the same as the sum of the airflow of all associated diffusers due to resistances in the ductwork.
So, I always measure all existing diffusers and record their airflow prior to adjusting and fine-tuning their damper for the air balancing work and it should be a very quick task.
3. Write down the airflow of each diffuser on your drawing
If you're working on existing HVAC units, write down the airflow of each diffuser you've just measured with your helper on your drawing.
If you're dealing with new HVAC units, your drawing should already indicate the original airflow. Otherwise, indicate the designed airflow on your drawing.
Below is an example of the measured airflow of an existing HVAC unit:
Once you've measured the airflow of all diffusers, check and see if the airflow on each diffuser makes sense. Ask questions like:
- Which diffusers have a very low airflow?
- Which diffusers have a very high airflow?
- Which diffusers you can reduce more airflow?
- Which diffusers do not need so much airflow?
If all of your diffusers have very low airflow (below 100 cfm), you may need to check either the fan motor of the HVAC unit or the inside of the duct. Excessive dust collected inside the duct will reduce the airflow.
4. Set a Target Airflow for Each Diffuser
The hardest part of the air balancing work is that every time you adjust one diffuser, the rest of the diffusers are affected. So, the air balancing work can be very frustrating sometimes.
However, if you set the target airflow at 80% of the original airflow, you'll find yourself less frustrated and get to completion much quicker.
When I was first started to work on HVAC air balancing, I tend to adjust the first diffuser to the original airflow. Then, I realize that with every diffuser I adjust, the airflow of the first diffuser becomes more and more. Then, I noticed that if I adjust each diffuser to 80% of its original airflow, I tend to get a better result.
Below is an example of the target airflow for each diffuser:
Once you've done writing the target airflow on your drawing, measure a diffuser using a flow hood. Then, adjust the damper of the diffuser and then measure the airflow again until the airflow matches with the target airflow before you move on to the next diffuser.
- Measure the airflow at diffuser
- Tune the damper of the diffuser
- Measure the airflow at the diffuser
- Repeat until achieve the target airflow
By the time you finish adjusting the last diffuser, the first diffuser should be increased from 80% of the original airflow to around 100% of the original airflow which is roughly what you want to achieve.
Using this method, it doesn't matter which diffuser you start. After you've done it, the result should be relatively close to the original airflow that you have written on your drawing.
However, I also noticed that if you are dealing with large HVAC units (airflow more than 5000 cfm), I need to set the target airflow to about 50% or even 40% of the original airflow for diffusers that are closer to their HVAC unit.
Still, the above does not guarantee you a one-time success because there are other factors such as duct length, internal cleanliness of the duct and etceteras that will affect the behaviour of the airflow. Nonetheless, it is what I've been doing for over 20 large HVAC systems in my previous project.
That's why people say that HVAC air balancing work is very much dependent on the experience of the balancer. Experienced technicians are able to close the damper of each diffuser at a much higher accuracy.
6. Verify if the air balancing work is well done
After you've adjusted the damper of all diffusers to the target airflow, go around and measure all diffusers again to see if their airflow is now acceptable (what you want to achieve).
For people that are working on existing HVAC units, if you have problems with certain rooms colder than other rooms, go around and check the temperature in those rooms again to verify if the air balancing work is well done.
To do HVAC air balancing work efficiently, you need to have tools and procedures but more importantly, you need to have a good feeling on how much you need to close the damper at each diffuser. With practice, you'll be doing it more and more efficiently.
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