R-134a PT Chart

About R-134a

R-134a is a single-component HFC used across car AC, medium-temperature refrigeration and chillers. It runs at much lower pressure than R-410A or R-404A, and at low temperatures it drops into a vacuum.

As a single component, R-134a has no glide, so liquid and vapor share one pressure column. Below about -15°F its saturation pressure falls under atmospheric pressure, shown in the chart as inches of mercury (inHg) vacuum.

• Car and light truck AC in most vehicles built before the switch to R-1234yf
• Medium-temperature commercial refrigeration, vending machines and older household refrigerators
• Centrifugal and screw chillers

R-134a Properties

Key facts for identifying R-134a and choosing tools, oil and replacements.

• R-1234yf PT chart

Typical R-134a Operating Pressures

Suction pressure is the vapor (dew) pressure at the evaporator temperature; head pressure is the liquid (bubble) pressure at the condensing temperature. The temperatures are common design points, not charging targets.

• Check R-134a superheat and subcooling

How to Read the R-134a PT Chart

The chart covers -40°F to 150°F. Pressures are gauge pressure (psig), the number your manifold shows; switch to °C and kPa above if you work in metric. At the cold end, R-134a saturates below atmospheric pressure, so those rows read in inches of mercury vacuum (inHg vac), the same as the vacuum scale on a compound gauge.

The chart has one pressure column because liquid and vapor share the same saturation pressure at each temperature. Use the same column for superheat (suction side) and subcooling (liquid side).

To read it, find the pressure on your gauge and read across to the saturation temperature, or start from a temperature and read the pressure. The lookup above interpolates between rows for you.

Superheat and Subcooling Formulas

The PT chart turns a gauge pressure into a saturation temperature. Compare that with a line temperature to get superheat or subcooling.

Tsuction

Suction line temperature near the outdoor unit or compressor, °F

Tliquid

Liquid line temperature leaving the condenser, °F

Tsat

Saturation temperature at the gauge pressure, °F

Gauge pressure (psig) is pressure above atmospheric. Below 0 psig, the chart shows vacuum in inches of mercury.

Worked Example: R-134a Superheat and Subcooling

Walk-in cooler (medium temp): an R-134a system reads 18.4 psig suction with a 30°F suction line, and 147.0 psig on the liquid line with a 100°F liquid line.

1. Saturation temperature at 18.4 psig: 20.0°F.
2. Superheat: 30°F − 20.0°F = 10.0°F.
3. Saturation temperature at 147.0 psig: 110.0°F.
4. Subcooling: 110.0°F − 100°F = 10.0°F.

Result: 10.0°F superheat and 10.0°F subcooling. Compare them with the manufacturer’s targets before adding or recovering refrigerant.

Is R-134a Being Phased Out?

Automakers have moved new light vehicles to R-1234yf, and EPA’s Technology Transitions rule limits high-GWP HFCs in many types of new equipment. R-134a is still made and sold for service.

In stationary equipment, lower-GWP alternatives include R-513A and R-450A, and in some new chillers R-1234ze. In cars, R-1234yf is the replacement, and the two refrigerants use different service fittings.

R-134a Field Tips

Practical points for working on R-134a systems, from the sources below.

1. 1

   Check for vacuum on low-temp work

   Below about -15°F, R-134a saturation pressure is a vacuum. A low side reading in inches of mercury is normal at low evaporator temperatures; read it on the vacuum scale, not as zero psig.

2. 2

   Match the oil to the system

   Car AC compressors use PAG oil, while stationary systems use POE. Both absorb moisture. Add only the type and amount the equipment maker specifies.

3. 3

   Keep car and stationary gear separate

   Car AC service uses SAE J2788 recovery machines and quick-couplers, while stationary work uses standard 1/4 inch flare fittings. Do not cross-connect them.

Sources: Vehicle Service Pros: Moving from R-134a to R-1234yf, EPA: Section 608 refrigerant management