You know how you can look at a tire and say, “That tire is a little low on air!”?
Or, look at a truck loaded with wet sand and say, “That truck looks overloaded!”?
My Mom can look at a plant and say, “That plant has been over-watered”!
Well, I’m going to show you in this brief article, how you can look at your air-handler and know the potential air-flow of your air duct system.
Is this important? Well, you tell me.
1. Do you wish your vents had more airflow coming out of them?
2. Do you think you need more air vents in certain places of your home?
3. Do you think certain vents in your home are too small or too large for the area they serve?
4. Have you had this conversation one or two times already with HVAC techs, and are tired of being brushed off?
5. Was your original HVAC system replaced with a larger one, attached to the same ducts that were sized for the smaller system?
Many years ago my very good life-friend Raymond DeBlois at Vital Air Services taught me this simple way of measuring the potential air flow of an air duct system. Thank you Raymond!
Know your Potential
First, notice I am using the word “potential” air-flow.
It’s potential air flow because you will be measuring/estimating the diameter of the collars where each air duct attaches to the air handler. This is very important to know.
No air duct system is 100% efficient. Meaning it won’t deliver the total amount of potential air flow it’s capable of delivering, when that potential is measured at the machine.
Here’s why: The air ducts create what is called “friction loss”. As the airflow travels through the long length of twisted ducts, turns, elbows, connections, reducers, leaks out into the attic or crawlspace through holes, tears, and poor connections, and squeezes through vents to finally spill into your room, it loses velocity and quantity.
Friction loss is why it’s so important to have your air ducts as straight as possible, with as few turns, elbows, reducers, splices as possible, and sealed.
Decorative Vent Covers… Oh No!
And those decorative vent covers? They are NOT your friend! Here at Bird Family Insulation, when we see decorative vents we use our Flowhood to measure the airflow. We then remove the decorative cover, and measure the airflow. Our homeowner is always BLOWN AWAY by the extra 40% MORE airflow coming from the vent after we remove the decorative cover.
Then we hear something like, “Well, my wife (or husband) will never go along with changing them… we’ll have to live with it.”
That air handler has the potential to move at least 40% more airflow than what is being delivered to the living space.
That’s why I use the word “potential” to qualify my remarks.
ALSO, my article is slanted to work best for those with round ducts attached to the air handler, (not square or rectangular).
Here is how to measure the potential airflow of your air duct system.
1. First, you need to identify the Supply Plenum and the Return Plenum of your air handler. A plenum is nothing more than a large, insulated box attached to the end of the metal cabinet that comprises your air handler.
The plenum is necessary to provide enough surface area to attach the air ducts.
Look for your filter. Generally the filter tray is located adjacent to the Return Plenum. If you see your filter tray, it is sandwiched between the Return Plenum and your air handler. Now you know where your Return Plenum is, and on the opposite end of the machine is your Supply Plenum.
Return air ducts attach to your Return Plenum, and Supply air ducts attach to your Supply Plenum.
2. You need a measuring tape for this next step. Measure the diameter of each duct attached to the Return Plenum.
If you have flex ducts attached to the plenum, you may find the diameter printed on the wrapper. If you find it printed on the duct, be certain that it’s the same-size duct at the plenum. In other words, it hasn’t been spliced or reduced between the spot where you see the printed diameter and the plenum.
In essence, you are needing to know the diameter of the hole that the air must pass through where a duct is attached to the Return Plenum.
For instance, you may measure the outer diameter of a duct to be 14”. However, at least 2” of that is duct wrap and insulation, (One inch thick on each side as you measure across the duct for diameter).
So, as a rule of thumb, I always recommend subtracting 1” or 2” from your measured diameter, to account for the insulation.
So, if you measured 14”, I would write down 12”, and so forth as you do this for each duct.
Let’s say your Return Plenum has two ducts attached, and they are each labeled with 10” stamped on the outer wrapper. The diameter of the interior of that air duct is 10”.
This means that all of the airflow produced by this piece of expensive equipment, must pass through two holes, 10” in diameter. That is the constriction point.
Fewer Return Air Ducts vs Supply
If you compare your Return Plenum to the Supply Plenum, you understand why I say this! Your Supply Plenum most always has MORE air ducts attached to it, than the Return Plenum. Or, perhaps in your system the Supply air ducts are sourced by a large trunk-line that moves a huge amount of airflow.
The point is, in 20 years of doing this, I rarely if ever see more potential airflow in the Return air ducts when compared to the Supply air ducts. I honestly cannot recall one time.
This is why about 1/3rd of my clients, year-after-year, when they see this – employ me to ADD A RETURN VENT (or two). I have read articles by experts who estimate that 80% of the residential hvac systems in America need additional Return Air Flow!
Fewer Return Air Ducts vs Supply
If you compare your Return Plenum to the Supply Plenum, you understand why I say this! Your Supply Plenum most always has MORE air ducts attached to it, than the Return Plenum. Or, perhaps in your system the Supply air ducts are sourced by a large trunk-line that moves a huge amount of airflow.
The point is, in 20 years of doing this, I rarely if ever see more potential airflow in the Return air ducts when compared to the Supply air ducts. I honestly cannot recall one time.
This is why about 1/3rd of my clients, year-after-year, when they see this – employ me to ADD A RETURN VENT (or two). I have read articles by experts who estimate that 80% of the residential hvac systems in America need additional Return Air Flow!
- Here comes the math!
Notice this chart:
| Duct Diameter (inches) | Maximum Capacity
Flex Duct (cfm) |
Maximum Capacity
Metal Duct (cfm) |
| 4” | 25 | 35 |
| 5” | 45 | 60 |
| 6” | 70 | 100 |
| 7” | 100 | 150 |
| 8” | 150 | 200 |
| 9” | 200 | 280 |
| 10” | 270 | 360 |
| 12” | 450 | 600 |
| 14” | 650 | 900 |
| 16” | 900 | Intentionally Blank |
CFM = Cubic Feet per Minute
The maximum airflow for a 10” flex duct is 270 cfm. CFM is “cubic feet per minute” of airflow. This is how we measure the quantity of airflow passing through a pipe, or a hole.
In our example, we have two, 10” flex ducts attached to our Return Plenum. Those two Return-air ducts have the potential to move (270 + 270 ) 540 total cfm.
It doesn’t matter if downstream from the Return plenum, these two 10” air ducts split into four ducts, or any other multiplication of more ducts and subsequent vents, because the quantity of air is limited to how much can pass through those two initial 10” ducts attached to the plenum.
By the way, where a duct is attached to a plenum is called the “collar”. A collar is the device used as the connection point. So the efficiency of your entire air duct system is limited by how much airflow can pass through those two 10” collars!
Bob’s Metaphors 🙂
Remember, the Return Air is the air that your machine is sucking from the house. So think of Return Airflow as the “inhale”. Think of the airflow in your Supply Air Ducts as the “exhale”.
You can only exhale as much air as you inhale!
Or think of it this way: The Return Air is the entrance into your machine, (air handler). The airflow leaving the exit (Supply Air Ducts) to heat and cool your home, is limited to the amount of air coming through the entrance, right?
4. Now we compare the potential airflow of your Return Air Ducts to the potential airflow of your airhandler (equipment).
To do this, you need to know the size (BTU) of your AC. Find the ID plate (or sticker) on your AC and read the Model number. Sometimes this can be tricky, but what you are looking for is a two-digit number that is divisible by 12.
Usually the model # a long combination of numbers, letters with maybe a dash. Somewhere in that string, look for any two numbers that can be divided by 12.
Most likely it will be 24, or 30, or 36, 42, or 48, etc. If you don’t see those numbers, search your entire model # on the internet. You need to know the tons of capacity.
A ton of AC capacity is 12,000 BTU. So if you see 24 in your model #, you divide 24 by 12, and get 2. So you have a 2-ton AC system attached to that set of air ducts. If you see 30, you have a 2.5 ton AC.
5. Convert your tons into airflow. Ideally, your system needs 400 cfm per ton (+/- 50 cfm) for proper operation and efficiency. So if we have a 2-ton system, you want about 800 cfm of airflow through your air ducts. If it’s 2.5 ton, you want around 1000 cfm, (2.5 x 400).
In our example, we have 540 cfm of potential airflow. In real life, measured at the vents, it will be somewhat lower, due to friction-loss of the flex duct system, the vent covers, and of course, the dirty filter.
When we do the math, we find that the potential air flow in the air duct system of my example is 68% airflow, at best.
Let’s Add A Vent!
We might add a new 10” Return, or two new 8” Return vents. Two 8” return vents would increase potential airflow by 300 cfm! With the two new vents, we’re at 840 cfm of potential airflow.
Air-in = Air-out! By increasing Return Airflow, we positively influence Supply Airflow, rewarding you with more airflow from your Supply Vents.
Install a clean, fresh, high-efficiency air filter, and benefit with even more airflow!
This is Real Life
My example is a real-life system that I found in the attic of a two story home on a site visit this week. Mr Homeowner isn’t happy with the cooling in his upstairs bedrooms, and called us for an insulation upgrade.
We are improving his 20 yr old insulation. After I explained his potential for more airflow, he readily agreed to us installing two additional dedicated Return Air Vents! When we make these improvements, I currently charge around $450 per vent.
If you read much of my material, or watch my Youtube and Facebook videos, (search Bird Family Insulation) you know I constantly encourage homeowners to employ a Home Performance company who is SOLUTION DRIVEN, opposed to employing an insulation or HVAC contractor who is simply product driven. Why hire a lazy contractor?
After all, you’re smart enough to know that it takes BOTH the HVAC system doing it’s best job, and the insulation doing it’s best job, for your home to be comfortable and efficient, and for you to enjoy healthy indoor air quality.
You know a leaky tire when you see it. You know an overloaded truck when you see it. And after living in your home this long, you probably have a hunch that your air ducts aren’t living up to the potential of your HVAC equipment.
Now you know how to find out!
Bob Bird
Bird Family Insulation
