Superheat and subcooling are the two most important measurements an HVAC technician takes in the field. They tell you whether the refrigerant charge is correct, whether the metering device is functioning, and whether airflow across the evaporator and condenser is adequate. Yet they are also the two most commonly misunderstood measurements, leading to unnecessary refrigerant adjustments that mask real problems.
This guide covers what superheat and subcooling actually mean in thermodynamic terms, how to measure them correctly, how to interpret the readings for both TXV and fixed-orifice systems, and how to diagnose the most common field scenarios including low charge, overcharge, airflow restrictions, and metering device failures.
What Is Superheat and Why Does It Matter?
Superheat is the temperature of refrigerant vapor above its boiling point at the current pressure. When liquid refrigerant enters the evaporator, it absorbs heat from the air passing over the coil and transitions from liquid to vapor. The point where the last drop of liquid evaporates is the saturation point. Any additional heat absorbed beyond that point raises the temperature of the vapor above saturation. That additional temperature rise is superheat.
Mathematically: superheat = suction line temperature minus saturation temperature at the measured suction pressure. If your suction pressure corresponds to a 40\x{00B0}F saturation temperature and your suction line temperature is 50\x{00B0}F, you have 10\x{00B0}F of superheat.
Superheat matters because it tells you whether liquid refrigerant has been fully evaporated before reaching the compressor. Compressors are designed to compress vapor, not liquid. If superheat is too low (near zero), liquid refrigerant can reach the compressor and cause slugging, which damages valves, rods, and bearings. If superheat is too high (over 20\x{00B0}F), the evaporator is starved, cooling capacity drops, and the compressor runs hotter than designed because suction vapor is not providing adequate motor cooling.
The ideal superheat depends on the metering device type, system design, and operating conditions. TXV systems typically maintain 8-12\x{00B0}F of superheat at the evaporator outlet regardless of charge level, because the TXV adjusts to maintain its setpoint. Fixed-orifice systems require the target superheat method, which varies with outdoor ambient and indoor wet-bulb temperature.
What Is Subcooling and How to Measure It
Subcooling is the temperature of liquid refrigerant below its condensing (saturation) temperature at the current pressure. After refrigerant vapor condenses to liquid in the condenser, any additional heat removal drops the liquid temperature below the saturation point. That temperature difference is subcooling.
Mathematically: subcooling = saturation temperature at the measured liquid (high-side) pressure minus liquid line temperature. If your liquid pressure corresponds to a 110\x{00B0}F saturation temperature and your liquid line reads 100\x{00B0}F, you have 10\x{00B0}F of subcooling.
Subcooling tells you how much liquid refrigerant reserve exists at the metering device inlet. Adequate subcooling ensures the metering device receives a solid column of liquid, not a mixture of liquid and flash gas. If subcooling is too low, the liquid line may contain flash gas, which reduces metering device capacity and system performance. If subcooling is too high, it usually indicates overcharge, with excess refrigerant backing up in the condenser and reducing its effective surface area.
For TXV systems, the manufacturer target subcooling is typically 10-15\x{00B0}F. This is the primary indicator for charge adjustment. For fixed-orifice systems, subcooling is still useful as a secondary diagnostic but the target superheat method is the primary charge indicator.
TXV vs Fixed-Orifice: Different Charging Methods
The metering device type determines which charging method you use. This is not optional or a matter of preference. Using the wrong method leads to incorrect charge and masked problems.
TXV (Thermostatic Expansion Valve) systems: Use subcooling as the primary charge indicator. The TXV adjusts its opening to maintain a constant superheat at the evaporator outlet (typically 8-12\x{00B0}F). Because the TXV compensates for charge variations, superheat remains relatively stable even when the system is over or undercharged. Subcooling changes directly with charge: low subcooling = undercharge, high subcooling = overcharge. Charge to the manufacturer's specified subcooling target.
Fixed-orifice (piston or capillary tube) systems: Use the target superheat method. Since the orifice is fixed, it cannot adjust for changing conditions. Superheat changes directly with charge level. The target superheat depends on outdoor ambient temperature and indoor wet-bulb temperature, which is why you need both measurements. Charge to match the calculated target superheat within 3-5\x{00B0}F.
If you use the subcooling method on a fixed-orifice system, you will get an unreliable reading because there is no receiver or TXV to establish a stable liquid column. If you use the superheat method on a TXV system, you will chase a number that the TXV is actively controlling, and you may overcharge the system while trying to adjust superheat.
How to Measure Superheat and Subcooling Correctly
Accurate measurements require proper technique and equipment. The two most common sources of error are thermocouple placement and inadequate stabilization time.
Thermocouple placement: Use a pipe clamp thermocouple, not a strap-on. Clamp it directly to the copper suction line 4-6 inches from the compressor for superheat. Clamp it to the liquid line as close to the condenser outlet as practical for subcooling. Make sure the thermocouple has good thermal contact with the pipe. Insulate the thermocouple with closed-cell foam to prevent ambient air from biasing the reading.
Pressure measurement: Use calibrated manifold gauges or a digital manifold. Zero your gauges before connecting. For R-410A, use gauges rated for high pressures (500+ PSIG). Read the pressure at the service valve, not at the end of a long hose. Long hoses introduce pressure drop that skews your saturation temperature lookup.
Stabilization time: Wait at least 15 minutes after startup before taking readings. Superheat and subcooling are meaningless during the initial transient. The system needs to reach steady-state conditions with a stable load on the evaporator and condenser. If the system is short-cycling, the readings will bounce and cannot be trusted.
Saturation temperature lookup: Use the pressure-temperature chart for the specific refrigerant in the system. Do not use an R-22 chart for an R-410A system. Digital manifolds handle this automatically. If using analog gauges, use a separate PT chart or app. For zeotropic blends (R-407C, R-404A), use the dew point temperature for superheat and the bubble point temperature for subcooling.
Common Diagnostic Scenarios
High superheat + low subcooling: The system is undercharged. The evaporator is starved for refrigerant, causing high superheat. The condenser does not have enough refrigerant to fully subcool the liquid. Add refrigerant in small increments and recheck after each addition stabilizes.
Low superheat + high subcooling: The system is overcharged. Excess refrigerant floods the evaporator, dropping superheat. The condenser is backed up with liquid, increasing subcooling. Recover refrigerant in small increments. On TXV systems, the TXV may mask this by maintaining its superheat setpoint while subcooling climbs.
High superheat + normal subcooling: The metering device is restricted or the evaporator airflow is low. The charge is likely correct (subcooling proves it), but the refrigerant is not reaching the evaporator efficiently. Check for a clogged filter-drier, iced TXV, restricted orifice, dirty evaporator coil, or collapsed duct restricting airflow.
Low superheat + normal subcooling: The metering device is overfeed or the evaporator is oversized for the current load. On a TXV system, the TXV sensing bulb may have lost its charge or be improperly mounted, causing the valve to open too wide. Check bulb contact and insulation.
Normal superheat + low subcooling: On TXV systems, this indicates borderline undercharge. The TXV is maintaining its superheat setpoint by throttling further closed, but there is not enough refrigerant in the condenser for proper subcooling. Add a small amount of refrigerant and recheck subcooling.
The Target Superheat Charging Chart
For fixed-orifice systems, the target superheat depends on two variables: outdoor ambient dry-bulb temperature and indoor return-air wet-bulb temperature. The relationship is captured in a charging chart that most manufacturers provide with their equipment. The chart gives you the target superheat for any combination of outdoor and indoor conditions.
The general trend: as outdoor temperature increases, target superheat increases. As indoor wet-bulb increases, target superheat decreases. At typical summer conditions (95\x{00B0}F outdoor, 67\x{00B0}F wet-bulb), the target is usually 10-15\x{00B0}F. At mild conditions (75\x{00B0}F outdoor, 62\x{00B0}F wet-bulb), the target may be 5-10\x{00B0}F.
If you do not have the manufacturer's chart, use the standard industry formula built into most superheat calculators. Measure outdoor ambient with a thermometer in the shade near the condensing unit. Measure indoor wet-bulb at the return-air grille with a sling psychrometer or a digital psychrometer. Enter both values and the calculator determines the target.
Compare actual superheat to the target. If actual is more than 5\x{00B0}F above the target, the system is likely undercharged. If actual is more than 5\x{00B0}F below the target, the system may be overcharged. Adjust charge in small increments (2-4 oz at a time on residential systems) and wait 5-10 minutes between adjustments for the system to stabilize.
Common Measurement Mistakes
Not insulating the thermocouple: A bare thermocouple on a cold suction line in a hot attic can read 5-10\x{00B0}F high because ambient heat radiates into the sensor. Always wrap the thermocouple and surrounding pipe with closed-cell foam insulation for accurate readings.
Using the wrong PT chart: R-410A at 118 PSIG corresponds to 40\x{00B0}F saturation. R-22 at 69 PSIG corresponds to 40\x{00B0}F. If you accidentally use the R-22 chart on an R-410A system, every calculation will be wrong. Double-check the nameplate before pulling out the chart.
Measuring before the system stabilizes: During the first 10-15 minutes after startup, pressures and temperatures are in flux as the system reaches equilibrium. Readings taken during this window are unreliable. Wait for steady-state operation.
Ignoring the metering device type: Using the superheat method to charge a TXV system or the subcooling method to charge a fixed-orifice system will produce incorrect charge levels. Always identify the metering device before selecting a charging method.
Not accounting for line losses: Long suction and liquid lines between the outdoor unit and the evaporator add pressure drop and temperature change. If the suction line runs 50 feet through a hot attic, the temperature at the compressor will be higher than at the evaporator outlet. Measure as close to the relevant component as possible for each reading.