PT Chart — Superheat & Subcooling Calculator

Superheat and subcooling in one paragraph each

Superheat is how many degrees the refrigerant vapor has been heated above the temperature at which it finished boiling. You measure it on the suction line. It exists to guarantee that everything reaching the compressor is vapor, because liquid does not compress and a compressor asked to compress liquid destroys itself.

Subcooling is how many degrees the liquid has been cooled below the temperature at which it finished condensing. You measure it on the liquid line. It exists to guarantee that what arrives at the metering device is solid liquid with no bubbles, because a metering device fed flash gas cannot meter properly.

Why this chart has two pressure columns

Most PT charts online give one pressure per temperature. That is correct for a pure refrigerant and wrong for a blend.

A pure substance — R-22, R-32, R-134a — boils at a single temperature for a given pressure. Water at sea level boils at 212 °F, start to finish. One number.

A zeotropic blend is a mixture of refrigerants with different boiling points, and it does not do that. As it evaporates, the more volatile component boils off first, the remaining liquid becomes richer in the less volatile component, and the boiling temperature climbs as evaporation proceeds. It starts boiling at the bubble point and finishes at the dew point. The spread between them is called glide.

Glide is not a fixed property — it varies with temperature and with how much of the charge has evaporated. The values above are computed at 40 °F saturated conditions, which is representative of a cooling evaporator.

The rule: superheat uses the vapor / dew column. Subcooling uses the liquid / bubble column. On R-410A you can be sloppy about this and lose two tenths of a degree. On R-407C you cannot.

R-454B is not R-410A, and your gauges will lie to you

New residential and light commercial equipment has moved to A2L refrigerants under the HFC phasedown — principally R-454B (roughly 68.9% R-32 / 31.1% R-1234yf by mass) and R-32. For a technician whose instincts were built on R-410A, the pressures are close enough to feel familiar and different enough to produce wrong answers.

R-454B runs lower than R-410A everywhere, and the gap widens with temperature. R-32 runs slightly higher. Now read it the way you actually work — gauge first:

A2L refrigerants are mildly flammable. That changes leak detection, ventilation, storage, recovery equipment and service procedure, and it is governed by codes and manufacturer instructions that are outside the scope of a pressure chart. Do not treat A2L service as R-410A service with a different number.

Which one tells you about the charge?

This depends entirely on the metering device, and getting it backwards produces confident wrong conclusions.

Fixed orifice or piston → charge by superheat

A fixed orifice has no feedback mechanism. It passes what pressure difference pushes through it. Superheat therefore floats with charge: add refrigerant and superheat falls, remove it and superheat rises. That responsiveness is exactly what makes it a usable charging indicator.

It also means superheat on a fixed-orifice system depends heavily on load — indoor wet bulb and outdoor dry bulb. This is why manufacturers publish a target superheat table rather than a single number, and why charging one of these on a mild day is unreliable.

TXV or EEV → charge by subcooling

A thermostatic or electronic expansion valve exists specifically to hold superheat constant. It modulates to maintain its setpoint regardless of charge, right up until it runs out of range. So on a properly functioning TXV system, superheat tells you the valve is working — it does not tell you the charge. Subcooling does, because surplus refrigerant backs up in the condenser and raises it.

Target values — where they come from

Use the equipment's data plate or charging chart. Not a rule of thumb, and not this page. Targets are specific to the equipment, the line set, the metering device and the operating conditions, and manufacturers publish them for exactly that reason.

The calculator above flags broad ranges only — roughly 8–12 °F subcooling and 8–14 °F evaporator superheat are common design targets — and they are there to catch gross errors, not to charge a system. A reading far outside them means something is wrong. A reading inside them does not mean the charge is right.

What the numbers point at when they're wrong

Note that low superheat and low subcooling together frequently means airflow, not refrigerant — a dirty filter, a collapsed flex duct, an undersized return. Which is a good reason to check static pressure before you connect gauges at all. If that describes the system in front of you, the duct sizing and static pressure calculator is the more useful tool.

Measurement errors that ruin the reading

• Thermocouple not insulated from ambient. An uninsulated clamp on a suction line in 95 °F sun reads high. On a cold liquid line it reads low. Both push you toward adding charge you do not need.
• Measuring temperature and pressure at different points. There is real pressure drop along a line set. Take both at the same location.
• Reading a system that has not stabilized. Give it 10–15 minutes of steady run time. Numbers taken during pulldown are meaningless.
• Charging in mild weather. Superheat on a fixed-orifice system is load-dependent. Below roughly 70 °F outdoors, charging by superheat is not reliable.
• Gauge set calibration. A few psi of error at 400 psig is a degree or more of saturation temperature.
• Altitude. These charts are gauge pressure at sea level. At elevation, gauge readings shift relative to absolute pressure.

What this page does not do

• It does not tell you how much refrigerant to add. Weigh in charge per the manufacturer where possible.
• It does not replace manufacturer target superheat and subcooling tables.
• It does not cover A2L safety requirements, which are governed by code and manufacturer instruction.
• It is not a substitute for EPA 608 certification, which is legally required to handle refrigerant.