Target Superheat Chart
Fixed-orifice / piston HVAC charging method. Target superheat is a function of indoor wet-bulb and outdoor dry-bulb — refrigerant-independent. For TXV / EEV systems, charge by subcooling instead.
Interactive lookup and matrix
Measured at the return-air grille with a wet-wick psychrometer. Typical cooling: 60–72°F WB.
Shaded thermometer near the condenser. Design condition: 95°F.
| WB \ DB | 55°F | 60°F | 65°F | 70°F | 75°F | 80°F | 85°F | 90°F | 95°F | 100°F | 105°F | 110°F | 115°F |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50°F | 7.5 | 5.0 | — | — | — | — | — | — | — | — | — | — | — |
| 52°F | 10.5 | 8.0 | 5.5 | — | — | — | — | — | — | — | — | — | — |
| 54°F | 13.5 | 11.0 | 8.5 | 6.0 | — | — | — | — | — | — | — | — | — |
| 56°F | 16.5 | 14.0 | 11.5 | 9.0 | 6.5 | — | — | — | — | — | — | — | — |
| 58°F | 19.5 | 17.0 | 14.5 | 12.0 | 9.5 | 7.0 | — | — | — | — | — | — | — |
| 60°F | 22.5 | 20.0 | 17.5 | 15.0 | 12.5 | 10.0 | 7.5 | 5.0 | — | — | — | — | — |
| 62°F | 25.5 | 23.0 | 20.5 | 18.0 | 15.5 | 13.0 | 10.5 | 8.0 | 5.5 | — | — | — | — |
| 64°F | 28.5 | 26.0 | 23.5 | 21.0 | 18.5 | 16.0 | 13.5 | 11.0 | 8.5 | 6.0 | — | — | — |
| 66°F | 31.5 | 29.0 | 26.5 | 24.0 | 21.5 | 19.0 | 16.5 | 14.0 | 11.5 | 9.0 | 6.5 | — | — |
| 68°F | 34.5 | 32.0 | 29.5 | 27.0 | 24.5 | 22.0 | 19.5 | 17.0 | 14.5 | 12.0 | 9.5 | 7.0 | — |
| 70°F | 37.5 | 35.0 | 32.5 | 30.0 | 27.5 | 25.0 | 22.5 | 20.0 | 17.5 | 15.0 | 12.5 | 10.0 | 7.5 |
| 72°F | 40.5 | 38.0 | 35.5 | 33.0 | 30.5 | 28.0 | 25.5 | 23.0 | 20.5 | 18.0 | 15.5 | 13.0 | 10.5 |
| 74°F | 43.5 | 41.0 | 38.5 | 36.0 | 33.5 | 31.0 | 28.5 | 26.0 | 23.5 | 21.0 | 18.5 | 16.0 | 13.5 |
| 76°F | 46.5 | 44.0 | 41.5 | 39.0 | 36.5 | 34.0 | 31.5 | 29.0 | 26.5 | 24.0 | 21.5 | 19.0 | 16.5 |
— = target below 5°F; superheat charging not recommended at these conditions. Industry convention on Trane and Carrier bead charts.
Where this chart comes from — and when to trust the formula
The target superheat chart looks like the output of a specification, but its history is looser than that suggests. Its precise origin was never recorded — "to the best of anyone's knowledge" is as far as the trade's memory reaches — and it survives not by certification but because it's close enough in the heart of the chart, where most residential charging happens.
The best account we have is recounted by HVAC School, who put the question to Wayne Pendergast, keeper of several published versions of the chart. The story he passed on traces to Carrier: a residential AC charged perfectly by weight on the lab bench, then run across a matrix of indoor and outdoor conditions, with the resulting superheat plotted at each point. That plot of measured points — not a derived formula — is the ancestor of every target superheat chart in the trade. Treat this as an industry account, not audited history: the story is consistent and it explains why the chart's shape is empirical rather than analytic.
The familiar formula TSH = ((3 × WB) − 80 − DB) / 2 came later. HVAC School describes it as "likely reverse engineered" from the chart — a linear fit through the plotted points, chosen because it's easy to compute in the field. That inversion matters: the chart is the primary artifact and the formula approximates it, not the other way around. Every chart-vs-formula discrepancy is the formula falling short of the chart, never the chart drifting from the formula.
AC Service Tech's target-superheat article says the same thing in different words: the formula "may not match exactly" the chart it approximates. In practice, the small discrepancies near the middle of the chart widen substantially toward its corners — enough that the formula stops being trustworthy exactly where charging is most delicate.
That's why this page publishes the matrix, not just the formula. The lookup renders the formula's output, but the intent is that techs read the matrix cell rather than compute the formula, because the matrix's structure — blank cells where the formula returns unreliable low targets, tighter accuracy near the center — carries information the formula alone doesn't. The blank sub-5°F cells aren't a coding choice; they're the honest boundary of a chart that was measured, not derived.
Method boundaries — fixed-orifice only, climate limits
The chart applies to fixed-orifice metering (pistons, capillary tubes, accurators) and to nothing else. The mechanism is clean: on a fixed-orifice system, superheat is what the charge produces — add refrigerant and SH falls, remove refrigerant and SH rises, so SH IS the charging indicator. On a TXV or EEV system, the valve actively holds SH near a fixed setpoint by throttling; SH tells you the valve is working, not what the charge is. HVAC School's fixed-orifice-only rule reduces to that mechanical distinction: use superheat to charge a system whose superheat responds to charge, and use subcooling for a system whose subcooling does. See R-410A charging chart for the TXV / subcooling side.
Some climates push the target chronically low. Dry-air regions with warm outdoor DB (typical Southwest US, high-elevation summers) sit in the low-WB / high-DB corner of the chart where targets fall below 5°F and the formula stops being trustworthy. AC Service Tech's guidance for chronically-low-target sites is to consider a TXV conversion or an accumulator on the suction line — either restores enough operating margin that the system can be charged and diagnosed with confidence instead of at the noisy edge of the chart.
The 95°F outdoor dry-bulb rating condition that anchors the middle of the chart isn't arbitrary — it's the AHRI Standard 210/240 cooling rating condition, which is why residential AC specs and charging charts across the industry converge on the same DB axis.
- Fixed-orifice: piston, capillary tube, or accurator devices — the target-superheat method applies.
- TXV / EEV: the valve controls superheat directly. Superheat tells you the valve is working, not the charge. Charge to a subcooling target instead.
- Distributor + fixed-orifice: same as bare fixed-orifice; use this chart.
- Multi-circuit condensing units: use the chart on the coil serving the current load; measure superheat downstream of the accumulator.
Where measurement error moves the target — and which way
The formula's coefficient on WB is 3× the DB coefficient's magnitude. A 1°F wet-bulb probe error moves target SH by 1.5°F (from ∂TSH/∂WB = 3/2); a 1°F dry-bulb error moves it by 0.5°F (from ∂TSH/∂DB = −1/2). That's why WB measurement discipline (a properly wetted wick given time to stabilize at the return grille) dominates the accuracy of a target-superheat charge. A wick that's gone dry reads 3°F high — that lifts the computed target from 12°F to 16.5°F, and a tech chasing that phantom target pulls refrigerant until the system is undercharged — an error that quietly sabotages an otherwise careful charge.
The direction of the error depends on which way each reading is off. The formula puts WB and DB on opposite signs (+3×WB vs −DB), so under-reading WB and over-reading DB both LOWER the computed target — the tech chases a target below the true one, adds refrigerant to reach the lower SH on the fixed-orifice system, and lands the machine overcharged. Over-reading WB and under-reading DB do the reverse: computed target above the true target, tech under-charges. Both errors produce the same operational fingerprint (SC drifts away from spec) but demand opposite corrections. The field rule: measure WB with care first (its coefficient is 3× DB's), DB with care second; if the computed target moves more than 2°F on re-measurement, one of the readings is dirty.
When the target reads "—"
Cells below 5°F render as "—" in the matrix above. The rendered value is the mathematical output of the formula, but it's unreliable to charge by superheat when the target is that low: a 1–2°F thermometer or manifold-gauge error swamps the setpoint, and the operating condition itself (very low WB with high DB, or very high WB with low DB) is often outside the fixed-orifice envelope.
- Verify indoor WB measurement — a dry psychrometer wick reads 2–3°F low; wet it and re-read.
- Verify outdoor DB — measure in shade at the condenser intake, not in direct sun.
- Check whether the system is really fixed-orifice — many modern systems (post ~2015) are TXV even on entry-level residential AC. If so, use subcooling.
- If conditions are legitimately extreme (winter charging, very dry indoors), accept a nameplate weight charge without SH verification. Come back on a more moderate day to fine-tune.
How to use the chart in the field
- Steady-state the system for 10–15 minutes at design conditions (or as close as ambient allows).
- Measure indoor WB at the return-air grille with a digital or sling psychrometer.
- Measure outdoor DB in shade, near the condenser intake, 4–6 feet off the ground.
- Look up target superheat in the matrix above (or enter WB + DB in the interactive lookup).
- Read suction pressure at the manifold gauge, convert to saturation temp on the PT chart for the specific refrigerant (see linked pages below).
- Measure suction-line temperature at the service port with a contact probe. Subtract sat temp to get measured superheat.
- Add refrigerant in 1–2 oz increments if measured SH is above target (undercharge); recover in the same increments if measured SH is below target (overcharge). Re-check after 5 minutes to allow re-steadying.
Fluid-specific target-superheat pages
The universal chart above works for any fixed-orifice fluid. The per-fluid pages combine the target with a saturation-pressure quick table so you don't need a separate PT chart in hand:
- R-410A superheat chart — target matrix + R-410A saturation at 35–55°F evap.
- R-22 superheat chart — target matrix + R-22 saturation at 35–55°F evap.
- R-410A charging chart — both methods (subcooling for TXV, superheat for fixed-orifice) on one page.
Related tools
- Superheat Calculator — computes measured superheat from suction PSIG and line temperature for any of the 49 CoolProp-modeled refrigerants.
- Subcooling Calculator — the corresponding tool for TXV/EEV systems that charge by subcooling instead.
- Superheat & Subcooling Fundamentals — theory, measurement, target values by system type, diagnostic patterns.
Frequently asked
›What is a target superheat chart?
A two-dimensional lookup table that gives the target superheat setpoint for a fixed-orifice HVAC system as a function of indoor wet-bulb (WB) and outdoor dry-bulb (DB) temperatures. The chart is the operational form of the formula TSH = ((3 × WB) − 80 − DB) / 2 — a formula that circulates through OEM and technician charging references, its precise origin unrecorded (see the provenance section on this page for the recounted Carrier / HVAC School account). Techs charging fixed-orifice equipment measure WB and DB, look up the target, then add or recover refrigerant until the measured superheat at the suction line matches the target within a few degrees.
›Does the same chart work for R-410A, R-22, R-32, and R-454B?
Yes — the target superheat formula is refrigerant-independent. It sets the operating point at the evaporator based on load conditions (WB) and heat-rejection conditions (DB). The refrigerant only enters when you convert measured suction pressure to saturation temperature: use the PT chart for the specific refrigerant. See the R-410A-specific and R-22-specific pages linked below for combined lookups.
›Why isn't this chart used for TXV or EEV systems?
TXV and EEV metering devices actively regulate refrigerant flow to hold suction superheat near a fixed setpoint (typically 8–15°F at the TXV outlet). Superheat on those systems tells you the valve is working — it does NOT tell you the charge. On a TXV system, undercharge shows up as low subcooling on the liquid line, not as high superheat. Charge TXV / EEV systems to a subcooling target (typically 8–12°F per the OEM nameplate).
›Where does the (3 × WB) − 80 − DB formula come from?
The formula circulates through OEM and technician charging references; its precise origin is unrecorded (see the provenance section above for the recounted Carrier / HVAC School account). The same math appears on major OEM (Carrier, Trane, Lennox, Rheem) fixed-orifice equipment charging labels — the bead charts on the outdoor unit's access panel are point renderings of the same underlying data, sometimes with 1–2°F OEM adjustments for coil geometry.
›What happens when the target comes out below 5°F?
Below 5°F the target becomes unreliable — a routine 1–2°F probe-error swamps the setpoint, and the operating condition is often outside the fixed-orifice envelope (low WB with high DB, or high WB with low DB). Industry charging charts blank these cells; charging in this region requires either running the system to a more central operating point (raise indoor load or wait for cooler ambient) or accepting a nameplate weight charge without SH verification.
›How do I measure indoor wet-bulb accurately?
Use a digital psychrometer or a sling psychrometer at the return-air grille after the system has run for 15+ minutes at design conditions. Wet the wick with distilled water; wait for the reading to stabilize (30–60 seconds). Do not measure at the supply grille (post-conditioning) or in an unconditioned space. Some techs use a wet-bulb probe on the return duct just upstream of the coil — that's equivalent.
›How do I use this chart in the field?
Steady-state the system for 10–15 minutes. Measure indoor WB at the return grille and outdoor DB (shaded, near the condenser). Look up the target superheat in the matrix above. Read the suction pressure at the manifold gauge, convert to saturation temp on the PT chart for the refrigerant, then measure the suction-line temperature and subtract. Add or recover refrigerant in small increments until measured SH matches target SH.