PT / Superheat / Subcooling Calculator
One form for both the low side (suction, superheat) and the high side (liquid, subcooling). The combined SH × SC banner classifies 6 charge patterns; the fault tables below show how common faults move each reading.
Updated October 26, 2026
Low side — suction line (superheat)
Saturation temp at suction P: 45.0°F
High side — liquid line (subcooling)
Saturation temp at liquid P: 112.7°F
Why both measurements together — the diagnostic synthesis
Superheat measures the suction side: how much vapor margin you have above saturation, how well the evaporator is being fed. Subcooling measures the condenser side: how much liquid column you have below saturation, how much refrigerant is backed up in the condenser. Each alone gives you half the picture.
Together, they form a coordinate system. Plot a system's state in the SH × SC plane and the position tells you the root cause: bottom-right (low SH, high SC) is overcharge; top-left (high SH, low SC) is undercharge; center is properly charged; corners and edges tell other stories.
How faults move the readings
Faults show up as a pattern across suction, head pressure, superheat and subcooling, compared with the same system running normally.[1] NIST measured these patterns on a 2.5-ton R-410A heat pump with a TXV in cooling mode, and its report also reproduces Breuker and Braun's chart for a rooftop unit with a fixed orifice.[1]
| Fault | Suction | Head | Superheat | Subcooling |
|---|---|---|---|---|
| Undercharge, superheat still under 16°F | no change detected | lower | no change detected | lower |
| Undercharge, superheat above 16°F | lower | lower | higher | lower |
| Overcharge | no change detected | higher | no change detected | higher |
| Liquid-line restriction, superheat above 16°F | lower | lower | higher | higher |
| Low indoor airflow | lower | lower | no change detected | no change detected |
| Low outdoor airflow, superheat under 16°F | higher | higher | no change detected | lower |
| Low outdoor airflow, superheat above 16°F | lower | higher | higher | lower |
| Compressor valve leakage | higher | lower | no change detected | lower |
NIST split its results at 9°C (16.2°F) of superheat, where the TXV runs out of control range. A liquid-line restriction caused no detectable change below that point. Overcharge, low indoor airflow and compressor leakage never pushed superheat past it at the fault levels tested.[1]
| Fault | Suction | Head | Superheat | Subcooling |
|---|---|---|---|---|
| Refrigerant leak (undercharge) | lower | lower | higher | lower |
| Liquid-line restriction | lower | lower | higher | higher |
| Condenser fault (blocked coil) | higher | higher | lower | lower |
| Evaporator fault | lower | lower | lower | lower |
| Compressor valve leakage | higher | lower | lower | lower |
Neither NIST chart covers an overcharged fixed-orifice system. DOE's flowchart below does.
DOE's order of checks
DOE's Building America diagnostics guide works through the readings in order once the indoor coil's temperature split runs more than 3°F below its target, with separate charts for fixed-orifice and TXV systems.[2]
| Reading | Fixed orifice | TXV |
|---|---|---|
| Condensing temperature too far above outdoor air | Non-condensables, fouled condenser coil, overcharge or liquid-line restriction | Same |
| Evaporator saturation temperature too low | Liquid-line restriction, fouled evaporator coil or low charge | Same, or the TXV is out of adjustment |
| Low subcooling, low superheat | Expansion device overfeed or fouled evaporator coil | TXV overfeed: unsecured bulb or adjustment |
| Low subcooling, high superheat | Undercharge | Undercharge, or TXV underfeed: bad power element or adjustment |
| High subcooling, low superheat | Overcharge | Overcharge, or TXV overfeed |
| High subcooling, high superheat | Low outdoor temperature | Undercharge, or TXV underfeed |
DOE gives its limits as starting points. Its Table 3 lists a maximum condensing-over-ambient of 25°F above 12 SEER, 30°F at 10–12 SEER and 35°F below 10 SEER, and a minimum evaporator saturation temperature of 35°F, 40°F and 45°F alongside indoor wet bulbs of 62–63°F, 64–65°F and 66–67°F.[2] It adds that non-condensables or a liquid-line restriction also raise subcooling.[2]
Charging procedure cheat sheet — TXV vs fixed orifice
- Verify equipment is clean and properly installed; clear nameplate target SC value.
- Run system for 10-20 minutes to reach steady state.
- Measure SC. Compare to nameplate (typically 10°F ±2°F).
- If SC is low: add refrigerant in 1-2 oz increments, re-test after each.
- If SC is high: recover refrigerant in 1-2 oz increments.
- Cross-check SH lands in 8-15°F (verifies TXV is regulating).
- Document final reading on service log.
- Verify equipment is clean; airflow is correct (400 CFM/ton standard).
- Measure indoor wet-bulb (entering evaporator) and outdoor dry-bulb (entering condenser).
- Look up target SH from the fixed-orifice charging-chart field approximation (3 × indoor WB − 80 − outdoor DB) / 2 for the WB / DB combination.
- Run system 10-20 min to steady state.
- Measure SH. Compare to chart target.
- If SH is high: add refrigerant. If SH is low: recover refrigerant.
- Adjust in 2-4 oz increments, re-test SH after each.
- SC on fixed-orifice systems is informational; don't charge by it.
When the combined readings don't fit a clean pattern
Real-world systems sometimes show patterns that don't match any single matrix row cleanly. Multiple faults can stack (e.g., dirty condenser plus mild undercharge), and zeotropic blends with wide glide can confuse pattern detection if curve selection is wrong. Two principles for ambiguous readings:
- Verify curve selection first. If using a zeotropic blend (R-407C, R-454C, R-455A), confirm your SH calculation uses dew and SC uses bubble. Wrong-curve errors can shift readings by 9–18°F and confuse pattern matching.
- Look at the trend, not just the snapshot. If this is a recurring service visit, compare today's readings to previous service logs. A slowly drifting pattern (Pattern 6 fingerprint) tells you something different than a snapshot that just happens to be off.
When to use this calculator vs the others
- Combined SH / SC / PT (this page) — full diagnostic synthesis. Best for system commissioning, post-repair verification, and complex troubleshooting where you have all four readings and want to identify the root cause from the fault tables.
- Superheat Calculator — focused single-result. Use for fixed-orifice charging, quick TXV operation check, or when you only have the suction-side readings.
- Subcooling Calculator — focused liquid-side. Use for TXV / EEV charging where SC is the primary metric, or condenser-side troubleshooting.
- PT Calculator — raw saturation lookup. Use for reference, retrofit comparisons, or as a building block in manual calculations.
- System Pressure Diagnostic — decision-tree fingerprint matcher with ranked root cause output. Use after computing SH and SC here if you want a richer decision-tree analysis with ranked suspect causes.
Primary sources behind the calculator and content
- CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999) — REFPROP-compatible Helmholtz EOS for all saturation temperatures.
- Fixed-orifice charging-chart target superheat — standard field approximation of OEM fixed-orifice charging charts, (3 × indoor WB − 80 − outdoor DB) / 2 (within about ±3°F of the Carrier/Bryant table in normal conditions; see California Title 24 Reference Appendix RA3.2, Table RA3.2-2). The chart or label on the unit always takes precedence.
- ASHRAE Handbook—Refrigeration (2022) — “Equipment and System Dehydrating, Charging, and Testing” (service procedures and non-condensable detection).
- NIST SP 1087 and DOE Building America — fault-direction tables for TXV and fixed-orifice systems and the order-of-checks reproduced in the fault section above.
- EPA Section 608 (40 CFR Part 82 Subpart F) — refrigerant handling certification, leak repair requirements before adding refrigerant.
- OEM service literature — Carrier, Trane, Lennox, Daikin, Goodman, Mitsubishi service manuals for equipment-specific SC and SH targets.
- [1]NIST Special Publication 1087, Cooling Mode Fault Detection and Diagnosis Method for a Residential Heat Pump (M. Kim, S. H. Yoon, W. V. Payne, P. A. Domanski). National Institute of Standards and Technology. October 2008. https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication1087.pdf (accessed October 20, 2026). Table 5.1(a) p. 40 and Table 5.2 p. 41, fault-direction charts; Section 5.4.1 p. 58 — residential heat-pump cooling-mode fault patterns.
- [2]Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement (D. Springer, B. Dakin), NREL/SR-5500-56283. U.S. DOE Building America. March 2013. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf (accessed October 20, 2026). Section 3.4 pp. 32–33, Table 3 p. 32, Figures 17–18 p. 33 — order-of-checks diagnostic flowcharts for fixed-orifice and TXV systems.
How to read your results
- Superheat (°F/°C)
- Suction-line temperature minus the dew-point saturation temperature at your suction pressure. Residential TXV systems target roughly 8–15°F and fixed-orifice systems follow the charging-chart value; high superheat points to undercharge, low evaporator airflow, or a liquid-line restriction, while near-zero or negative superheat means liquid is reaching the compressor (slugging risk — stop and diagnose).
- Saturation temperature at suction pressure (dew curve)
- The temperature at which the refrigerant boils at the measured suction pressure, read off the dew curve (the correct curve for a vapor-side measurement, which matters on wide-glide zeotropic blends). It is the baseline the suction-line temperature is compared against to produce superheat, not a target on its own.
- Subcooling (°F/°C)
- The bubble-point saturation temperature at your liquid pressure minus the liquid-line temperature. TXV/EEV systems are charged to the nameplate value (commonly 8–12°F); high subcooling indicates overcharge or a condenser rejecting heat poorly, and low or negative subcooling indicates undercharge or flash gas in the liquid line.
- Saturation temperature at liquid pressure (bubble curve)
- The condensing temperature at the measured liquid pressure, read off the bubble curve (correct for a liquid-side measurement on blends). It is the reference the liquid-line temperature is subtracted from to get subcooling; compared against outdoor air it also reflects condenser approach — condensing temperature 15–25°F above outdoor air (high-efficiency units near the low end).
- Combined diagnostic banner
- Reads the combined superheat × subcooling fingerprint against fixed thresholds (8–25°F superheat, 3–15°F subcooling), not manufacturer targets, and names the likely condition: properly charged, undercharge (high SH + low SC), overcharge (low SH + high SC), restriction (high SH + high SC), or a stuck-open TXV flooding the evaporator (low SH + low SC). The banner color signals severity, and a negative SH or SC raises a stop-and-diagnose alarm.
How to use this calculator
- Pick the refrigerant. Defaults to R-410A.
- On the low-side panel: enter suction-line pressure (PSIG) and suction-line temperature (°F).
- On the high-side panel: enter liquid-line pressure and liquid-line temperature.
- Read superheat (dew curve, suction), subcooling (bubble curve, liquid), and the combined-pattern diagnostic banner.
- Compare against your equipment's targets — TXV system 8-12°F SC, fixed-orifice per the charging-chart field-approximation SH target for the WB/DB.
Common errors
- Measuring at the wrong service port — suction is the LOW-side port on the larger insulated line; liquid is the HIGH-side port on the smaller uninsulated line.
- Reading before steady state — let the system run 10-20 minutes after compressor start.
- Probing without insulating thermocouples — ambient pickup inflates SH and depresses SC.
- Adjusting charge based on a single reading. Identifying the right root cause needs both SH and SC and the pressure trends together.
Underlying math
Formula
Superheat = T_suction_line − T_sat(P_suction, dew) Subcooling = T_sat(P_liquid, bubble) − T_liquid_line Diagnostic pattern from {SH, SC, P_suction, P_liquid} read against the NIST SP 1087 fault-direction tables.
Source
Saturation values from CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999). Target ranges per the ASHRAE Handbook—Refrigeration (2022), “Equipment and System Dehydrating, Charging, and Testing”, and equipment-specific manufacturer charging procedures (Carrier, Trane, Lennox, Daikin, Goodman).
Worked example
R-410A residential TXV system, 95°F outdoor: Suction 130 PSIG / line 60°F → SH = 60 − 45 = 15°F (in 8-15°F TXV range) Liquid 380 PSIG / line 100°F → SC = 111 − 100 = 11°F (in 8-12°F TXV range) Diagnostic pattern: SH normal, SC normal, pressures normal Verdict: properly charged.
Related tools
Superheat alone
Focused single-result view of just the suction side.
Subcooling alone
Just the liquid side.
PT Calculator
Raw pressure-temperature lookup without measurements.
System Pressure Diagnostic
Decision-tree fingerprint matcher with ranked root cause output.
High head pressure causes
Decision tree for high SC / high discharge pressure cases.
Frequently asked
›Why measure both superheat and subcooling together?
Each one alone tells half the story; together they pin down the system's charge state and isolate root causes. The classic combinations: high SH + low SC = undercharge, low SH + high SC = overcharge, high SH + high SC = airflow problem, low SH + low SC = restriction or TXV failure. No single measurement gives you these patterns — you need both.
›Which one should I trust more for charging?
Depends on the metering device. Fixed-orifice systems are charged by superheat (target from the fixed-orifice charging-chart field approximation, indexed on indoor wet-bulb and outdoor dry-bulb). TXV / EEV systems are charged by subcooling (8-12°F target per OEM nameplate). On a TXV system, superheat hovers near the TXV setpoint regardless of charge — even an overcharged TXV system reads normal SH — so subcooling is the primary metric. Use the other measurement as a cross-check.
›What if both superheat and subcooling are off in the same direction?
Both high typically means low indoor airflow (raises evaporator temperature, raises SH) plus condenser fouling or low ambient airflow (raises SC). Both low typically means a restricted metering device flow path — TXV stuck open, oversized orifice, or distributor nozzle missing. Cross-check airflow on both sides before adjusting charge.
›Does this work for zeotropic blends?
Yes. The calculator uses the dew curve at suction pressure for superheat (correct for vapor-side measurement) and the bubble curve at discharge pressure for subcooling (correct for liquid-side measurement). For wide-glide blends like R-407C (~11°F), R-454C (~14°F), R-455A (~22°F), this avoids errors equal to the glide value that would invalidate charging decisions.
›Can I use this for commercial refrigeration?
Yes, but apply commercial target ranges: 10-20°F superheat and 5-15°F subcooling depending on application (walk-in cooler, walk-in freezer, refrigerated transport). The diagnostic banner uses residential HVAC ranges by default — interpret commercial readings against the equipment OEM spec rather than the banner. Walk-in / commercial reference targets are tabulated below.
›How do faults show up across the four readings?
The SH × SC banner alone classifies the core charge patterns (properly charged, undercharge, overcharge, restriction, airflow/metering). Suction and discharge pressure trends add context. NIST SP 1087 measured how each reading moves under common faults on a residential TXV system, and reproduces Breuker and Braun's chart for a fixed-orifice unit; the fault tables below reproduce both, with DOE's order-of-checks.
›How does this relate to the system pressure diagnostic calculator?
Both tools synthesize SH + SC + pressure into a fingerprint. This combined calculator focuses on measurement entry and pattern interpretation; the System Pressure Diagnostic tool is structured around the decision tree (input four readings, get a ranked list of suspected causes with confidence scores). Use this calculator first to get clean SH/SC values, then feed them into the diagnostic if you want deeper decision-tree analysis.
›Why are the target ranges so specific to the equipment?
Different OEMs design their TXVs, condensers, and evaporators to specific operating points. Carrier targets 10°F SC, Trane targets 8°F SC, some Lennox models target 12°F — there's no universal residential AC value. The fixed-orifice charging chart gives generic SH targets indexed on WB/DB conditions via the field approximation (3 × indoor WB − 80 − outdoor DB) / 2, but every TXV system is charged to its specific nameplate SC value. Always read the nameplate before charging.