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 the formula comes from
The formula TSH = ((3 × WB) − 80 − DB) / 2 is codified in the Air Conditioning Contractors of America Manual T (Air Distribution Basics), and is the same math printed on the fixed-orifice charging labels of major OEMs (Carrier, Trane, Lennox, Rheem). The OEM "bead charts" on the outdoor unit access panel are discrete-point renderings of the formula, sometimes with 1–2°F adjustments for coil geometry.
The physical meaning: at higher indoor WB (more latent + sensible load), the evaporator absorbs more heat and superheat rises for a fixed charge — so the target rises. At higher outdoor DB (harder heat rejection), the condenser saturates hotter, subcooling rises for a fixed charge, and the target superheat falls. The formula balances those two so that a properly-charged system sits at the target across the operating envelope.
- 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.
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 ACCA Manual T charging formula: TSH = ((3 × WB) − 80 − DB) / 2. 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 is codified in ACCA Manual T (Air Distribution Basics for Residential and Small Commercial Buildings) and is the same math the major OEMs (Carrier, Trane, Lennox, Rheem) print on their fixed-orifice equipment charging labels. The bead charts on the outdoor unit's access panel are discrete-point renderings of this formula — 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.