PT / Superheat / Subcooling Calculator
One form for both the low side (suction, superheat) and the high side (liquid, subcooling). The eight-pattern diagnostic matrix maps the combined SH × SC × pressure fingerprint to the root cause: properly charged, undercharge, overcharge, restriction, fouling, TXV failure, or non-condensables.
Low side — suction line (superheat)
Saturation temp at suction P: 45.0°F
High side — liquid line (subcooling)
Saturation temp at liquid P: 112.6°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.
The eight-pattern diagnostic matrix
Combining SH × SC × pressure trends yields eight common fingerprints that cover the majority of HVAC service issues. Each fingerprint has a distinct root cause and a corresponding service action.
| # | SH | SC | Suction P | Discharge P | Root cause |
|---|---|---|---|---|---|
| 1 | Normal | Normal | Normal | Normal | Properly charged. No action. |
| 2 | High | Low / neg | Low | Low | Undercharge. Find leak, repair, recharge by weight. |
| 3 | Low / zero | High | High | High | Overcharge. Recover refrigerant in increments. |
| 4 | High | Normal | Low | Normal | Liquid-line restriction (filter-drier, kinked line, TXV stuck closed). |
| 5 | Normal | High | Normal | High | Condenser fouling / low ambient airflow. Clean coil, check fan. |
| 6 | Slight high | Slight low | Slight low | Slight low | Slow leak (early stage). Leak search before adding refrigerant. |
| 7 | Low / zero | Low / neg | Variable | Variable | TXV stuck open + undercharge. Replace TXV, recharge. |
| 8 | High | High | Normal | Very high | Non-condensables in system. Recover, evacuate, recharge. |
The SH × SC plane visualized: each quadrant corresponds to a different root cause family. The center region (10°F SH ±5, 10°F SC ±3) is the "properly charged" window for residential TXV systems. Source: ACCA Manual T (2017), ASHRAE Handbook of Refrigeration 2022.
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 on the ACCA Manual T chart 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.
Real service problems — eight-pattern matrix in action
Eight scenarios — one per pattern in the matrix — show what each fingerprint looks like in actual field readings and how to use the combined SH + SC + pressure data to identify the root cause and choose the right service action.
Pattern 1: properly charged — no action
Scenario · R-410A TXV residential AC, 95°F outdoor day, 75°F indoor / 63°F WB. Just charged. You measure all four values to verify.
Pattern 2: undercharge — the textbook leak fingerprint
Scenario · Same R-410A TXV system, six months later. Customer reports poor cooling on a 95°F day. You take the full readings to confirm what's going on.
Pattern 3: overcharge — recover refrigerant
Scenario · R-410A TXV system. Previous tech added refrigerant by gauge feel. Compressor is running noisy and the customer reports higher power bills.
Pattern 4: liquid-line restriction — partially clogged filter-drier
Scenario · R-410A TXV system. Recent customer complaint of weak cooling, but pressures look only slightly off and the unit has plenty of refrigerant (no recent service add or leak history).
Pattern 5: condenser fouling — high SC but normal SH
Scenario · R-410A TXV system at end of summer. Customer reports the AC isn't keeping up during peak heat. You measure and find SC and discharge pressure are both elevated, but SH and suction pressure look normal.
Pattern 6: slow leak (early stage) — subtle pattern shift
Scenario · R-410A TXV residential AC. System is two years old, customer says cooling seems slightly weaker than last summer. All four readings are only slightly off — easy to miss without comparing to baseline.
Pattern 7: TXV stuck open + low refrigerant — confused pattern
Scenario · R-410A TXV system. Customer reports compressor noise and weak cooling. The pattern doesn't match clean undercharge or clean overcharge — both SH and SC are low simultaneously, which is the TXV-flooding fingerprint.
Pattern 8: non-condensables in system — air contamination
Scenario · R-410A TXV system, recent commissioning but not properly evacuated before charging. Discharge pressure is unusually high despite normal-ish other readings. Both SH and SC are slightly high.
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. Three 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 11-22°F and confuse pattern matching.
- Check airflow on both sides. Low indoor airflow (dirty filter, failed blower wheel, closed dampers) raises evap temperature and SH; low condenser airflow raises condenser temperature and SC. Many ambiguous patterns resolve once airflow is corrected.
- 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 matrix.
- 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. Accuracy typically better than ±0.5% across operating range.
- ACCA Manual T "Air-Side and Refrigerant-Side Diagnostics" (2017) — combined SH × SC × pressure pattern matrix, charging procedures for TXV and fixed-orifice systems, ambient-corrected target SH.
- ASHRAE Handbook of Refrigeration 2022 — Chapter 23 (service procedures), eight-pattern fingerprint discussion, non-condensable detection.
- AHRI Standard 540-2020 — compressor protection minimum return-gas superheat (20°F hermetic, 30°F semi-hermetic).
- 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.
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 ACCA Manual T chart.
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. The eight-pattern matrix needs both SH and SC to identify the right root cause.
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} via the eight-pattern matrix.
Source
Saturation values from CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999). Target ranges per ACCA Manual T (2017), ASHRAE Handbook of Refrigeration 2022 (Chapter 23), AHRI Standard 540-2020 (compressor protection minimums), 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 (per ACCA Manual T chart). 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.
›What about the eight-pattern diagnostic matrix?
The matrix correlates SH × SC × pressure patterns to root causes. There are eight common fingerprints: properly charged, undercharge, overcharge, liquid-line restriction, condenser fouling, slow leak (early stage), TXV failure, non-condensables. Each has a distinct combined pattern that this calculator's diagnostic banner detects. The full matrix is in the reference table below.
›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 ACCA Manual T chart gives generic SH targets indexed on WB/DB conditions for fixed-orifice systems, but every TXV system is charged to its specific nameplate SC value. Always read the nameplate before charging.