MeasureQuick SCFM vs. ACFM: The Airflow Metric That Actually Matters
You're on a job site, tools connected, and measureQuick is streaming live data to your phone. As you review the system's operation, you navigate to the performance calculations and see two different airflow numbers: one for SCFM and one for ACFM. They're close, but not the same. It’s a common point of confusion: which number is right, and which one truly matters for diagnosing the system?
While your tools measure the volume of air moving through the system, the actual work of heating and cooling is tied to the mass of that air. Cubic Feet per Minute (CFM) is a measure of volume, but Standard Cubic Feet per Minute (SCFM) is our proxy for mass because it represents a fixed quantity of air molecules by standardizing its volume to a known, constant density. Both numbers appear in measureQuick for a very specific reason, and understanding the difference is crucial for an accurate diagnosis.
This article will explain why focusing on the mass of the air—represented by SCFM—is the key to understanding a system's true performance. We'll explore why your system's heat-carrying capacity is tied to mass, not volume, and how measureQuick uses this principle to give you a clearer picture of what's really happening.
CFM is a Measure of Volume, but Heat Moves with Mass
Cubic Feet per Minute (CFM), often referred to as Actual CFM (ACFM), is a straightforward measurement of the volume of air moving past a point every minute. It simply answers the question, "How much space is this air occupying as it moves?" However, the critical concept for any HVAC technician is that the heat-carrying capacity of air depends on its mass—how much "stuff" is in that volume—not just the volume itself.
This distinction is fundamental to understanding system performance. As Bryan Orr of HVAC School points out, our initial understanding often needs refinement.
"I once heard someone say, and have since repeated many times that BTU's ride on the back of Cubic Feet of Air or CFM. As I've learned more I've come to the conclusion that the weight or mass of the air passing over the evaporator coil is far more important to understand both in terms of delivering BTUs and in understanding how different motors work." - Bryan Orr, HVAC School
Think of the air moving across a coil like trucks delivering cargo on a highway. The cargo (BTUs) is what matters, not just the number of trucks. ACFM is like the number of trucks passing a point each minute. SCFM, however, represents the actual amount of cargo being delivered. You could have many small, mostly empty trucks (like thin, low-density air) or fewer, fully loaded trucks (like dense, sea-level air). Both scenarios might look like 'a lot of trucks' (high ACFM), but only the second one delivers a significant amount of cargo (high mass flow, represented by SCFM). This is why mass flow is what truly matters for heat transfer. SCFM is the industry's method for discussing mass flow because it standardizes the 'truck' to a fixed, known cargo capacity (a fixed, known density). (Note: While these are common "standard" conditions, they can vary slightly by organization, such as CAGI. The core principle of standardizing to a fixed density for mass comparison remains the same.)
A Tale of Two Cities: Why 100 SCFM Isn't the Same Everywhere
Air is a compressible fluid. Its density—the mass packed into each cubic foot—is significantly affected by temperature, humidity, and, most dramatically, altitude (atmospheric pressure). The higher the altitude, the lower the atmospheric pressure and the less dense the air becomes.
Consider an example comparing two cities: Cartagena, Colombia, at sea level, and Medellin, Colombia, at an altitude of 5,000 feet. The air in Medellin is significantly less dense ("thinner") than the air in Cartagena. In physical terms, a cubic foot of air in Cartagena weighs 0.075 lbs, while in Medellin it only weighs 0.062 lbs. To deliver the same amount of heat-carrying capacity (the same mass of air), the fan in each location has to move a different volume of air.
Cartagena (Sea Level): To deliver 100 SCFM of air, the system must move 106.6 ACFM (Actual Cubic Feet per Minute).
Medellin (5000 ft): To deliver the same 100 SCFM (the same mass of air), the system must move 121.0 ACFM.
This comparison highlights a critical point: to perform the same amount of work, the fan in Medellin has to move a much larger volume of thinner air. If a technician in Medellin was only looking for a target ACFM value from a sea-level manual, they would misdiagnose the system. Relying on an ACFM reading alone is misleading because the "actual" volume required to do the job changes with every location.
Side Note: Sea Level Air vs. High Altitude Air
Generally, sea-level air, even when very humid, is significantly denser than high-altitude air.
This is due to two competing factors: air pressure and humidity.
• Air Pressure (The Dominant Factor): The most significant factor determining air density is barometric pressure. At sea level, the entire column of Earth's atmosphere is pressing down, creating high pressure (around 14.7 psi). This pressure compresses the air, packing more molecules into every cubic foot. At high altitudes, there is less atmosphere above, so the pressure is much lower (e.g., ~10.1 psi at 10,000 ft), and the air is far less compressed.
• Humidity (The Minor Factor): This is a common point of confusion.Many people, including some HVAC professionals assume that the density of air rises with humidity. Humid air is actually less dense than dry air at the same temperature and pressure. This is because a molecule of water vapor (H₂O, atomic mass ≈ 18) is lighter than the molecules it displaces in dry air—primarily Nitrogen (N₂, atomic mass ≈ 28) and Oxygen (O₂, atomic mass ≈ 32).
Conclusion: The massive increase in density from sea-level pressure far outweighs the minor decrease in density from humidity.
Inside the Brain of measureQuick: It’s a Mass Flow Calculation
When you look at the "Performance Calculations" screen in measureQuick, you'll see both "Airflow (estimated): SCFM" and "Airflow (estimated): ACFM". This is not a mistake or a redundancy. The app provides both because it is fundamentally built on a mass flow calculation.
In the measureQuick user forums, a technician once asked if the airflow estimate was based on Delta T. The answer from the development team was clear and direct: "It’s a mass flow calculation."
This means that measureQuick is analyzing the system based on the mass of air required for proper heat exchange under the current operating conditions. It calculates this required mass flow and presents it as SCFM—the stable, standardized metric for performance. Then, it translates that for you. The app also calculates what that required mass of air looks like in terms of volume at your specific job site's temperature and elevation, displaying that as ACFM.
Think of these two values as a diagnostic pair. SCFM is the target for system performance (the "what"—how much heat-carrying capacity is required), while ACFM reveals the fan's actual volumetric effort to meet that target under site conditions (the "how"—how much space that air takes up). A significant difference between the two can be a diagnostic clue itself, pointing to issues like low-density air at high altitudes.
The Danger of a "Good" CFM Reading
Every seasoned technician knows the rule of "Airflow Before Charge" (ABC). You must establish correct airflow before the refrigerant charge can be properly evaluated or set. But "correct airflow" means correct mass flow, not just a certain volume.
Focusing only on ACFM can hide serious problems. Low mass flow, even if the air feels cold at the vent, will cripple a system. Here is the chain reaction that occurs:
Colder Coil: With less air mass moving across the evaporator, less heat is absorbed from the air. The coil gets colder than it should.
Lower Suction Pressure: The excessively cold evaporator coil causes the refrigerant boiling inside it to drop in pressure.
Lighter Refrigerant Gas: As the suction pressure drops, the density of the refrigerant vapor returning to the compressor also drops.
Reduced Efficiency & Capacity: The compressor is a fixed-volume pump. With each stroke, it now moves a lighter, less-dense volume of refrigerant gas. This means less refrigerant mass is circulated per stroke, crippling the system's ability to move heat and drastically reducing both capacity and efficiency.
The opposite can also be true. In a humid climate, an airflow that is too high (too much mass) can prevent the evaporator coil from getting cold enough to reach the dew point. When this happens, the system stops dehumidifying (latent cooling), which can crash its total cooling capacity.
SCFM gives you the stable, mass-based number you need to verify that the system is meeting its design performance. ACFM tells you the volumetric reality your fan is dealing with at that specific site to achieve that performance.
Conclusion
As technicians, we live in a world of volume. Our tools measure pressure and temperature, and we calculate volumetric flow (ACFM). However, the real work of air conditioning—the transfer of BTUs—happens at the mass level. Standard Cubic Feet per Minute (SCFM) is our window into that world, providing a standardized benchmark for the mass of air a system is moving.
The next time you see the SCFM and ACFM values in measureQuick, don't just see two numbers. See the difference between the work being done and the effort it's taking to do it. Is your system just moving a lot of air, or is it moving enough heat? Understanding this difference is the key to making consistently accurate and defensible diagnoses.
Note: The formula for sensible heat transfer is: q = mass flow rate × specific heat × temperature differenceIn symbols: q = ṁ × cp × ΔT.
The common HVAC shortcut:q = 1.08 × CFM × ΔTis actually a mass-based formula in disguise. The constant “1.08” comes from multiplying the standard air density (0.075 pounds per cubic foot) by the specific heat of air (0.24 BTU per pound per degree Fahrenheit). Therefore, the “CFM” in this formula is implicitly Standard CFM (SCFM), not Actual CFM (ACFM).
Refrences: https://www.hvacrschool.com/does-mass-or-volume-move-heat/
https://www.pdblowers.com/wp-content/uploads/2016/11/scfm_vs_acfm_2.pdf
https://www.prominser.com/html/fs_scfm_vs_acfm.pdf
Premier Ac and Heating Services Inc
1412 17th St, #216
Bakersfield, CA 93301