2.3 R-407C: Glide, Fractionation & Service

Why R-407C Is in an R-410A Course

The ESCO manual covers safe service of systems containing R-410A and R-407C, the two leading HCFC-22 replacement candidates of its time. They took different paths:

• R-410A has much higher pressures and capacity, so it required new equipment designed for it.
• R-407C was formulated to run at pressures and capacity close to R-22, so manufacturers could use it in R-22-like designs, and it has been used to retrofit some R-22 systems after converting them to POE oil.

Because R-407C has a large temperature glide, it is the best example in the manual of how blends behave—and why liquid charging and bubble/dew calculations matter.

Composition and Properties

The pressures above come from standard property data (rounded). Notice that R-407C's dew-point pressure tracks R-22 closely, while its bubble-point (liquid-line) pressure runs about 10% to 15% higher.

Temperature Glide in the Coils

R-134a boils at a much higher temperature than R-32 and R-125. At constant pressure, the blend starts boiling at its bubble point and finishes at its dew point:

• In the evaporator, refrigerant temperature rises as it boils—from near the bubble temperature at the inlet to near the dew temperature at the outlet.
• In the condenser, temperature falls as it condenses—from the dew point to the bubble point.

That is why R-407C P-T charts carry two columns. Some manufacturers also quote a mid-point temperature for coil performance, but charging calculations use the bubble and dew columns.

Superheat and Subcooling with Glide

Worked Example

An R-407C system shows 70 psig suction and 55°F suction-line temperature, and 260 psig liquid pressure with a 100°F liquid line.

1. Dew point at 70 psig ≈ 44.5°F → superheat = 55 − 44.5 ≈ 10.5°F ✔
2. Bubble point at 260 psig ≈ 110°F → subcooling = 110 − 100 = 10°F ✔

If the technician wrongly used the bubble temperature at 70 psig (about 33.5°F) for superheat, the result would be about 21.5°F—enough to suggest a false undercharge and lead to overcharging. For R-410A, the two columns are nearly identical, which is why R-410A charts show one column.

Fractionation, Charging, and Leaks

• Charge as a liquid. The components have different boiling points, so drawing vapor from a cylinder removes more of the volatile components (R-32 and R-125) and leaves an R-134a-rich liquid behind. Withdraw liquid and meter it into the suction side slowly so it flashes before reaching the compressor.
• Vapor leaks change composition. A leak from the vapor space of a system or cylinder can shift the remaining blend. Follow the equipment and refrigerant manufacturers' guidance on whether to top off after the leak is repaired or to recover the charge and weigh in a fresh one.
• Leak detection. Use detectors rated for HFC refrigerants (heated diode, infrared, or ultrasonic). A halide torch cannot detect R-407C because it contains no chlorine.

Retrofit and Regulatory Status

When R-407C has been used to retrofit R-22 equipment, the refrigerant makers' procedures replace most of the mineral oil with POE, install a new filter-drier, adjust or replace components as needed, and label the system with the new refrigerant and oil. Never mix R-407C with R-22 or R-410A.

R-407C's GWP of 1,774 is above the 700 limit in EPA's Technology Transitions rule, so newly manufactured residential and light-commercial air conditioners and heat pumps cannot use it from January 1, 2025. Existing R-407C systems may still be serviced, and venting R-407C is prohibited under Section 608 just like R-410A.

Charging R-407C by Subcooling and Superheat

R-407C systems are charged the same way as R-410A systems once the correct saturation column is used:

• TXV systems: Charge to the manufacturer's subcooling target using the bubble-point temperature at liquid-line pressure.
• Fixed-orifice systems: Charge to the manufacturer's target superheat using the dew-point temperature at suction pressure and the charging chart for indoor wet-bulb and outdoor dry-bulb conditions.
• Weigh-in first: On new installations or after recovery, weigh in the nameplate charge plus any line-set adder before fine-tuning.

Common Exam Traps

• Using a single-column P-T chart (or an R-22 chart) for an R-407C system.
• Assuming R-407C can be vapor-charged like R-22 because their pressures are similar.
• Calling R-407C "near-azeotropic"—that description fits R-410A, not R-407C's wide glide.
• Forgetting that a halide torch cannot find R-407C leaks because the blend contains no chlorine.