R-134a (HFC)
R-134a Superheat & Subcooling Calculator
Check charge on R-134a equipment: reach-in coolers, vending machines, chillers, and automotive AC. The calculator opens preloaded for R-134a with a full PT chart and targets tuned to the low-pressure refrigerant that runs half the refrigeration on any commercial block.
R-134a Pressure-Temperature Chart
| Pressure (psig) | Sat. Temp (°F) |
|---|---|
| 0 | -15°F |
| 2 | -10°F |
| 4 | -5°F |
| 7 | 0°F |
| 9 | 5°F |
| 12 | 10°F |
| 15 | 15°F |
| 18 | 20°F |
| 22 | 25°F |
| 26 | 30°F |
| 30 | 35°F |
| 35 | 40°F |
| 40 | 45°F |
| 45 | 50°F |
| 51 | 55°F |
| 57 | 60°F |
| 64 | 65°F |
| 71 | 70°F |
| 79 | 75°F |
| 87 | 80°F |
| 95 | 85°F |
| 104 | 90°F |
| 114 | 95°F |
| 124 | 100°F |
| 135 | 105°F |
| 146 | 110°F |
| 159 | 115°F |
| 171 | 120°F |
Two anchors cover most calls: 22 psig is a 25°F coil, the workhorse medium-temp refrigeration point, and 124 psig is a 100°F condenser. Note how flat the low end is; a few psi of gauge error moves the saturation temperature more on R-134a than on high-pressure refrigerants, so zero your gauges before trusting a marginal reading.
R-134a Target Superheat & Subcooling
| Metering device | Target superheat | Target subcooling | Charging method |
|---|---|---|---|
| TXV | 8–14°F | 8–14°F | Charge by subcooling; the TXV holds superheat |
| Fixed orifice (piston) | 5–18°F | 4–10°F | Charge by superheat from the manufacturer charging chart |
Refrigeration equipment often publishes its own evaporator-outlet superheat spec, and automotive systems are charged by weight to the underhood label. Treat the ranges above as the general-purpose field targets and defer to equipment-specific data when it exists.
Where R-134a Shows Up
R-134a is a pure single-component HFC with zero glide and a GWP of 1,430. You will not find it in residential split systems, but it is everywhere else: reach-in coolers and merchandisers, vending and ice machines, water coolers, refrigerated transport, large centrifugal chillers, and every light-duty vehicle built from the mid-1990s until the R-1234yf transition. For techs who cross between HVAC and refrigeration work, R-134a is the low-pressure end of the toolbag.
Its defining trait on the gauges is how low everything reads. A medium-temp case runs 20-something psig suction, and even a hot condenser stays under 200 psig. The flip side: low-temperature applications can pull the low side into vacuum, and any leak on a system operating below 0 psig lets air and moisture in rather than refrigerant out, which is how R-134a circuits end up with non-condensables and acid.
R-134a Charging Notes
As a pure compound, R-134a can be charged as vapor or liquid without fractionation; liquid charging is simply faster on larger charges. Stationary equipment runs POE oil (automotive uses PAG), and POE's affinity for moisture makes a good vacuum and a fresh filter-drier non-negotiable on any opened R-134a circuit. On TXV systems charge to subcooling; on cap-tube and orifice equipment, which covers most small refrigeration boxes, charge to the manufacturer superheat or performance data, and weigh the charge on critically charged units.
One habit worth building on refrigeration calls: use the static pressure check before starting the unit. With the system off and equalized, gauge pressure should match the PT chart at ambient temperature (about 87 psig at 80°F). A static reading far below the chart means the charge is critically low or gone, and you just saved a compressor from running on nothing.
Worked Example: Reach-In Cooler, Medium Temp
A commercial reach-in cooler on R-134a with a TXV, holding 37°F product temperature. Field readings: suction pressure 22 psig, suction line temperature 35°F, liquid pressure 124 psig, liquid line temperature 90°F.
Saturated suction temp at 22 psig = 25°F
Superheat = 35°F − 25°F = 10°F
Saturated liquid temp at 124 psig = 100°F
Subcooling = 100°F − 90°F = 10°F
Diagnosis: Superheat 10°F and subcooling 10°F both sit inside the 8–14°F targets, and a 25°F coil is exactly right for 37°F product. The refrigeration circuit is healthy. If the box still struggles to hold temperature, look at door gaskets, defrost operation, and condenser cleanliness rather than the charge.
R-134a Charging FAQs
What is normal superheat for R-134a?
With a TXV, target superheat for R-134a is 8-14°F at the evaporator outlet. With a fixed metering device (cap tube or orifice), typical targets run 5-18°F depending on load and ambient, so use the equipment manufacturer's specification. Medium-temp refrigeration cases often specify their own evaporator-outlet superheat, commonly in the 6-10°F range, which overrides generic targets.
Can I use this calculator for car AC?
Yes for the PT math: saturation pressure and temperature are properties of the refrigerant, not the machine. The targets differ though. Automotive systems are charged by weight to the underhood label, then verified with gauge pressures against the vehicle's ambient-temperature chart and center-vent temperature. Use this page's PT chart to convert your readings; use the vehicle service data for pass/fail.
What should R-134a static pressure be?
With the system off and pressures equalized, static pressure equals the saturation pressure at the current temperature, as long as some liquid refrigerant is present. At 80°F, that is about 87 psig; at 70°F, about 71 psig. If static pressure matches the PT chart for ambient, the system contains liquid refrigerant, but static pressure alone cannot tell you how much. It only reliably reveals a near-empty system or a non-condensables problem.
Why are R-134a pressures so much lower than R-410A?
R-134a simply boils at a much higher temperature at a given pressure: a 40°F evaporator needs only 35 psig on R-134a versus 118 psig on R-410A. That low-pressure character is why it suited automotive AC, medium-temp refrigeration, and large centrifugal chillers. It also means a low-side leak on an R-134a system pulling below 0 psig can go into vacuum and draw in air and moisture.
Is R-134a being phased out?
It is being phased down, not banned. With a GWP of 1,430, R-134a falls under the AIM Act HFC step-downs, and new light-duty vehicles moved to R-1234yf years ago. Stationary refrigeration is migrating toward lower-GWP options like R-513A and R-450A. Existing R-134a equipment remains legal to service, and supply remains available, at rising cost.
Is R-1234yf the same as R-134a?
No. R-1234yf is the low-GWP A2L replacement for R-134a in automotive AC. Operating pressures are similar, which was the point, but the refrigerants must not be mixed, use different service fittings and machines, and R-1234yf is mildly flammable. A vehicle's underhood label tells you which one it takes; there is no legal drop-in in either direction.
What temperature is R-134a at 22 psig?
About 25°F saturated. That is a typical medium-temp refrigeration evaporator: a reach-in cooler holding 35-38°F product usually runs a coil around 25°F, which is exactly 22 psig on R-134a. Add your measured suction line temperature and this calculator gives superheat instantly.
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