Debunking a Misconception: Low Airflow and Compressor Flooding
Why an intermittent blower failure can be a greater danger to the compressor than no airflow.
This article synthesizes the key findings from a diagnostic demonstration conducted by Jim Bergman of measureQuick, which investigates the effects of low airflow on a fixed orifice HVAC system. The central thesis is a refutation of the common misconception that low airflow immediately causes catastrophic compressor flooding. Instead, the demonstration reveals a more nuanced process of "charge migration," where refrigerant accumulates in the evaporator coil while the system maintains a minimal, positive superheat.
The true danger of compressor damage arises not from the low airflow state itself, but from the sudden restoration of airflow to the liquid-filled evaporator, a condition termed "hunting and flooding." This event causes a violent boiling of refrigerant that sends liquid back to the compressor. The measureQuick software successfully diagnosed the simulated fault, identifying "Low load on the evaporator" and pointing to airflow as the likely cause. This underscores measureQuick's engineering philosophy, which relies on physically creating and verifying faults in a lab environment rather than relying on theoretical models.
I. Debunking a Common Misconception: Low Airflow and Compressor Flooding
A prevalent belief in the HVAC industry is that a low airflow condition will invariably lead to zero superheat, causing liquid refrigerant to "flood" the compressor. The demonstration establishes that this is "not necessarily true."
The Misconception: Low airflow causes an immediate return of liquid refrigerant to the compressor.
The Demonstrated Reality: Even in a "worst case scenario" with a fixed orifice metering device and virtually no airflow across the evaporator coil, the system can stabilize and maintain a positive superheat of 3 to 5 degrees. This prevents immediate compressor damage.
"a lot of people always think that when you have low air flow that you're going to flood the compressor you're going to have no superheat and that is not necessarily true"
The analysis shows that systems are more resilient to this specific fault than commonly believed, and that if this were not the case, every instance of a bad blower motor would result in a failed compressor.
II. The Low Airflow Simulation and Its Effects
The experiment was conducted on a 1.5-ton system with a fixed orifice metering device. A low airflow condition was simulated by removing the air handler door, causing supply air to bypass the evaporator coil entirely.
Initial System State (Normal Operation):
Superheat 17.2° F
Target/Range 12.6° F
Subcooling 7.0° F. Within Range
System Reaction to Simulated Low Airflow:
Initial Superheat Spike: Upon removing the door, the superheat value shot up dramatically. This is an artifact of the calculation. System pressure drops much faster than refrigerant line temperature. The saturation temperature, which is derived from pressure, plummeted instantly, while the suction line temperature lagged, causing a temporarily high calculated superheat.
Stabilized Low Airflow State: After the initial spike, the system settled into a new, stable state characterized by:
Superheat: Dropped significantly but remained positive, stabilizing between 3.3°F and 6°F.
Pressures: Both head pressure and suction pressure dropped.
Subcooling: Trended downwards.
Coil Temperature: Dropped to approximately 25-26°F.
This demonstrates that while performance is severely degraded, the system does not immediately enter a zero-superheat state.
III. The Phenomenon of "Charge Migration"
The core process occurring during the low airflow condition is charge migration.
Definition: With no heat load on the evaporator coil (due to the lack of airflow), the liquid refrigerant entering the coil fails to boil off.
Process: The refrigerant begins to accumulate, or "stack," inside the evaporator coil. This effectively moves the system's refrigerant charge out of the condenser and into the evaporator.
Consequences:
Flooding the Coil: The evaporator coil becomes filled with liquid refrigerant. This is distinct from flooding the compressor.
Starving the Condenser: As refrigerant moves to the evaporator, the condenser is deprived of the refrigerant it needs to function, leading to a drop in head pressure and subcooling.
"refrigerant is basically coming out of the condenser going into the evaporator there's no heat energy to boil that refrigerant away so the refrigerant is stacking in the evaporator coil... The condenser is starting to starve because it's not getting that refrigerant back so the charge has migrated over into the evaporator coil."
IV. The True Danger: Sudden Restoration of Airflow ("Hunting and Flooding")
The most critical and dangerous phase of the experiment occurred when the air handler door was replaced, suddenly restoring massive airflow across the liquid-filled evaporator coil.
Immediate Effect: The superheat instantly dropped to 0°F.
Underlying Cause: The rush of warm air caused the large volume of liquid refrigerant in the coil to boil violently and rapidly. This surge of boiling refrigerant and entrained liquid was carried down the suction line directly toward the compressor.
Term for the Condition: This event is described as "hunting and flooding."
Real-World Parallel: This scenario is analogous to a failing blower motor with a bad capacitor that cycles on and off. The period of no airflow allows the coil to flood with liquid, and the subsequent restart of the fan forces that liquid into the compressor, leading to mechanical failure.
System Protection: The demonstration unit was protected by an accumulator, which caught the liquid slug before it could enter the compressor. Without this component, "really, really loud noises from the compressor" or immediate damage would have been likely.
V. System Diagnostics and Engineering Philosophy
The demonstration highlighted the capabilities of the measureQuick diagnostic software and the company's underlying engineering methodology.
measureQuick Software Analysis
During the simulated fault, the software provided the following accurate diagnosis:
Top-Line Fault: "Low load on the evaporator."
Potential Causes Identified:
Blocked filter
Blocked evaporator
Airflow issue
Symptoms Identified:
The coil will make ice
System capacity is low
measureQuick's Engineering Approach
The company's diagnostic algorithms are built on empirical data derived from lab-based testing, not theoretical assumptions.
Methodology: Faults are physically and deliberately created on real equipment. This includes installing oversized metering devices, physically blocking evaporator coils, and inducing low airflow states.
Goal: To understand precisely how HVAC equipment behaves under specific fault conditions and then verify that the software can accurately detect and diagnose those conditions.
Direct Quote: "It's not guessing how equipment operates it's knowing how the equipment operates and then very verifying the software can pick it up."
VI. System-Specific Considerations: Fixed Orifice vs. TXV
The experiment was intentionally performed on a fixed orifice system, which was described as the "worst case scenario." A system with a Thermostatic Expansion Valve (TXV) would have behaved differently.
TXV Function: A TXV's primary job is to actively regulate refrigerant flow to maintain a target superheat.
Projected TXV Behavior: In a low airflow situation, a TXV would sense the low superheat and begin to close down, restricting refrigerant flow into the flooded evaporator. This action would likely maintain a superheat of 8°F to 12°F, offering a greater degree of protection against liquid return compared to the fixed orifice device.
See Measurequick video https://youtu.be/gs5X0TkYjjM
Premier Ac and Heating Services Inc
1412 17th St, 2nd Floor #216
Bakersfield, CA 93301