HVAC Calculator
Superheat & Subcooling Calculator
Intry VerifiedA worked default reading, traceable end to end: what it was calculated from, what it was run with, how it was checked, and who has final say.
Where this number comes fromIntry Verified
- Calculated from
- R-410A PT chart, TXV
- Run with
- RefrigerantR-410A
- MeteringTXV
- Suction118 psig, 52°F
- Liquid350 psig, 96°F
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the locked modules each figure is traced to: the manufacturer PT tables behind the saturation temperatures, superheat and subcooling, and the transcribed code and service-literature constants behind the charge, metering, airflow, approach and discharge line figures, each quoted with its own source in the panel it appears in before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- The unit nameplate and the manufacturer's charging chart have final say.
Intry Verified · Build AE2FC52 · 2026-08-21
By refrigerant: dedicated calculators & PT charts
Charging a piston or cap-tube system? Use the target superheat calculator to set the charge from indoor wet-bulb and outdoor dry-bulb. Just need the numbers? See normal superheat & subcooling values by refrigerant, learn how to calculate superheat & subcooling, or print the superheat & subcooling chart. Just checking subcooling on a TXV system? Use the dedicated subcooling calculator.
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Target Ranges by Refrigerant & Metering Device
| Refrigerant | Type | TXV Superheat | TXV Subcooling | Fixed Superheat | Fixed Subcooling |
|---|---|---|---|---|---|
| R-410A | HFC | 8–15°F | 8–14°F | 5–20°F | 4–10°F |
| R-454B | A2L | 8–15°F | 8–14°F | 5–20°F | 4–10°F |
| R-32 | A2L | 8–15°F | 8–14°F | 5–20°F | 4–10°F |
| R-22 | HCFC | 10–18°F | 8–14°F | 5–25°F | 4–12°F |
| R-134a | HFC | 8–14°F | 8–14°F | 5–18°F | 4–10°F |
| R-407C | HFC | 8–15°F | 8–14°F | 5–20°F | 4–10°F |
R-410A Pressure-Temperature Reference
| Pressure (psig) | Sat. Temp (°F) |
|---|---|
| 60 | 8°F |
| 80 | 21°F |
| 100 | 32°F |
| 120 | 41°F |
| 140 | 49°F |
| 160 | 57°F |
| 180 | 63°F |
| 200 | 70°F |
| 250 | 84°F |
| 300 | 96°F |
| 350 | 107°F |
| 400 | 117°F |
What Is Superheat?
Superheat is the temperature difference between the actual measured temperature of refrigerant vapor and its saturation temperature at the same pressure. It tells you how much heat the vapor has absorbed beyond the point where the last drop of liquid boiled off. In practical terms, superheat is measured at the suction line, the low-pressure vapor line between the evaporator outlet and the compressor inlet.
To measure superheat in the field, you need two readings: the suction pressure (from a gauge on the suction service valve) and the actual suction line temperature (from a pipe clamp thermometer or thermocouple on the suction line near the service valve). Convert the suction pressure to its corresponding saturation temperature using a PT chart for your specific refrigerant. Then subtract: Superheat = Measured Suction Temp − Saturation Suction Temp.
For example, if you read 118 psig on the suction gauge of an R-410A system, the saturation temperature is 40°F. If your pipe clamp reads 50°F, the superheat is 50 − 40 = 10°F. This means the refrigerant vapor has been heated 10 degrees beyond its boiling point at that pressure.
Why does superheat matter? It protects the compressor. Compressors are designed to pump vapor, not liquid. If liquid refrigerant reaches the compressor (a condition called liquid slugging or floodback), it can cause immediate mechanical damage: broken valves, damaged scrolls, and catastrophic failure. Superheat confirms that all liquid has fully evaporated before reaching the compressor. Too little superheat (below 5°F) means liquid may be present. Too much superheat (above 20°F) means the evaporator is starved and system capacity drops because the refrigerant is picking up useless heat in the suction line instead of absorbing heat from the conditioned space.
What Is Subcooling?
Subcooling is the temperature difference between the saturation temperature of the refrigerant at the liquid line pressure and the actual measured temperature of the liquid refrigerant. It tells you how far below its condensing temperature the liquid has been cooled. Subcooling is measured at the liquid line, the high-pressure liquid line between the condenser outlet and the metering device inlet.
The measurement process mirrors superheat but on the high side: read the liquid line pressure, convert to saturation temperature, then measure the actual liquid line temperature. The formula is reversed: Subcooling = Saturation Liquid Temp − Measured Liquid Temp. Note the subtraction order: subcooling is saturation minus actual, because the liquid is cooler than its saturation point.
For example, if you read 350 psig on the liquid line of an R-410A system, the saturation temperature is 107°F. If your pipe clamp reads 97°F, the subcooling is 107 − 97 = 10°F. This means the liquid refrigerant has been cooled 10 degrees below its condensing temperature.
Subcooling matters because it indicates how much liquid refrigerant is stacked in the condenser. Higher subcooling means more liquid is present in the bottom of the condenser, a full column of liquid that ensures only pure liquid reaches the metering device. If subcooling is too low, flash gas (vapor bubbles) can form in the liquid line, starving the metering device and reducing system capacity. If subcooling is too high, the condenser is overfilled with liquid, raising head pressure and increasing compressor work.
Why Read Them Together
Neither superheat nor subcooling alone gives you the complete picture of system operation. A system can have normal superheat but abnormal subcooling, or vice versa, and each combination points to a different root cause. Reading both together creates a diagnostic matrix that narrows the problem quickly.
Consider two systems that both show 15°F superheat. On system A, subcooling is 5°F, which points to low charge (not enough refrigerant to fill the condenser). On system B, subcooling is 20°F, which points to a restriction or non-condensable gases (the condenser is overfull, but the evaporator is still starved because refrigerant cannot flow freely). The superheat alone looks identical, but the subcooling tells you completely different stories.
This is why experienced technicians always take four readings: suction pressure, suction temperature, liquid pressure, and liquid temperature. From these four measurements you calculate both superheat and subcooling, and the pair of values together points you toward the correct diagnosis.
TXV vs Fixed Orifice
The type of metering device determines which measurement is your primary charging indicator. A thermostatic expansion valve (TXV) actively regulates superheat by adjusting its orifice based on the sensing bulb temperature. Because the TXV controls superheat, adding or removing charge does not significantly change superheat because the valve compensates. Instead, charge level shows up in subcooling. With a TXV system, subcooling is your primary charging indicator: low subcooling means low charge, high subcooling means overcharge. Target subcooling is typically 8–14°F for R-410A.
A fixed orifice (piston or capillary tube) has no active regulation. The orifice size is fixed, so both superheat and subcooling respond to charge level. With a fixed orifice, superheat is your primary charging indicator. Target superheat varies with outdoor ambient temperature and indoor wet bulb temperature, so you must use the manufacturer's charging chart, which typically plots these two variables to give a specific superheat target (usually 5–20°F depending on conditions). Charging a fixed-orifice system to a single superheat number without consulting the chart is a common field error.
Most modern residential equipment uses TXVs, but millions of systems with fixed orifices are still in service. Always identify the metering device before interpreting your readings. Look for the TXV body on the indoor coil. It has a distinctive shape with a sensing bulb clamped to the suction line. If you see a small brass piston or cartridge, it is a fixed orifice.
Target Superheat: The Fixed-Orifice Charging Method
On a fixed-orifice system you do not charge to a fixed number. You charge to a target superheat that depends on two load conditions: the indoor return-air wet-bulb temperature and the outdoor dry-bulb temperature at the condenser. A humid indoor coil and a mild outdoor day call for a high superheat target; a dry coil on a hot day calls for a low one. You measure the actual superheat, compare it to the target, and adjust: measured above target means the system is undercharged (add refrigerant); measured below target means it is overcharged (recover). Set the metering device to Fixed Orifice above and the calculator does this for you, live.
Target values come from the California Energy Commission Title 24 Reference Appendix RA3, Table RA3.2-2, the state HERS charging reference. Cells are machine-parsed from the CEC's published Reference Appendices rather than hand-typed, and the shipped table is spliced from two printings (the 2019 appendices for the milder outdoor rows, the 2022 for the hot ones) and re-checked against the 2025 edition. Those are all one publisher, so agreement between them catches a transcription slip and is not an independent confirmation of the values.
For an outside check, 168 of this table's 1054 cells were compared against manufacturer piston charging grids on 2026-08-04, which is about a sixth of it, and the rest carry no manufacturer check at all. ICP's R-410A air-conditioner grid (421 01 5103 00, Figure 12) agrees on all 115 cells it shares with this table, a mean of 0.08F and never more than 1F apart. Goodman's grid (IOG-4048C) does not: across the 53 cells it shares, this table runs a mean 4.77F and a worst 10F ABOVE Goodman's, and 15 of those 53 cells are further apart than the whole plus or minus 5F band a fixed-orifice charge is graded against. The gap is not flat: it widens from about 3F at mild outdoor temperatures to about 8F at 100F and above, so it is worst in the hot humid weather when most charging calls happen.
Do not read the ICP agreement as proof this table is right. Agreement shows two publishers print the same targets, and both may inherit one lineage; on these cells the CEC and ICP grids both track the classic trade shortcut closely while Goodman's does not, so Goodman may be the genuinely independent chart here rather than the outlier. The two manufacturer grids share no cell directly, since ICP publishes even wet-bulb columns and Goodman odd ones, though that rests on reading one ambiguous ICP column label as 66 rather than 67, a reading confirmed by interpolating its neighbours rather than by fitting it to this table. Interpolating ICP's own grid onto Goodman's columns reproduces the same 4.77F gap with this table nowhere in the path, so the disagreement is between the manufacturers and is not an artifact of ours.
A widely-shared field shortcut, [(3 × WB) − 80 − DB] ÷ 2, approximates that chart, but it drifts by a degree or two near typical conditions and by more at the edges, so this tool reads the RA3.2-2 table directly instead of the shortcut. Worked example: at an indoor wet-bulb of 66°F and an outdoor dry-bulb of 90°F, the target superheat is 13°F. If your suction line reads 20°F of superheat, that is 7°F high, so the system is low on charge. Where the table shows a dash (target below 5°F, which happens when the outdoor air is very warm or the indoor coil is dry), a valid superheat charge check cannot be performed and you should use subcooling, weigh-in, or wait for conditions to change.
For the full RA3.2-2 chart, a worked example, and a calculator that opens straight in fixed-orifice mode, see the dedicated target superheat calculator.
The A2L Transition
The HVAC industry is transitioning from R-410A (a high-GWP HFC) to lower-GWP A2L refrigerants under the AIM Act and EPA regulations. R-454B (marketed as Puron Advance by Carrier) is the primary replacement for R-410A in ducted residential and light commercial systems. Starting January 1, 2025, new residential and commercial AC and heat pump equipment manufactured in the US must use lower-GWP alternatives.
R-454B operates at similar pressures to R-410A, so superheat and subcooling target ranges are comparable (8–15°F superheat with TXV, 8–14°F subcooling). However, R-454B is a mildly flammable (A2L) refrigerant, which means equipment requires leak detection, reduced charge limits, and modified installation practices per updated UL standards. Existing R-410A systems continue to be serviced with R-410A. R-454B is not a drop-in replacement.
R-32 is another A2L refrigerant gaining ground, primarily in ductless mini-split systems. It has a lower GWP than R-410A (675 vs 2088) and higher volumetric capacity, meaning systems can use smaller charge sizes. R-32 is already widely deployed in Asia and Europe. In the US, Daikin, Mitsubishi, and Fujitsu now offer R-32 mini-splits. Superheat and subcooling ranges are similar to R-410A. R-32 is a single-component refrigerant (not a blend), so there is no zeotropic glide to account for.
Diagnostic Patterns (4 Quadrants)
When you plot superheat and subcooling together, four diagnostic quadrants emerge. Each combination of high/low superheat and high/low subcooling points to a specific class of problems. Learning these patterns lets you diagnose systems quickly in the field.
| Superheat | Subcooling | Most likely cause | First thing to check |
|---|---|---|---|
| High | Low | Undercharge (low on refrigerant) | Find the leak before adding charge, then weigh in to the target subcooling |
| Low | High | Overcharge | Rule out low condenser airflow, then recover in small steps to the target subcooling |
| High | High | Restriction (liquid line or metering device) | Filter-drier, TXV, or a cold spot on the liquid line |
| Low | Low | Weak compressor or an overfeeding TXV | Compressor amp draw vs rated load amps, and the TXV sensing bulb charge |
High Superheat + Low Subcooling = Low Charge
This is the most common pattern. The evaporator is starved because there is not enough refrigerant in the system. Liquid boils off early in the evaporator coil, and the remaining coil surface superheats the vapor excessively. On the high side, the condenser does not have enough refrigerant to fill its lower portion with liquid, so subcooling is low. Check for leaks at service valves, flare connections, the evaporator coil, and the line set before adding charge. A system that lost charge did not lose it to nowhere: there is a leak.
Low Superheat + High Subcooling = Overcharge
Too much refrigerant in the system. The condenser is flooded with liquid (high subcooling), which backs up head pressure. The evaporator is oversupplied with liquid, so vapor barely superheats before reaching the compressor (low superheat). If superheat drops below 5°F, liquid floodback to the compressor is a risk. The fix is to recover excess charge to bring subcooling into the target range. On TXV systems, the valve may mask this condition by maintaining superheat while subcooling climbs, so always check subcooling on TXV systems.
High Superheat + High Subcooling = Restriction
A restriction in the liquid line or metering device traps refrigerant on the high side (high subcooling) while starving the evaporator (high superheat). Common restrictions include a clogged filter-drier, a kinked liquid line, a partially blocked TXV, or wax buildup in the metering device from contaminated refrigerant. The pressure drop across the restriction is visible as an abnormal temperature drop at the restriction point. Feel along the liquid line for a cold spot. Replace the filter-drier or metering device as indicated.
Low Superheat + Low Subcooling = Compressor or TXV Issue
When both readings are low, the system is not moving refrigerant effectively. The compressor may have weak valves (low compression ratio, poor pumping), or the TXV may be stuck open (flooding the evaporator while the condenser cannot build adequate head pressure to subcool the liquid). Check compressor amp draw versus rated load amps. If amps are significantly low, the compressor is likely failing. If the TXV sensing bulb has lost its charge, the valve may hang open. Also check for a faulty reversing valve on heat pump systems, which can internally bypass refrigerant between the high and low sides.
Target Temperature Split: the full CEC table (2019 and 2022 Joint Appendices)
The four quadrants above tell you what a superheat and subcooling pair means. They assume the air side is already right, and that assumption is where most charging mistakes start. A restricted coil and an overcharge both push superheat down, so the gauges alone cannot separate them. The air side can, and it takes two thermometers: return air dry-bulb and return air wet-bulb.
Below is Table JA6.1-1 in full, all 312 published values. Find your return dry-bulb down the left, your return wet-bulb across the top, and read the target split where they meet. Subtract your supply-air temperature from your return-air temperature to get the actual split. If the actual is more than 5°F above the target, the system fails the minimum airflow criterion and no charge conclusion is valid until that is fixed. The criterion is one-sided: it says nothing about a split that comes in under target. Enter both readings in the calculator above and it does this lookup and the comparison for you.
Take the reading under the conditions the criterion assumes, or it will lie to you. The same section states them: the system must have run for 15 minutes, and the air entering the condenser must be at or above 65°F. Both matter in the same direction. A coil that has not run long enough is still dry and doing little latent work, and cool condenser air raises capacity, so each one widens the split and can manufacture a failed airflow test on a system whose airflow is fine. That is how you end up pulling a clean filter. The calculator refuses to draw an airflow conclusion below 65°F rather than reporting one, and it states the 15 minute assumption every time, because it cannot check runtime for you.
| DB \ WB | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 70 | 20.9 | 20.7 | 20.6 | 20.4 | 20.1 | 19.9 | 19.5 | 19.1 | 18.7 | 18.2 | 17.7 | 17.2 | 16.5 | 15.9 | 15.2 | 14.4 | 13.7 | 12.8 | - | - | - | - | - | - | - | - | - |
| 71 | 21.4 | 21.3 | 21.1 | 20.9 | 20.7 | 20.4 | 20.1 | 19.7 | 19.3 | 18.8 | 18.3 | 17.7 | 17.1 | 16.4 | 15.7 | 15.0 | 14.2 | 13.4 | 12.5 | - | - | - | - | - | - | - | - |
| 72 | 21.9 | 21.8 | 21.7 | 21.5 | 21.2 | 20.9 | 20.6 | 20.2 | 19.8 | 19.3 | 18.8 | 18.2 | 17.6 | 17.0 | 16.3 | 15.5 | 14.7 | 13.9 | 13.0 | 12.1 | - | - | - | - | - | - | - |
| 73 | 22.5 | 22.4 | 22.2 | 22.0 | 21.8 | 21.5 | 21.2 | 20.8 | 20.3 | 19.9 | 19.4 | 18.8 | 18.2 | 17.5 | 16.8 | 16.1 | 15.3 | 14.4 | 13.6 | 12.6 | 11.7 | - | - | - | - | - | - |
| 74 | 23.0 | 22.9 | 22.8 | 22.6 | 22.3 | 22.0 | 21.7 | 21.3 | 20.9 | 20.4 | 19.9 | 19.3 | 18.7 | 18.1 | 17.4 | 16.6 | 15.8 | 15.0 | 14.1 | 13.2 | 12.2 | 11.2 | - | - | - | - | - |
| 75 | 23.6 | 23.5 | 23.3 | 23.1 | 22.9 | 22.6 | 22.2 | 21.9 | 21.4 | 21.0 | 20.4 | 19.9 | 19.3 | 18.6 | 17.9 | 17.2 | 16.4 | 15.5 | 14.7 | 13.7 | 12.7 | 11.7 | 10.7 | - | - | - | - |
| 76 | 24.1 | 24.0 | 23.9 | 23.7 | 23.4 | 23.1 | 22.8 | 22.4 | 22.0 | 21.5 | 21.0 | 20.4 | 19.8 | 19.2 | 18.5 | 17.7 | 16.9 | 16.1 | 15.2 | 14.3 | 13.3 | 12.3 | 11.2 | 10.1 | - | - | - |
| 77 | - | 24.6 | 24.4 | 24.2 | 24.0 | 23.7 | 23.3 | 22.9 | 22.5 | 22.0 | 21.5 | 21.0 | 20.4 | 19.7 | 19.0 | 18.3 | 17.5 | 16.6 | 15.7 | 14.8 | 13.8 | 12.8 | 11.7 | 10.6 | 9.5 | - | - |
| 78 | - | - | - | 24.7 | 24.5 | 24.2 | 23.9 | 23.5 | 23.1 | 22.6 | 22.1 | 21.5 | 20.9 | 20.2 | 19.5 | 18.8 | 18.0 | 17.2 | 16.3 | 15.4 | 14.4 | 13.4 | 12.3 | 11.2 | 10.0 | 8.8 | - |
| 79 | - | - | - | - | - | 24.8 | 24.4 | 24.0 | 23.6 | 23.1 | 22.6 | 22.1 | 21.4 | 20.8 | 20.1 | 19.3 | 18.5 | 17.7 | 16.8 | 15.9 | 14.9 | 13.9 | 12.8 | 11.7 | 10.6 | 9.4 | 8.1 |
| 80 | - | - | - | - | - | - | 25.0 | 24.6 | 24.2 | 23.7 | 23.2 | 22.6 | 22.0 | 21.3 | 20.6 | 19.9 | 19.1 | 18.3 | 17.4 | 16.4 | 15.5 | 14.4 | 13.4 | 12.3 | 11.1 | 9.9 | 8.7 |
| 81 | - | - | - | - | - | - | - | 25.1 | 24.7 | 24.2 | 23.7 | 23.1 | 22.5 | 21.9 | 21.2 | 20.4 | 19.6 | 18.8 | 17.9 | 17.0 | 16.0 | 15.0 | 13.9 | 12.8 | 11.7 | 10.4 | 9.2 |
| 82 | - | - | - | - | - | - | - | - | 25.2 | 24.8 | 24.2 | 23.7 | 23.1 | 22.4 | 21.7 | 21.0 | 20.2 | 19.3 | 18.5 | 17.5 | 16.6 | 15.5 | 14.5 | 13.4 | 12.2 | 11.0 | 9.7 |
| 83 | - | - | - | - | - | - | - | - | - | 25.3 | 24.8 | 24.2 | 23.6 | 23.0 | 22.3 | 21.5 | 20.7 | 19.9 | 19.0 | 18.1 | 17.1 | 16.1 | 15.0 | 13.9 | 12.7 | 11.5 | 10.3 |
| 84 | - | - | - | - | - | - | - | - | - | 25.9 | 25.3 | 24.8 | 24.2 | 23.5 | 22.8 | 22.1 | 21.3 | 20.4 | 19.5 | 18.6 | 17.6 | 16.6 | 15.6 | 14.4 | 13.3 | 12.1 | 10.8 |
Read the shape, not just your cell. Target split falls as the return air gets more humid, because a wet coil spends more of its capacity condensing moisture and less of it dropping temperature. At 75°F return air the target runs 23.6°F at a dry 50°F wet-bulb and only 10.7°F at a humid 72°F wet-bulb. That is why a single remembered number fails: a 15°F split is right on target at a 68°F wet-bulb and sits nearly 9°F under target at a 50°F one, on the same equipment, on the same thermostat setting, in different weather. Note which direction that is. A split under target does not fail the airflow criterion, because the criterion is one-sided; the field procedure reads a much narrower split than target as lost capacity from some fault other than airflow, and this tool states it that way rather than sending you to check a filter that is fine. Proctor Engineering, whose field sheet is one of the sources this table was checked against, puts the underlying point plainly: contrary to common knowledge, the split should not universally be between 18°F and 20°F.
A tighter threshold exists, and the source we checked against publishes it. The 5°F above is JA6’s. The Proctor Engineering field sheet uses 3°F, and it uses it on both sides: more than 3°F above target and the airflow is low, more than 3°F below and the unit “probably has low capacity due to some fault other than airflow.” So a split sitting 4°F over target passes the criterion this tool applies and would not pass Proctor’s. We report the JA6 number because it is the one written into a code appendix and we can quote it exactly, and we are telling you the field sheet is tighter so you can use your own judgment on a borderline reading. The minus side is worth knowing for the same reason: a split well under target is not an airflow fault under either source, and the direction it points is lost capacity from something else.
Provenance, including the part most sources leave out. These values are transcribed from the 2019 California Joint Appendices (CEC-400-2018-021-CMF, folio JA6-6), which typesets the table without the digit clipping present in the 2022 printing, and every cell was checked against the 2022 edition and against an independent Carrier and CheckMe! lineage printing published by Proctor Engineering. All three agree at all 312 cells. Table JA6.1-1 is not in the 2025 edition. The 2025 Reference Appendices mark section JA6.1 RESERVED, and the California Energy Commission states that buildings whose permit applications are filed on or after 1 January 2026 must comply with the 2025 Energy Code. Direct measurement of airflow is not new and did not replace this: it is in Appendix RA3.3 of the 2019, 2022 and 2025 editions alike, and it was always the compliance path. What 2025 removed is JA6.1, the specification for an automated fault indicator display. That is also the scope worth knowing when you read the criterion: JA6.1.6 describes what a permanently installed display must compute, and the numeric threshold transfers cleanly to a reading you take by hand, but the appendix was not written as a manual field procedure. The table remains sound engineering and remains what most equipment literature is built on. It is a criterion from a named superseded edition rather than current California regulation, and you should know that before you cite it.
Zeotropic Glide
Zeotropic refrigerant blends (also called zeotropes) are mixtures of two or more refrigerants that have different boiling points. Unlike azeotropic blends (which behave like a single refrigerant), zeotropes boil and condense over a range of temperatures at the same pressure. This temperature range is called glide.
R-407C is the most common zeotropic blend in HVAC service, with a glide of approximately 9–11°F. At any given pressure, R-407C has two relevant saturation temperatures: the bubble point (where the first bubble of vapor forms as liquid heats up) and the dew point (where the last drop of liquid condenses as vapor cools down). The bubble point is always lower than the dew point by the amount of the glide.
For accurate superheat and subcooling measurements on zeotropic refrigerants, you must use the correct reference point. Use the dew point temperature for superheat calculations (because you are measuring vapor that has passed the dew point) and the bubble point temperature for subcooling calculations (because you are measuring liquid that has cooled below the bubble point). Using the wrong reference point introduces an error equal to the glide (9–11°F on R-407C), which is enough to completely misdiagnose a system. This calculator handles glide automatically when you select a zeotropic refrigerant.
R-410A and R-32 have negligible or zero glide (R-410A is technically a near-azeotropic blend with less than 0.3°F glide, and R-32 is a pure component), so a single saturation temperature is fine. R-454B is a near-azeotrope with a small glide of about 2°F, so this calculator still splits it into bubble and dew points the same way it handles R-407C. Glide is a large concern with R-407C, R-448A, and R-449A (8–11°F) and a small but real one with R-454B.
Cold-Weather Charging: Trust the Gauges, or Weigh It In?
Below 70°F outdoor ambient, do not charge R-454B by subcooling. That is the manufacturer's own boundary, and it is stricter than the 55°F figure this page carried until 2026-08-03. Adjust the charge for line-set length only, and check the charge again when both temperatures are back inside the window. The subcooling method assumes conditions inside the manufacturer's charging window, and cold weather falls outside it.
The subcooling charging method only works inside a window of favorable conditions. Carrier's R-454B installation instructions (PH5SAN5) state it verbatim: “Charging method is shown on information plate inside unit. For TXV, use subcooling method. For piston, use superheat method. To properly check or adjust charge, conditions must be favorable for subcooling or superheat charging. Favorable conditions exist when the outdoor temperature is between 70°F and 100°F (21.1°C and 37.8°C), and the indoor temperature is between 70°F and 80°F (21.1°C and 26.7°C).” Outside that window Carrier directs the technician to adjust the charge for line-set length above or below 15 ft only, and to check the charge at another time when both temperatures are in a more favorable range. Cold-weather service calls sit outside the window, so the gauge-based subcooling number is not trustworthy no matter how carefully you read it.
Two corrections were made here on 2026-08-03, and they are stated rather than quietly swapped. This page used to attribute the window to “York, Johnson Controls, and Daikin”. York is Johnson Controls, so that named one manufacturer twice and read as three; and Daikin does not publish an R-454B manual at all, because Daikin, Goodman and Amana chose R-32 for the A2L transition. The window above is Carrier's.
Two things break the reading in the cold. First, low outdoor ambient drops the condensing pressure, which compresses and distorts the subcooling number the same way it does on any system. Second, R-454B is a zeotropic blend (68.9% R-32, 31.1% R-1234yf) with about a 2°F temperature glide between its bubble and dew points, so the saturation temperature you convert to depends on which point you use. In cold conditions that glide is enough to push a borderline reading across the line and tempt an overcharge.
There is a published way to charge in the cold, and it is not a lower ambient threshold. The Johnson Controls (York) R-454B split-system installation manual gives a charging tent procedure: enclose the outdoor unit, then “based on the outdoor ambient temperature, begin adjusting windows in order to achieve a stable temperature above 55°F inside the tent”, read the charging chart against the temperature inside the tent rather than the outdoor ambient, and let the unit stabilise at least 15 minutes. That 55°F is the temperature you create inside the tent, not an outdoor ambient you are allowed to charge down to. This page previously published it as the latter, which is the same figure attached to the wrong rule, and it made us less conservative than the manufacturer between 55 and 70°F.
There is also a patience problem. An A2L system can take noticeably longer to stabilize enough for an accurate subcooling reading in cold weather than a comparable R-410A system. We have not sourced a manufacturer-published stabilization time and so do not print one. A reading taken before the system settles runs low, which again tempts an overcharge that shows up as high head pressure once the weather warms. If you are below the window, the weigh-in skips the wait entirely: recover, evacuate if needed, and charge to the nameplate weight plus the per-foot line-set adjustment.
| Outdoor ambient | Method | Why |
|---|---|---|
| 70–100°F | Subcooling (TXV) or manufacturer chart (fixed orifice) | Inside the charging window; gauges are reliable |
| Below 70°F | Adjust for line-set length only, or build a charging tent and hold it above 55°F | Outside the manufacturer's favorable window; glide plus low condensing pressure make subcooling unreliable. Carrier says check the charge again when conditions are favorable |
| Indoor outside 70–80°F | Above 80°F, adjust by weight and let the indoor drop to 80°F first; below 70°F, line-set adjustment only | The window has an indoor half, and it is the half most often ignored |
Worked Examples
Example 1: R-410A Residential System with TXV
A residential R-410A split system with a TXV metering device. Field readings: suction pressure 118 psig, suction line temperature 50°F, liquid pressure 340 psig, liquid line temperature 100°F.
Suction sat. temp at 118 psig = 40°F
Superheat = 50°F − 40°F = 10°F
Liquid sat. temp at 340 psig = 105°F
Subcooling = 105°F − 100°F = 5°F
Diagnosis: Superheat is 10°F (within the 8–15°F TXV target). Subcooling is 5°F (below the 8–14°F target). This is a normal superheat with low subcooling pattern: the system is slightly low on charge. The TXV is maintaining superheat by throttling down, but the condenser does not have enough liquid to produce adequate subcooling. Check for leaks, confirm airflow first, and add charge until subcooling reaches the target this unit's nameplate or charging chart prints.
Example 2: R-22 Legacy System with Fixed Orifice
A legacy R-22 package unit with a fixed orifice (piston). Field readings: suction pressure 70 psig, suction line temperature 50°F, liquid pressure 200 psig, liquid line temperature 80°F.
R-22 suction sat. temp at 70 psig ≈ 41°F
Superheat = 50°F − 41°F = 9°F
R-22 liquid sat. temp at 200 psig ≈ 101°F
Subcooling = 101°F − 80°F = 21°F
Diagnosis: Superheat is 9°F, inside the wide 5–25°F fixed-orifice window for R-22 (the 10–18°F range applies to TXV systems) but near its low end. Subcooling is 21°F (well above the 4–12°F target). Low-end superheat with high subcooling points to overcharge: the condenser is flooded with excess liquid, driving head pressure up. On a fixed-orifice system, recover refrigerant until superheat rises to the manufacturer's chart target for current conditions.
Frequently Asked Questions
Is the Intry Superheat & Subcooling Calculator accurate?
Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the locked modules each figure is traced to: the manufacturer PT tables behind the saturation temperatures, superheat and subcooling, and the transcribed code and service-literature constants behind the charge, metering, airflow, approach and discharge line figures, each quoted with its own source in the panel it appears in before each deploy. This is our own deterministic gate, not a third-party audit. The per-tool receipt is public at https://www.intrysys.com/verified.
What does high superheat with low subcooling mean?
High superheat with low subcooling means the system is undercharged (low on refrigerant), the classic four-quadrant signature of a low charge. With too little refrigerant the evaporator runs starved, so vapor leaves fully boiled off and picks up extra heat and superheat climbs above the typical 8 to 15°F target, while the condenser holds too little liquid to stack a proper column, so subcooling falls below the roughly 8 to 14°F range and often trends toward zero. Confirm against the PT chart for your specific refrigerant (R-410A, R-454B, R-22) and weigh in charge to the manufacturer's subcooling target before adding gas. First rule out a restriction such as a plugged filter-drier or a stuck TXV, since that also raises superheat but is not fixed by adding refrigerant.
What does low superheat with high subcooling mean?
Low superheat with high subcooling is the classic overcharge signature in the four-quadrant diagnostic. Excess refrigerant backs up and floods the condenser, raising subcooling above the manufacturer's target (often around 10-12F), while the overfilled circuit pushes liquid toward the evaporator outlet and drops superheat below target (a fixed-orifice system reads low against its charging chart; a TXV holds superheat near target and shows overcharge mainly as high subcooling). Recover refrigerant in small increments and recheck superheat and subcooling against the equipment's rated PT values and nameplate or manufacturer charging chart until both return to range. Before recovering, confirm the reading by ruling out low or restricted condenser airflow, which also backs up liquid and inflates subcooling.
What is a normal superheat for R-410A?
For R-410A with a TXV, target superheat is 8-15°F. With a fixed orifice (piston), superheat depends on outdoor ambient and indoor wet bulb, so use the manufacturer’s charging chart. Typical targets range from 5-20°F. Superheat below 5°F risks liquid slugging at the compressor.
What is a normal subcooling for R-410A?
For R-410A with a TXV, target subcooling is 8-14°F. Subcooling is the primary charging indicator when a TXV is present because the TXV controls superheat. Low subcooling with normal superheat usually means low charge. High subcooling with normal superheat usually means overcharge.
How do you calculate subcool?
Subcooling = saturation temperature at the liquid-line pressure minus the measured liquid-line temperature. Read the liquid-line pressure at the service valve, convert it to a saturation (condensing) temperature with the PT chart for your refrigerant, clamp a thermometer to the liquid line near the condenser outlet, and subtract. Example: R-410A liquid saturation of 105°F minus a measured 93°F liquid line = 12°F of subcooling. On a TXV system, subcooling is the primary charging indicator.
What is 10 degrees of subcooling?
10 degrees of subcooling means the liquid refrigerant leaving the condenser is 10°F colder than its saturation (condensing) temperature at that pressure. For a typical R-410A TXV system the target is 8 to 14°F, so 10°F sits right in range and usually indicates a properly charged system. Always confirm against the manufacturer's rated subcooling on the nameplate or charging chart.
Is 16 subcooling too high?
For most R-410A and R-454B TXV systems, yes. Typical target subcooling is 8 to 14°F, so 16°F is above range and usually points to an overcharge, though restricted or low condenser airflow can inflate it the same way. Confirm the airflow first, then recover refrigerant in small increments and recheck superheat and subcooling against the manufacturer's target before deciding.
How do I measure superheat in the field?
Attach a refrigerant gauge to the suction service valve and read the pressure. Convert suction pressure to saturation temperature using a PT chart or this calculator. Clamp a pipe thermometer to the suction line near the service valve and read the actual temperature. Superheat = measured suction temperature minus saturation suction temperature.
How do I measure subcooling in the field?
Attach a refrigerant gauge to the liquid service valve (or the liquid line access port) and read the pressure. Convert liquid pressure to saturation temperature. Clamp a pipe thermometer to the liquid line near the condenser outlet. Subcooling = saturation liquid temperature minus measured liquid temperature.
Do you charge by superheat or subcooling?
It depends on the metering device, so identify it first. On a thermostatic expansion valve (TXV), charge by subcooling: the valve holds superheat steady, so charge level shows up in the condenser as subcooling, and you weigh in to the manufacturer's target (commonly 8 to 14°F). On a fixed orifice or piston, charge by superheat, comparing measured superheat to the manufacturer's charging-chart target, which is set by the indoor wet-bulb and outdoor dry-bulb conditions. Either way, read both numbers together to confirm the diagnosis before adding or recovering refrigerant.
What causes low subcooling?
Low subcooling means too little liquid is stacked in the condenser, and what it points to depends on the superheat reading beside it. With low subcooling, read superheat first: if superheat is high, it is a low charge (find and fix the leak before adding refrigerant); if superheat is also low, the charge is not the problem and it points to a weak compressor or an overfeeding TXV. A liquid-line restriction does the opposite and raises subcooling, so it is never the cause of a low reading.
Do I read superheat and subcooling together, and which one wins?
Read both, then let the metering device decide which one you charge to. A fixed-orifice or piston system is charged by superheat, and a TXV or EXV system is charged by subcooling, because a TXV already regulates superheat itself so superheat tells you about the valve rather than the charge. You still read both, because the PAIR is what identifies the fault: high superheat with low subcooling is undercharge, low superheat with high subcooling is overcharge, high with high is a restriction in the liquid line or metering device, and low with low points at a weak compressor or an overfeeding TXV. One reading alone cannot separate those four. Read the quadrants with one caution on a TXV: a working valve holds superheat near target, so an overcharged TXV system often shows itself as high subcooling with superheat that still looks normal rather than low. And subcooling is not a charge indicator at all on a system with a liquid receiver, because the receiver holds the surplus; charge those to the manufacturer's procedure.
Is my subcooling reading valid if the airflow is bad?
No, and this is the step most charging procedures skip. On a TXV system the charge is verified by subcooling, but the California Joint Appendices (2019 and 2022 editions; section JA6.1 is RESERVED in the 2025 edition) gate that test on airflow: items 1 to 3 of JA6.1.6.3 each require that the minimum airflow requirement is satisfied (alongside a 15 minute runtime and condenser entering air at or above 65F). So if the measured temperature split is more than 5F above the target from Table JA6.1-1, the charge test is not merely suspect, it is not testable, and adding or recovering refrigerant on that reading corrects a number that was never valid. Check the return dry-bulb and wet-bulb, get the target split, compare it to your actual split, and fix airflow first. This tool reports the charge and metering tests as not testable rather than ranking them below the airflow fault, because that is the order the primary sets.
Why is my superheat high and subcooling low?
High superheat combined with low subcooling is the classic symptom of low refrigerant charge. The evaporator is starved (not enough liquid reaching it, so vapor superheats excessively) and the condenser has less liquid backing up (low subcooling). Check for leaks before adding charge.
My superheat is low. Is the system overcharged, or is the airflow bad?
The gauges alone cannot tell you, because both put liquid closer to the compressor. The air side is what separates them. Measure return air and supply air dry-bulb, subtract to get the temperature split, and compare it to the target for that equipment. Low airflow WIDENS the split: less air over the coil means the air that does pass is cooled further. So a split more than plus 5F over target indicts airflow rather than the charge, per CEC-400-2022-010-AP Joint Appendix JA6.1.6.3 item 4. That criterion is one-sided and says nothing about the minus side. Getting this backwards is the expensive mistake: recovering refrigerant from a system whose real fault is a dirty filter or a collapsed duct drops superheat further toward floodback and leaves the actual problem in place.
What is the temperature split, and what should mine be?
The temperature split is return air temperature minus supply air temperature across the evaporator coil, sometimes called delta T. There is no single correct value, and any source giving you one number is oversimplifying: the target depends on both return dry-bulb and return wet-bulb, because a humid return does more latent work and produces a smaller sensible split. That is why Table JA6.1-1 of the California Joint Appendices carries 312 published values across a 15 by 27 grid rather than one number. Enter your return dry-bulb and wet-bulb and this tool reads your target off that table for you, cites it, and compares your measured split against it; your equipment's own figure always overrides it, and where the table prints a dash we print nothing rather than invent one. Worked example: at 75F return dry-bulb and 63F wet-bulb the target split is 18.6F, so a measured 26F split deviates +7.4F, which fails the minimum airflow criterion (read after 15 minutes of runtime, with condenser entering air at or above 65F, which the criterion also requires). Note the edition: JA6.1-1 appears in the 2019 and 2022 Appendices and is NOT in the 2025 edition, which marks JA6.1 RESERVED. Proctor Engineering states the underlying point plainly: contrary to common knowledge, the split should not universally be between 18F and 20F.
Should I add refrigerant when the superheat is low?
Not on that reading alone, and not before checking airflow. A low superheat can mean overcharge, but it can equally mean the evaporator is not getting enough air, and the correct charge depends on the airflow being right first. The California criteria make this explicit: the charge and metering tests each require that the minimum airflow requirement is satisfied before their conditions are valid, so a charge call made while airflow is failing is a call made outside its own preconditions. And if superheat is at or below 3F, stop: liquid refrigerant is reaching the compressor, which is an active fault to correct rather than a charge to adjust.
What is approach temperature, and what is a good number?
Approach is the liquid line temperature minus the outdoor air temperature entering the condenser, per Lennox service literature. It tells you how effectively the condenser is rejecting heat. There is no general good number, and that is the honest answer: manufacturers publish target approach per equipment model, not as a universal band. The Lennox XC16 literature alone lists 8F, 10F, 8F and 4F across four sizes of a single product line. Compare your reading against the figures on that unit's own charging chart or service literature rather than against a rule of thumb.
Why is compression ratio taken on absolute pressure?
Because taking it on gauge pressure gives a wrong answer that still looks plausible. Compression ratio is absolute discharge pressure divided by absolute suction pressure, and absolute means gauge plus atmospheric at sea level (NIST SP 811 Appendix B.9 gives the exact conversions). On R-410A at 118 psig suction and 350 psig liquid, the absolute ratio is about 2.75 to 1; computed on gauge pressures alone you would get about 2.97, which is high by roughly eight percent and sits in a range you would not question.
What is zeotropic glide and how does it affect readings?
Zeotropic refrigerant blends like R-407C have different boiling and condensing temperatures at the same pressure. The bubble point (where liquid starts boiling) and dew point (where vapor finishes condensing) differ by several degrees. Use the bubble point for subcooling calculations and the dew point for superheat calculations to get accurate readings.
Does R-454B (Puron Advance) replace R-410A directly?
R-454B is one of the lower-GWP refrigerants now used in new residential and light commercial equipment that would previously have used R-410A. R-32 is another. EPA's Technology Transitions rules under the AIM Act impose a GWP limit on the equipment category rather than designating a single successor refrigerant, and the compliance dates differ for manufacture, import and installation. R-454B is not a drop-in replacement for an existing R-410A system: it requires equipment designed for A2L refrigerants with updated safety controls, leak detection, and revised charge limits per ASHRAE 15 and UL 60335-2-40.
Can you charge R-454B by subcooling in cold weather?
Not below 70°F outdoor, which is stricter than the 55°F figure this page used to give. Carrier's R-454B installation instructions (PH5SAN5) state it verbatim: "Favorable conditions exist when the outdoor temperature is between 70°F and 100°F (21.1°C and 37.8°C), and the indoor temperature is between 70°F and 80°F (21.1°C and 26.7°C)." Outside that window Carrier directs you to adjust the charge for line-set length only and to check the charge again later when both temperatures are in a more favorable range. Johnson Controls (York) publishes the cold-weather alternative in its R-454B split-system manual: build a charging tent around the outdoor unit and adjust its windows to hold a stable temperature above 55°F inside the tent, then charge against the chart using the tent temperature rather than the outdoor ambient.
How long does an R-454B system take to stabilize before reading the charge?
Longer than you expect in cold weather, and we do not publish a figure because we have not sourced one. Third-party field guidance reports substantially longer stabilization for R-454B in low ambient than for R-410A; we have not verified a manufacturer-published time in any primary we hold, so we will not print a number a technician would time a job against. What is sourced is the window: if the outdoor temperature is below the manufacturer's favorable window (70°F on the Carrier R-454B primary), adjust for line-set length instead of waiting for a gauge reading that will not settle reliably.
How do you calculate superheat?
Superheat is suction line temperature minus saturated suction temperature: read the suction pressure, convert it to a saturation temperature for that refrigerant, and subtract it from the temperature measured on the suction line at the same point. That arithmetic is the easy half and it is not what decides a charge. Two things have to be true before the number means anything about the charge. Airflow has to be right first. A starved evaporator moves the reading in a direction that depends on the machine: on a fixed orifice it drives superheat DOWN, so it reads like an overcharge and recovering refrigerant would undercharge the system; on a TXV the valve holds superheat and the fault shows in suction pressure and subcooling. Either way you would be grading an airflow fault rather than the charge. And superheat has to be the indicator for that machine: on a TXV or EXV the valve holds superheat at its setpoint by design, so it barely moves as charge is added and subcooling is the number that sets the charge instead. This tool converts the pressure, applies the target for your conditions, and states which of the two readings governs before it grades either one.
Do you charge a mini-split by superheat or subcooling?
Neither, and this is the case the usual two-way answer drops. An inverter mini-split is generally charged by weight plus a line-length adder rather than to a superheat or subcooling target, so no reading-based charge verdict is issued for ductless equipment unless that model's literature publishes one. The Trane guide we hold (88-M5MHWUN-1A-EN) publishes none: that is evidence about that guide, not about every ductless machine. Two other machines also route away from a reading for their own reasons: a ducted inverter whose controller manages superheat in firmware, and any system with a liquid receiver. The routing table on this page states what each one charges by.
How much refrigerant do you add at a time, and how long do you wait between readings?
York ST-022-12, Figures 1 and 2 states the increment and the wait on its example charging charts: "Add/remove charge in 1 ounce increments and let stabilize for 10 minutes. Repeat process until proper suction temperature is reached." on the superheat side, and "Add/remove charge in 1 ounce increments and let stabilize for 10 minutes. Repeat process until proper liquid pressure is reached." on the subcooling side. That wait is the part techs skip, and skipping it is how a system ends up overcharged: the reading has not caught up with the last dose, so the next dose is added against a stale number. York prints these as examples and says so, and the scope travels with the quantity: "Please note these are examples only. Refer to the chart supplied with the outdoor unit for actual superheat and subcooling values, and the installation manual for detailed step-by-step instructions." Converged means within 5F of target. Two documents publish that figure for the fixed-orifice superheat method: Energy OutWest, Weatherization Field Guide, section 3.8 "Evaluating Refrigerant Charge", steps 8 and 9 ("within 5F of target - Charge OK"), and California Title 24 Table RA3.2-1, which is the tolerance published with the RA3.2-2 target this loop converges to.
What does it mean if the superheat will not come into range?
It usually means the charge is not the fault. After about 3 adjustments with the reading still outside the band, a restriction, non-condensables, a failing metering device or an airflow problem will each hold the reading off target no matter how much refrigerant you move, and continuing to dose is how the system gets overcharged on top of the original fault. Stop adding and diagnose. Read the pair rather than the one number: high superheat with high subcooling points at a restriction in the liquid line or the metering device, and low with low points at a weak compressor or an overfeeding TXV. Neither of those is corrected by moving refrigerant. The iteration count above is Intry's own guidance rather than a manufacturer's published number, and this tool labels it that way wherever it appears.
Related Calculators
Target Superheat Calculator
Charging a fixed-orifice (piston or cap-tube) system? Set the charge by target superheat from indoor wet-bulb and outdoor dry-bulb, compared to your measured superheat.
Subcooling on a Heat Pump
A heat pump charges like an AC in cooling mode (target 8 to 14°F subcooling), but the reversing valve changes the rules in heating. Where to measure and when to weigh it in.
BTU / HVAC Load Calculator
Calculate cooling and heating load by climate zone, insulation, windows, and duct location. Whole-house or room-by-room breakdown with automatic tonnage sizing.
Duct Size Calculator
The air side of the job: round and rectangular duct size from CFM by the equal-friction method, with velocity and the rectangular equivalent. Computed, not looked up.
A2L Charge Limit Calculator
Maximum R-454B and R-32 charge for a room, or minimum room area for a charge, per UL 60335-2-40 and ASHRAE 15.2. Know the limit before you set the charge.
A2L Line Set Calculator
Before you charge, size the line set and check whether the existing R-410A set can be reused with R-454B. REUSE, FLUSH, or REPLACE verdict with the charge adjustment.
Nitrogen Pressure Test Calculator
Before the charge, pressure-test with dry nitrogen. Separate a real leak from a temperature-driven pressure change with the Gay-Lussac correction. Computed, not looked up.
EPA Refrigerant Leak Rate Calculator
Topping off a 15+ lb system triggers a leak rate calculation under 40 CFR 84.106 as of January 1, 2026. Run the EPA math and check the thresholds.
Dual-Fuel Switchover Calculator
Converting to a heat pump with a furnace left in place? Find the temperature where the furnace becomes cheaper to run, with the thermal balance point so a lockout never strands the load.
Charge is dialed in. Was the system sized right to begin with?
Chronic charge problems on a short-cycling unit often trace back to oversizing. The BTU calculator runs the load by climate zone, insulation, and windows so you can check the tonnage against the house, not the nameplate.