Refrigerant P-T & Superheat Calculator HVAC
Saturation pressure-temperature lookup and a superheat / subcooling check.
Inputs are seeded with an example — edit them to your numbers.
Input
Output
Worked example
An R-410A system, suction gauge reading 118 psig at the evaporator outlet.
- Pick the refrigerant and enter the gauge pressure. The tool reads the saturation temperature off that refrigerant's published P-T curve, interpolating between rows the way you would between lines on the paper chart.
- A pure refrigerant (R-22, R-134a) has one saturation temperature at each pressure. A zeotropic blend has two: the bubble point (where the liquid is fully saturated) and the dew point (where the vapor is). The gap between them is the glide.
- Which one you want depends on what you're measuring. Superheat is referenced to the dew temperature; subcooling to the bubble temperature. R-407C glide runs ~10 °F — average the two and every superheat reading is off by ~5 °F.
Pressures are gauge (psig / kPag), as read at the manifold. Switch the Superheat / Subcooling tab to turn a line temperature into an actual superheat or subcooling value.
Input
Suction line: read the pressure and a clamp-on temperature at the evaporator outlet. The tool references the dew point.
The reference flips with the line: superheat is
line − dew (vapor side); subcooling is
bubble − line (liquid side). Same gauge, the
subtraction reverses.
Output
Worked example
Same R-410A system: suction 118 psig, a clamp-on thermometer on the suction line reading 50 °F.
- Superheat (suction line): the tool
finds the dew-point temperature at the suction pressure,
then
superheat = line temp − dew temp. Here ~118 psig gives a ~40 °F dew point, so superheat ≈ 50 − 40 = 10 °F of vapor warmer than saturation. - Subcooling (liquid line): the tool finds
the bubble-point temperature at the liquid pressure, then
subcooling = bubble temp − line temp— liquid cooler than saturation. - Low superheat warns of liquid floodback to the compressor; high superheat of a starved evaporator. Low subcooling points to undercharge or a liquid-line restriction. The target numbers are system-specific — a TXV (thermostatic expansion valve) holds a different superheat than a fixed orifice — so treat the verdict as a direction, not a setpoint.
This is the same procedure printed on a manufacturer P-T chart: superheat off the dew column, subcooling off the bubble column.
About this data
The pressure-temperature tables are transcribed from published manufacturer P-T charts — Honeywell's Genetron / Solstice charts for R-22, R-134a, R-407C, R-404A and R-454B, and an iGas chart for R-410A. The tool interpolates linearly between chart rows, the same move a tech makes reading between lines on the paper card. R-32 and other refrigerants will be added as charts for them are sourced.
Bubble vs. dew. A pure refrigerant boils at one temperature for a given pressure. A zeotropic blend — R-407C most of all — boils across a range: the bubble point is where the last liquid would boil off, the dew point where the last vapor would condense. Superheat is measured against the dew point, subcooling against the bubble point. A single-column P-T card averages the two and is quietly wrong on every glide blend.
Pressures are gauge (psig / kPag), as read at the manifold — not absolute. Target superheat and subcooling depend on the system — metering device, equipment manufacturer, operating conditions. This tool gives you the measured value and a general direction; the equipment data always wins over a rule of thumb.
And a number here is a cross-check, not a charging procedure. Use it to test a theory against gauge readings, to learn the relationships, and as a second opinion on a diagnosis — then let the manufacturer’s charging chart and commissioning data make the call. Acting on a single lookup is how a misread glide becomes a mischarge, and a mischarge is how liquid finds its way back to a compressor.