Superheat and subcooling are the two most commonly referenced refrigerant-side measurements. They are also the two most commonly misinterpreted.
High superheat does not always mean "add refrigerant." High subcooling does not always mean "remove refrigerant." Each deviation has multiple possible causes, and the only way to identify the correct one is to correlate superheat and subcooling with other measurements: condenser temperature over ambient (CTOA), design temperature difference (DTD), total external static pressure (TESP), and airflow.
From measureQuick's diagnostic database, 56.0% of piston-metered systems fail refrigerant charge evaluation across 115,706 quality-filtered cooling tests. Many of those failures trace back to problems that are not charge-related at all: restricted airflow, dirty coils, or metering device issues. Adding or removing refrigerant to "fix" the superheat or subcooling number without identifying the actual cause makes the system worse.
High superheat means the refrigerant is absorbing more heat than expected in the evaporator. The suction line is hotter than it should be relative to saturation temperature. This happens when:
Not enough refrigerant in the system. The evaporator starves, and the refrigerant fully evaporates before reaching the end of the coil. The remaining coil surface superheats the vapor.
Correlated indicators: Low subcooling, low suction pressure, low liquid pressure.
A partially blocked piston, clogged filter screen, or debris in the metering device restricts refrigerant flow into the evaporator. The evaporator is starved even though the charge may be correct.
Correlated indicators: Subcooling may be normal or high (refrigerant is backing up in the condenser because it cannot flow through the restriction). The temperature drop across the metering device is larger than expected.
When airflow across the evaporator coil drops, less heat is available for the refrigerant to absorb per unit of time. The evaporator pressure drops, saturation temperature drops, and the measured suction line temperature is further above saturation.
Correlated indicators: High TESP (above 0.50 inWC), low CFM per ton, low supply-to-return temperature split. Subcooling may be elevated because the condenser is overcondensing refrigerant that the evaporator cannot use at the reduced airflow.
The system has more capacity than the building load requires. On a mild day with low cooling demand, the evaporator does not have enough heat load to fully utilize the refrigerant, producing high superheat.
Correlated indicators: Short cycling, low suction pressure, capacity ratio below expectations for a mild day. Subcooling may be normal.
Low superheat means liquid refrigerant is getting close to (or reaching) the compressor. This is a compressor safety concern because liquid does not compress.
Too much refrigerant in the system. The evaporator is flooded with liquid, and the refrigerant does not fully evaporate before leaving the coil.
Correlated indicators: High subcooling, high suction pressure, high liquid pressure, elevated CTOA.
When outdoor airflow is restricted (dirty condenser coil, blocked fins, failed condenser fan), the condenser cannot reject heat efficiently. High head pressure forces more liquid into the evaporator than it can evaporate.
Correlated indicators: High CTOA (condenser temperature is far above outdoor ambient), high liquid pressure, high suction pressure. Subcooling may be low because the condenser is not fully condensing the refrigerant.
A thermostatic expansion valve (TXV) that is stuck open or oversized feeds too much refrigerant into the evaporator.
Correlated indicators: Normal or low subcooling (refrigerant is flowing freely, not backing up). CTOA may be normal if the condenser is functioning.
High subcooling means the liquid refrigerant leaving the condenser is cooler than expected relative to saturation temperature. Refrigerant is accumulating in the condenser.
Excess refrigerant has nowhere to go. It backs up in the condenser, subcooling further below saturation.
Correlated indicators: Low superheat, high suction pressure, high liquid pressure, elevated CTOA.
A kinked liquid line, clogged filter-drier, or partially closed service valve restricts flow between the condenser and the metering device. Refrigerant backs up in the condenser, driving subcooling high.
Correlated indicators: High superheat (the evaporator starves downstream of the restriction). The temperature drop across the restriction point is visible with contact thermometers on either side of the filter-drier. CTOA may be normal because the condenser itself is fine; the problem is downstream.
A TXV that is not opening fully restricts refrigerant flow into the evaporator. The effect is similar to a liquid line restriction.
Correlated indicators: High superheat, low suction pressure. The metering device is the restriction point.
Low subcooling means the condenser is not fully condensing the refrigerant. Some vapor may be present in the liquid line.
Not enough refrigerant to fill the condenser. The liquid level in the condenser is low, so the refrigerant exits without much subcooling.
Correlated indicators: High superheat, low suction pressure, low liquid pressure.
An overfeeding TXV drains the condenser faster than it can condense refrigerant, reducing the liquid level and subcooling.
Correlated indicators: Low superheat (evaporator is flooded). Suction pressure may be elevated.
A slow leak gradually reduces system charge. Early in the leak, subcooling drops before superheat rises noticeably. Low subcooling with gradually increasing superheat over time suggests a leak.
Correlated indicators: Trending data from previous visits showing declining subcooling.
These four scenarios show how correlating measurements identifies the root cause.
| Measurement | Value | Target |
|---|---|---|
| Superheat | 22F | 10-15F |
| Subcooling | 4F | 8-12F |
| CTOA | 18F | 15-25F |
| TESP | 0.42 inWC | < 0.50 |
Diagnosis: Undercharge. The condenser is working properly (normal CTOA), but both superheat is high and subcooling is low, indicating insufficient refrigerant. Airflow is adequate (TESP within range). Add refrigerant.
| Measurement | Value | Target |
|---|---|---|
| Superheat | 25F | 10-15F |
| Subcooling | 14F | 8-12F |
| CTOA | 38F | 15-25F |
| TESP | 0.78 inWC | < 0.50 |
Diagnosis: Restricted indoor airflow. High TESP confirms the duct system is restricting airflow. The evaporator cannot absorb enough heat (high superheat), and the condenser is overcondensing (high subcooling) because the system is out of balance. CTOA is elevated because the condenser is working harder to compensate. This is not a charge problem. Fix the airflow restriction first, then re-evaluate.
| Measurement | Value | Target |
|---|---|---|
| Superheat | 3F | 10-15F |
| Subcooling | 20F | 8-12F |
| CTOA | 35F | 15-25F |
| TESP | 0.45 inWC | < 0.50 |
Diagnosis: Overcharge. Both superheat is low and subcooling is high, with elevated CTOA. The system has too much refrigerant. The condenser is backed up (high subcooling), and the evaporator is flooded (low superheat). Remove refrigerant.
| Measurement | Value | Target |
|---|---|---|
| Superheat | 24F | 10-15F |
| Subcooling | 18F | 8-12F |
| CTOA | 20F | 15-25F |
| TESP | 0.40 inWC | < 0.50 |
Diagnosis: Liquid line restriction. The condenser is operating normally (CTOA fine), but subcooling is high because refrigerant is backing up behind a restriction. High superheat confirms the evaporator is starving downstream of the restriction. Check the filter-drier for a temperature drop across it. Inspect the liquid line for kinks. This is not a charge problem.
mQ Assist (covered in detail in I4) uses the full set of measurements from a completed test to identify fault patterns automatically. Rather than requiring you to mentally correlate superheat, subcooling, CTOA, DTD, TESP, and airflow, mQ Assist evaluates all of these together and identifies the most likely root cause.
This is especially valuable in ambiguous cases where multiple measurements are borderline. A superheat of 18F and a subcooling of 7F might be "slightly high" and "slightly low" respectively. Taken individually, neither is alarming. Taken together with a low liquid pressure, they point to a slow undercharge.
mQ Assist does not replace your diagnostic judgment. It gives you a second opinion grounded in the data, and it catches patterns you might miss when evaluating measurements one at a time.
If superheat is high because of restricted airflow, adding refrigerant will lower the superheat number, but the system will be overcharged once the airflow issue is corrected. Always verify airflow (TESP, CFM per ton) before making charge adjustments.
On TXV-metered systems, the valve actively regulates superheat. Subcooling becomes the primary charge indicator because the TXV adjusts to maintain its setpoint superheat. If superheat is abnormal on a TXV system, the TXV itself may be the problem (stuck, failed sensing bulb, or loose equalizer).
CTOA and subcooling targets shift with outdoor conditions. A CTOA of 30F at 95F outdoor is more concerning than a CTOA of 30F at 110F outdoor. measureQuick accounts for outdoor conditions in its target calculations. Use the app's target zones (I6) rather than memorized single numbers.
Air or nitrogen trapped in the system produces symptoms similar to overcharge: high head pressure, high subcooling, elevated CTOA. If the system was recently opened for service and shows these symptoms, recover and weigh the charge rather than adjusting based on superheat and subcooling alone.
When you identify a fault and make a correction, run a new test in measureQuick to capture the post-correction readings. The before/after comparison in the test-in/test-out pairing demonstrates the improvement and provides a record for the customer.
Prerequisites (complete these first):
Follow-up articles (next steps after this one):
Related in the same domain:
If you have questions about interpreting superheat or subcooling faults in measureQuick: