What You'll Learn

• What CFM per ton means and why 400 CFM/ton is the standard design target
• The acceptable range of 350-450 CFM/ton and what happens outside it
• How low CFM/ton affects humidity, efficiency, coil temperature, and comfort
• How high CFM/ton affects dehumidification and sensible capacity
• How measureQuick calculates and displays airflow using TrueFlow Grid, capture hood, and estimated airflow
• What the V12 data reveals about airflow problems in the field (70%+ exceed 0.5" TESP)

What You'll Need

• Device: iPhone (iOS 15+) or Android phone (Android 10+) with measureQuick installed
• Account: measureQuick account with active subscription
• For measured airflow (recommended): TrueFlow Grid paired with measureQuick, or a powered capture hood (such as the TEC TO420)
• For estimated airflow: Supply and return air temperature probes properly placed (supply probe inside the register, return probe at the face of the return)
• Manometer probes: For TESP measurement (recommended alongside airflow assessment)
• Understanding of: TESP (E9), superheat and subcooling (E3)
• Time: 15-20 minutes for airflow measurement and evaluation

What CFM per Ton Means

CFM per ton is the ratio of airflow volume to cooling capacity:

CFM per ton = Total system airflow (cubic feet per minute) / System tonnage

One ton of cooling capacity equals 12,000 BTU/hour. A 3-ton system is rated for 36,000 BTU/hour. At the standard design rate of 400 CFM per ton, that 3-ton system should move 1,200 CFM of air across the evaporator coil.

This ratio determines how much heat the air delivers to the evaporator coil per unit of refrigerant capacity. Too little air means the coil gets too cold. Too much air means the coil does not get cold enough. Both conditions degrade system performance, but in different ways.

As Jim Bergmann explains when setting up a system profile: "refrigerant type is it, run off the nominal airflow which typically is 400 CFM per ton - if I'm in Florida or a humid climate you can see here I might set up to 350." The target depends on the application.

The Target Range: 350-450 CFM/ton

400 CFM/ton: Standard Design

Most residential cooling systems are designed around 400 CFM per ton. This is the standard ARI test condition and the default that most manufacturers use for their performance ratings. When you see a system rated at a specific SEER or capacity, those numbers were measured at 400 CFM/ton.

measureQuick defaults to 400 CFM/ton in the system profile unless you change it. As shown in the benchmarking video: "I have a system set up and running, I've got a profile, this 410a, 400 CFM per ton, 13 to 16 SEER."

350 CFM/ton: Humid Climates

In hot, humid climates (Florida, Gulf Coast, Southeast), some contractors intentionally set airflow to 350 CFM/ton. Lower airflow makes the evaporator coil colder, which wrings more moisture out of the air. This improves latent capacity (dehumidification) at the cost of some sensible capacity (temperature reduction) and total efficiency.

As noted in the commissioning walkthrough: "we're realistically expecting the airflow you know like maybe 375 CFM per ton to get the latent sensible split right."

450 CFM/ton: Dry Climates

In dry climates where dehumidification is not a priority, higher airflow (up to 450 CFM/ton) can improve sensible capacity and efficiency. The coil runs warmer, which means less dehumidification but more heat transfer per BTU of capacity.

Below 350 or Above 450: Problems

Operating outside the 350-450 range creates measurable problems. Below 300 CFM/ton, evaporator freeze-up becomes likely. Above 500 CFM/ton, the coil cannot adequately cool the air, and the system may short-cycle on high head pressure.

What Happens at Low CFM per Ton

When airflow is below the design target, the evaporator coil receives less heat from the air. The coil temperature drops. This creates a cascade of effects:

Humidity Problems

A colder coil removes more moisture per cubic foot of air, but the total volume of air crossing the coil is reduced. The net result is often poor humidity control in the conditioned space. The system runs longer to meet the thermostat setpoint, and the home feels "cold and clammy" rather than comfortably cool.

Reduced Efficiency

The system's rated efficiency was tested at 400 CFM/ton. At lower airflow, the evaporator temperature drops, which reduces the temperature difference between the return air and the coil. Heat transfer efficiency decreases. The compressor works harder (lower suction pressure, higher compression ratio) for less useful cooling.

Evaporator Coil Freeze

Below about 300 CFM/ton, the evaporator coil temperature can drop below 32 degrees F. Moisture in the air freezes on the coil surface. Ice buildup further restricts airflow, creating a feedback loop that can completely block the coil and potentially damage the compressor through liquid slugging.

False Overcharge Symptoms

As covered in Charge & Airflow Balance, low airflow produces the same diagnostic pattern as overcharge: low superheat, high subcooling, high head pressure. If you evaluate charge without verifying airflow, you may remove refrigerant from a correctly charged system.

What Happens at High CFM per Ton

When airflow exceeds the design target, the evaporator coil receives more heat than designed. The coil temperature rises.

Poor Dehumidification

A warmer coil removes less moisture from the air. In humid climates, this means the system meets the thermostat setpoint (temperature) but leaves the home at uncomfortably high humidity. The homeowner may lower the thermostat to compensate, which wastes energy and overcools the space.

Higher Sensible Capacity

More airflow does improve sensible cooling (temperature reduction). In dry climates, this can be beneficial. The system cools the space faster with each run cycle.

False Undercharge Symptoms

High airflow produces the same diagnostic pattern as undercharge: high superheat, low subcooling, low suction pressure. Again, if you evaluate charge without verifying airflow, you may add refrigerant to a correctly charged system.

The Airflow Problem in the Field

measureQuick's V12 database shows that more than 70% of tested systems exceed 0.5" total external static pressure. High TESP is a direct indicator of restricted airflow. Systems with high static pressure are almost certainly delivering less than 400 CFM/ton.

This is not a measurement artifact. The most common causes are undersized ductwork, dirty filters, dirty evaporator coils, and undersized return grilles. Many of these are design problems that have been present since installation.

The implication: when you arrive at a service call, the probability that the system has correct airflow is less than 30%. Measuring or estimating CFM per ton is not optional if you want your charge diagnostics to be reliable.

How measureQuick Measures Airflow

measureQuick supports three methods for determining airflow, listed from most accurate to least:

1. TrueFlow Grid

The TrueFlow Grid is a calibrated plate that installs in the filter slot. measureQuick reads the pressure differential across the grid via a paired manometer and calculates CFM based on the grid's calibration data.

As Jim Bergmann describes: "I personally really love the true flow plate - true flow plate gives you a bunch of different sizes, typically... it does a duct traverse for you and it actually measures it out in total pressure."

To use TrueFlow in measureQuick:

1. Install the TrueFlow Grid in the filter slot (remove the existing filter).
2. Pair your manometer with measureQuick.
3. Activate TrueFlow from the home screen or within a project. measureQuick will pause the test and guide you through the measurement.
4. The app calculates CFM based on the manometer reading and the grid size you selected.

Technician using measureQuick app with TrueFlow grid installed for airflow measurement

The V12 database tracks airflow sources. Tests using TrueFlow Grid are tagged with airflow_source = 'trueflow', and the actual measured CFM is stored in the airflow_trueflow field. This is the most reliable airflow measurement available in the field.

2. Capture Hood

If you have a powered capture hood (such as the TEC TO420), measureQuick can read the airflow data via Bluetooth. The capture hood measures CFM at individual supply registers. You sum the register readings to get total system airflow.

This method is accurate for total system airflow but time-consuming on systems with many registers.

3. Estimated Airflow

measureQuick can estimate airflow using the enthalpy method. This calculation uses:

• Supply air dry bulb and wet bulb temperatures
• Return air dry bulb and wet bulb temperatures
• System capacity (from the profile)

The estimated airflow appears automatically when supply and return air probes are connected and the system profile includes tonnage.

This estimate is useful as a screening tool. It is less precise than TrueFlow or a capture hood, but it requires no additional equipment beyond the temperature probes you are already using for charge evaluation.

Probe placement for estimated airflow is critical. As the existing Zoho article notes: "insert the supply up into the closest supply register and the return air probe at the face of the return. You need to go inside the supply so you do not get a mixed air temperature at the face due to air entrainment. Do not get too close to the evaporator coil."

measureQuick diagnostics screen showing estimated airflow value alongside superheat, subcooling, and TESP

Reading Airflow Results in measureQuick

measureQuick displays airflow in CFM and calculates CFM per ton based on the system tonnage from your profile. The diagnostic screen shows:

• Airflow (CFM): The measured or estimated total airflow
• CFM per ton: The calculated ratio based on the profiled tonnage
• Airflow status indicator: Green if within the acceptable range for the profiled design, yellow or red if outside

If CFM per ton is below 350 or above 450, measureQuick flags it. This flag is a signal to investigate the duct system, filter, and coil before making any charge adjustments.

What the Numbers Tell You

Connecting Airflow to Charge and Static Pressure

These three measurements form a diagnostic triangle:

• TESP tells you whether the duct system is restricting airflow.
• CFM per ton tells you how much air is actually moving across the coil.
• Superheat and subcooling tell you the refrigerant charge condition, but only when CFM per ton is in the acceptable range.

The correct diagnostic sequence:

1. Measure TESP. If it exceeds the rated maximum, investigate the cause.
2. Measure or estimate CFM per ton. If it is outside 350-450, address the airflow problem.
3. Only after airflow is acceptable, evaluate superheat and subcooling for charge condition.

measureQuick shows all three on the same screen because they are interdependent. The app cannot separate them, and neither should you.

Video Walkthrough

• YouTube: (0:59, 2,889 views) - Quick explanation of the origin and rationale for the 400 CFM/ton standard

• YouTube: (1:00, 1,326 views) - Overview of diagnostic symptoms caused by insufficient airflow across the evaporator

• YouTube (HVAC School): (1:05:15, 23,171 views) - Jim Bergmann covers airflow measurement methodology in depth, including why "most technicians think of when they think of measuring air flow, they immediately think of static pressure, which is interesting because that's not" the direct measurement of airflow. Discusses TrueFlow, static pressure matching, and duct traverse methods

• YouTube: - Jim Bergmann explains the enthalpy method for airflow estimation and why measured airflow (TrueFlow) is preferred. Covers the formula "4.5 times CFM times change in enthalpy" and why any error in CFM multiplies straight through to capacity

• YouTube: (1:27:15, 1,663 views) - Includes TrueFlow report demonstration on an existing system and discusses available static pressure, airflow options, and sensible heat factor

Tips & Common Issues

measureQuick shows an estimated airflow that seems too high

The most common cause is incorrect probe placement. If the supply air probe is at the face of the register instead of inside the duct, it reads mixed air (partially room air, partially supply air). This higher temperature produces a smaller delta-T, which the enthalpy calculation interprets as higher airflow. Push the supply probe well inside the register, at least 6 inches into the supply boot.

TrueFlow reading differs significantly from the estimated airflow

The TrueFlow Grid is a direct measurement; the estimated airflow is a calculation. Differences of 10-15% are normal. If the difference is larger than 20%, check your temperature probe placement and verify that the system profile (tonnage, refrigerant type) is correct.

CFM per ton is low but TESP passes

TESP within the rated maximum does not guarantee correct airflow. A variable-speed or ECM blower may be running at a speed setting that is too low for the system's tonnage. TESP could be acceptable at that reduced airflow. Check the blower speed setting against the manufacturer's table for the installed tonnage. Also verify that the system is actually the tonnage you profiled; if the tonnage in the profile is wrong, the CFM/ton calculation will be off.

The customer has a high-MERV filter and complains about comfort

High-MERV filters (MERV 13+) in standard 1" filter racks create significant pressure drop. This restricts airflow and reduces CFM per ton. Measure TESP with the filter in place, then remove the filter and remeasure. The difference is the filter's contribution. If the filter alone adds more than 0.15" of static, the filter is too restrictive for this system's filter rack. Options include upgrading to a deeper filter rack (4" or 5"), adding more return filter area, or reducing the MERV rating.

Why does measureQuick show two different airflow values?

If you use both TrueFlow and estimated airflow in the same test, measureQuick records both. The TrueFlow value takes precedence for diagnostic evaluation because it is a direct measurement. The estimated airflow is stored separately for comparison.

Related Articles

Prerequisites (complete these first):

• Total External Static Pressure - How to measure TESP and what it means
• Superheat & Subcooling - How charge indicators work

Follow-up articles (next steps after this one):

• Charge & Airflow Balance - How airflow affects charge diagnostics
• TESP Budget & 140% Rule - Breaking TESP into component pressure drops

Related in other domains:

• Smart Tool Overview - TrueFlow Grid and capture hood compatibility
• Manometer Placement: Air Handler - Probe placement for TrueFlow measurement
• AC Cooling Commissioning Workflow - Where airflow measurement fits in the full workflow
• Static Pressure Screening - Quick airflow screening via static pressure

Need Help?

If you get stuck or this article does not answer your question:

• Check the Related Articles section above
• Contact measureQuick support: support@measurequick.com