Superheat Calculator
Enter your suction-line pressure and temperature for any refrigerant; get superheat plus diagnostic context. Correct dew-curve math for zeotropic blends so high-glide refrigerants (R-407C, R-454C, R-455A) don't read 11-22°F off.
What superheat is and why we measure it
Superheat is the temperature of refrigerant vapor above its saturation temperature at the same pressure. At any pressure, the saturation temperature is the boundary where liquid and vapor coexist; once the refrigerant has fully boiled, every degree above that saturation reading is one degree of superheat.
On a working HVAC system, suction-line superheat serves two simultaneous purposes. First, it protects the compressor: positive superheat guarantees no liquid is reaching the suction crankcase (liquid refrigerant in the cylinder is essentially incompressible — "slugging" damages valves, bearings, and rotors). Second, it tells you whether the evaporator is being fed correctly — too little superheat means liquid is leaving the evaporator unboiled (wasted capacity); too much means the evaporator is starved.
Schematic of a residential split-system suction line, showing where the thermocouple should clamp (within 6 inches of the compressor inlet, insulated from ambient) and where the suction pressure is read at the manifold service port. Source: Carrier / Trane / Lennox residential service literature.
Two metering devices, two different questions
What you do with a superheat reading depends entirely on whether the system uses a fixed-orifice metering device (piston, capillary tube, accurator) or a thermostatic expansion valve (TXV / EEV). Different devices, different procedures.
| Metering device | Charge by | Verify by | Typical SH |
|---|---|---|---|
| Fixed orifice (piston, captube) | Superheat (target from chart) | Match ACCA Manual T target | 5-25°F (variable) |
| TXV (thermostatic expansion valve) | Subcooling (charge to SC target) | Steady SH at TXV setpoint | 8-15°F (regulated) |
| EEV (electronic expansion valve) | Subcooling | EEV control board diagnostic | 5-15°F (regulated) |
Fixed-orifice devices have no feedback control — superheat varies with charge, ambient, and indoor load. Charging a fixed-orifice system means adjusting refrigerant mass until superheat lands on the ACCA Manual T target value for the current indoor wet bulb / outdoor dry bulb conditions. TXV and EEV systems have a sensing element that modulates flow to maintain a fixed superheat setpoint (typically 10°F at the bulb).
On TXV / EEV systems, charge is set by subcooling instead. Superheat measurement on these systems verifies the valve is operating in its target range; it does not determine charge directly. A TXV system reading 30°F superheat usually points to a stuck-closed valve or restriction, not undercharge (cross-check subcooling).
Target superheat reference — ACCA Manual T and OEM service literature
The ACCA Manual T (2017) charging chart for fixed-orifice systems targets superheat based on the indoor wet-bulb temperature entering the evaporator coil and the outdoor dry-bulb temperature at the condenser. Higher indoor WB and lower outdoor DB both raise the target.
| Indoor WB ↓ / Outdoor DB → | 75°F | 85°F | 95°F | 105°F | 115°F |
|---|---|---|---|---|---|
| 50°F | 6 | — | — | — | — |
| 55°F | 11 | 9 | 7 | 5 | — |
| 60°F | 18 | 16 | 14 | 12 | 9 |
| 65°F | 23 | 21 | 19 | 17 | 15 |
| 70°F | 28 | 26 | 24 | 22 | 20 |
| 75°F | 33 | 31 | 29 | 27 | 25 |
Source: ACCA Manual T "Air-Side and Refrigerant-Side Diagnostics" (2017 edition, Table 1). Representative values for R-22 and R-410A residential split systems with fixed-orifice metering. Cell "—" means the operating point is outside normal envelope — verify equipment is operating correctly before charging.
| Application | Target SH | Source |
|---|---|---|
| Residential AC, TXV / EEV | 8-15°F | Carrier, Trane, Lennox, Daikin OEM literature |
| Residential AC, fixed orifice | per ACCA chart | ACCA Manual T (2017) Table 1 |
| Walk-in cooler (MT), TXV | 6-12°F | ASHRAE Handbook of Refrigeration 2022 Ch. 23 |
| Walk-in freezer (LT), TXV | 8-15°F | ASHRAE Handbook of Refrigeration 2022 Ch. 23 |
| Heat pump, heating mode | 10-20°F | Carrier / Trane heat-pump service procedures |
| Centrifugal chiller at evap | 2-5°F | ASHRAE HVAC Systems & Equipment 2024 Ch. 43 |
| Hermetic compressor return-gas min | 20°F | AHRI Standard 540-2020 §6 |
| Semi-hermetic compressor return-gas min | 30°F | AHRI Standard 540-2020 §6 |
Target superheat ranges across HVAC applications. Compressor minimums (AHRI 540) are protection thresholds at the compressor inlet; service-line targets are usually lower because suction-line pickup adds further superheat between the line probe and the compressor crankcase.
Real service problems solved with the superheat measurement
Ten field scenarios spanning residential AC charging (fixed-orifice and TXV), walk-in commercial refrigeration with wide-glide zeotropic blends, heat pump heating mode, and slugging-risk pattern recognition. Each shows what gets measured, the PT chart lookup that converts suction pressure to saturation temperature, the superheat derivation, and a verdict on what to do.
Charging a new R-410A residential AC by superheat target
Scenario · Brand-new R-410A residential AC, piston metering device, 95°F outdoor dry bulb, 63°F indoor wet bulb (75°F return air, 50% RH). You need to set the charge by superheat per the manufacturer's charging instructions, cross-checked against ACCA Manual T.
Verifying TXV operation on a commissioned R-410A system
Scenario · R-410A TXV-equipped residential AC, just commissioned. Charge was set by subcooling (10°F target). You want to verify the TXV is regulating superheat correctly before signing off.
High superheat — diagnosing undercharge vs restriction
Scenario · Same R-410A TXV system, two months later. Customer reports poor cooling on a 95°F day. Suction pressure looks low and the suction line feels warmer than expected.
Low superheat — overcharge or TXV flooding the evaporator
Scenario · R-410A TXV system after a service add by a previous tech. The compressor is running noticeably louder than normal and you suspect liquid floodback.
Zero superheat — emergency slugging shutdown
Scenario · Legacy R-22 split system reported as 'making knocking noise'. You arrive on-site, hook up gauges, and find concerning numbers. The compressor sound is consistent with hydraulic events.
R-407C medium-temp commercial — why curve selection matters
Scenario · R-407C retrofit on a medium-temp commercial unit (reach-in deli case), 25°F evaporator target. R-407C is zeotropic with ~11°F glide, so the bubble vs dew distinction is significant for superheat calculation.
R-454C low-temp walk-in freezer superheat charging
Scenario · R-454C walk-in freezer (low-temp commercial), -20°F target evaporator temperature, 95°F ambient. R-454C is a wide-glide zeotropic blend used as a low-GWP replacement for R-404A in commercial refrigeration.
R-410A heat pump superheat at 35°F outdoor heating mode
Scenario · R-410A residential air-source heat pump in heating mode. 35°F outdoor (outdoor coil is now the evaporator), 70°F indoor return air (indoor coil is the condenser). Customer report of weak warm air — you're checking if the system is operating correctly.
R-134a centrifugal chiller — low evaporator superheat is normal
Scenario · Water-cooled R-134a centrifugal chiller, 45°F leaving chilled water, 85°F entering condenser water. You measure suction and want to verify the evaporator is operating correctly. Chillers are different from residential AC — much lower SH targets.
R-32 vs R-454B superheat targets — same as R-410A?
Scenario · New installation choosing between R-32 (pure) and R-454B (zeotropic ~3°F glide) for residential AC. Field tech asks: do these A2L refrigerants need different superheat targets than R-410A?
| Refrigerant | Glide | TXV target SH | Fixed-orifice approach |
|---|---|---|---|
| R-410A | ~0°F (near-az) | 8-15°F | ACCA Manual T target |
| R-32 (pure) | 0°F | 8-15°F | ACCA Manual T target |
| R-454B (zeotropic) | ~3°F | 8-15°F (dew curve) | ACCA Manual T target |
Glide-aware curve selection — why dew is the right curve for superheat
Zeotropic blends boil across a temperature range at constant pressure. As liquid refrigerant enters the evaporator (bubble point) and progresses to fully vaporized (dew point), the saturation temperature rises by the glide value — even though the pressure is unchanged.
Suction-line superheat is measured downstream of the evaporator, where refrigerant is fully vaporized. The relevant saturation reference is the dew point: the temperature at which the last drop of liquid disappeared. Using the bubble point would treat the entry-side saturation as if it were the exit-side reference, underestimating superheat by the glide value.
R-407C bubble and dew saturation curves over the service range, showing the consistent 11°F glide between the two. Use the dew curve at suction pressure for superheat (suction line); use the bubble curve at discharge pressure for subcooling (liquid line). Source: CoolProp 7.2.0 saturation data for R-407C.
Glide values across common HVAC blends: R-454B ≈ 3°F, R-448A ≈ 6°F, R-449A ≈ 6°F, R-407C ≈ 11°F, R-454C ≈ 14°F, R-455A ≈ 22°F. Wrong-curve selection on R-455A would shift superheat by 22°F — easily enough to invalidate a charging decision or trigger an unnecessary compressor protection shutdown.
Six common superheat measurement mistakes
- Wrong curve on zeotropes. Using bubble pressure for saturation temperature on R-407C / R-454C / R-455A underestimates superheat by the glide value (11-22°F). This calculator uses the dew curve automatically — verify any paper PT chart you reference shows both columns and use the dew column for SH.
- Thermocouple at the wrong location.Industry standard is within 6 inches of the compressor suction inlet on the suction line; the OEM service literature for your equipment specifies the exact location. Probing at the evaporator outlet, at random elbows mid-line, or at the compressor body itself gives different values that don't match the OEM's SH target.
- No insulation on the probe. An uninsulated clamp-on probe reads partly the line temperature and partly the ambient air temperature. Inside a warm attic this inflates apparent superheat by 5-10°F. Use closed-cell foam tape or insulation putty over the probe.
- Reading before steady state. Superheat takes 10-20 minutes after compressor start to stabilize as the system reaches steady-state operation. Brief after-start spikes or transient values during defrost / cycle changes are not charging-decision data.
- Confusing total vs evaporator superheat. Total Superheat is measured at the compressor suction (what manifold-based service procedures use); Evaporator Superheat is at the evap outlet (what the TXV bulb senses). Total SH is 2-5°F higher than Evap SH due to suction-line pickup. ACCA Manual T targets are Total SH; TXV setpoints are Evap SH.
- PSIG vs PSIA mix-up. Service gauges read PSIG (gauge pressure above atmospheric); some refrigerant property software wants PSIA (absolute, measured from vacuum). PSIA = PSIG + 14.696 at sea level. Confusing the two shifts the saturation lookup by 15 PSI which can swing a reading by 5°F or more at low-side pressures.
When to use this calculator vs the others
- Superheat Calculator (this page) — suction-line measurement. Charge fixed-orifice systems; verify TXV operation; diagnose evaporator-side issues (undercharge, restriction, flooding).
- Subcooling Calculator — liquid-line measurement. Charge TXV / EEV systems; diagnose condenser-side issues (fouling, overcharge, low ambient airflow). Always pair with SH.
- Combined SH / SC / PT — both sides plus pattern-matching diagnostic banner. Use for full system commissioning or comprehensive diagnostic.
- PT Calculator — raw saturation lookup, no measurement input. Reference tool for cross-checking or comparing refrigerants.
- System Pressure Diagnostic — high-low pressure × high-low SH / SC pattern matcher. Use when you have all four values and want a quick fingerprint identification (undercharge, overcharge, restriction, airflow problem, compressor issue).
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 the operating range.
- ACCA Manual T "Air-Side and Refrigerant-Side Diagnostics" (2017) — fixed-orifice charging chart (target superheat indexed on indoor wet-bulb and outdoor dry-bulb), measurement procedure, common error patterns. Industry-standard reference for residential service technicians.
- ASHRAE Handbook of Refrigeration 2022 — Chapter 1 (vapor-compression fundamentals), Chapter 23 (service procedures and target superheat by application). The reference text for commercial refrigeration service.
- AHRI Standard 540-2020 (Positive Displacement Refrigerant Compressors) — minimum return-gas superheat at the compressor inlet: 20°F hermetic, 30°F semi-hermetic. The compressor-protection floor.
- ASHRAE HVAC Systems & Equipment 2024 — Chapter 43 (chillers), centrifugal chiller evaporator approach and superheat targets.
- 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, Mitsubishi, Goodman charging procedures and target superheat ranges per equipment model.
- IEC 60335-2-40 (2022) — A2L refrigerant charge limits and installation requirements for R-32, R-454B equipment.
How to use this calculator
- Pick the refrigerant in the system. Defaults to R-410A.
- Read the suction-line pressure from the low-side manifold gauge — most service gauges read PSIG by default.
- Measure the suction-line temperature with a contact or clamp-on probe within 6 inches of the compressor inlet. Insulate from ambient air and let the reading stabilize (10-20 min after compressor start).
- Enter both values. The calculator returns superheat in °F (or °C if you toggle the unit) plus a diagnostic banner.
- Compare against your equipment's target superheat (OEM charging chart, TXV spec, or the ACCA Manual T reference table below).
Common errors
- Reading the discharge pressure instead of the suction pressure. The suction is the LOW side; discharge is the HIGH side.
- Probing the suction line without insulating — ambient air pulls the reading toward room temperature, inflating apparent superheat.
- On zeotropic blends, using the bubble pressure for saturation temperature — underestimates superheat by the temperature glide (11°F for R-407C, 14°F for R-454C, 22°F for R-455A). This calculator does dew-curve math automatically.
- Forgetting that fixed-orifice and TXV systems have very different target ranges. A fixed-orifice system reading 10°F superheat on a 95°F day may actually be undercharged per the ACCA Manual T chart.
- Reading SH before steady state. Allow 10-20 minutes after compressor start before the readings stabilize.
Underlying math
Formula
Superheat (°F) = T_suction_line − T_sat(P_suction) T_sat is read off the DEW curve at the measured suction pressure for zeotropic blends. For pure refrigerants and azeotropes, bubble ≡ dew, so the curve choice is moot.
Source
Saturation temperatures from CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999), REFPROP-compatible Helmholtz EOS. Target superheat per ACCA Manual T (Air Conditioning Contractors of America 2017), ASHRAE Handbook of Refrigeration 2022 (Chapter 1, 23), AHRI Standard 540-2020 (Positive Displacement Refrigerant Compressors), and equipment-specific manufacturer charging charts (Carrier, Trane, Lennox, Daikin, Goodman).
Worked example
R-410A residential AC, 95°F outdoor, TXV metering: Suction pressure (gauge): 130 PSIG Suction-line temperature: 60°F Saturation temperature at 130 PSIG: 45°F (CoolProp 7.2.0) Superheat = 60 − 45 = 15°F Within the typical 8-15°F TXV target range and comfortably above the slugging threshold. For a fixed-orifice system, cross-check against the ACCA Manual T target SH chart for the specific indoor wet-bulb / outdoor dry-bulb combination.
Related tools
Subcooling Calculator
Companion to superheat on the liquid line. Together they pin down a system's charge state.
Combined PT / SH / SC
All three measurements on one form with pattern-matching diagnostic banner.
PT Calculator
Raw saturation-pressure lookup for any refrigerant.
System Diagnostic
Pattern matcher for high/low pressure × high/low SH/SC fingerprints.
R-410A reference
Full PT chart, operating pressures, and lubricant guidance for the dominant residential AC refrigerant.
Frequently asked
›What is superheat?
Superheat is the temperature of refrigerant vapor above its saturation temperature at the same pressure. On a working HVAC system, it's measured at the suction line near the compressor: actual suction-line temperature minus the saturation temperature corresponding to the suction-line pressure. Positive superheat means the refrigerant has fully boiled; zero or negative superheat means liquid is reaching the compressor — slugging, which damages valves and bearings.
›What is the target superheat for an HVAC system?
Depends on the metering device. Fixed-orifice systems target a variable 5-25°F superheat calculated from a charging chart indexed on indoor wet-bulb and outdoor dry-bulb temperatures (ACCA Manual T, 2017). TXV / EEV systems target a fixed 8-15°F superheat regardless of ambient (the valve regulates to its setpoint, typically 10°F). Walk-in coolers target 6-12°F; walk-in freezers 8-15°F; heat-pump heating mode 10-20°F. Always cross-check against the manufacturer's service literature for the specific equipment.
›How do I measure superheat in the field?
Connect a manifold gauge to the suction service port and read the pressure in PSIG. Clamp a contact temperature probe on the suction line within 6 inches of the compressor inlet, insulate it from ambient air, and let the reading stabilize (10-20 minutes after start). Convert the suction pressure to saturation temperature using a PT chart for your refrigerant — use the dew curve for zeotropic blends. Subtract: superheat = T_line − T_sat. This calculator does the conversion and dew-curve selection automatically.
›What does low superheat indicate?
Low superheat (under 5°F on most systems) usually means the system is overcharged, the metering device is flooding the evaporator (TXV stuck open, oversized orifice, missing distributor nozzle), or indoor airflow is too low to fully boil the refrigerant. Liquid refrigerant reaching the compressor — slugging — causes valve damage and bearing failure. Cross-check with subcooling and verify indoor airflow before adjusting charge.
›What does high superheat indicate?
High superheat (over 25°F on most residential systems) usually means undercharge, a liquid-line restriction starving the evaporator, a TXV over-controlling or stuck partially closed, or low indoor load. Check subcooling first — low subcooling alongside high superheat strongly suggests undercharge. Verify indoor airflow and inspect the filter-drier (a partially clogged drier raises subcooling on the inlet side and superheat at the outlet) before adding refrigerant under EPA Section 608.
›Why does superheat math differ for zeotropic blends?
Zeotropic blends (R-407C, R-454C, R-455A, R-448A, R-449A) condense and evaporate across a temperature range at constant pressure. On the suction line the refrigerant has already passed through evaporation — the relevant saturation boundary is the dew temperature, not the bubble temperature. This calculator uses the dew curve automatically for zeotropic blends. Using the bubble curve for R-407C would underestimate superheat by approximately 11°F; for R-455A by approximately 22°F.
›Is this the same as Total Superheat versus Evaporator Superheat?
This calculator computes superheat at the measurement point — typically the suction line near the compressor, which is the 'Total Superheat' value most charging procedures reference. Evaporator Superheat (at the evaporator outlet, before line pickup) is 2-5°F higher than Total Superheat at the compressor. TXV setpoints control to Evaporator Superheat; ACCA Manual T charging charts target Total Superheat. The distinction matters most on systems with long suction line sets exposed to warm spaces.
›What is the AHRI 540 minimum return-gas superheat?
AHRI Standard 540 (Positive Displacement Refrigerant Compressors) specifies minimum return-gas superheat at the compressor suction to guarantee no liquid floodback under any operating condition: 20°F for hermetic compressors and 30°F for semi-hermetic compressors. These are compressor-protection minimums, not service-charging targets. A residential split system charged to 10°F TXV superheat at the compressor inlet still satisfies the AHRI 540 minimum because suction-line pickup adds further superheat between the line measurement point and the compressor crankcase.
›Does this calculator work with R-1234yf and R-454B?
Yes — R-1234yf (mobile AC) and R-454B (residential AC A2L replacement for R-410A) are both supported, along with the full 49 CoolProp-modeled refrigerants in the dataset. For R-454B (zeotropic, ~3°F glide) the calculator uses the dew curve automatically, though the glide is small enough that bubble vs dew rarely changes the SH reading by more than 2-3°F. R-1234yf is pure and uses a single saturation curve.