R-12 to R-134a on an older Sub-Zero: what a conversion actually involves

First, find out whether you actually have an R-12 machine

The data plate settles this and nothing else does. It names the refrigerant and gives the factory charge weight, and on a cabinet with two sealed systems it lists a charge for each one separately. On a built-in the plate sits inside the door frame on the hinge side or behind the lower grille. Age narrows the question without answering it: production of new CFC-12 in the United States ended at the start of 1996 under the Montreal Protocol phase-out, and domestic refrigeration had largely moved to HFC-134a before that deadline. Machines from the mid-1990s onward are almost all R-134a already. It is the units built around the changeover that catch people out, because two cabinets identical from the front can hold either one.

• The plate names the refrigerant and the charge weight for each sealed system
• A cabinet with two compressors has two independent circuits, so one can be flat while the other is perfect
• A previous retrofit is supposed to have left a label on the machine, so look before assuming
• If the plate is gone, the compressor's own label usually states the refrigerant it was built for

The oil is the whole problem

R-12 systems were designed around mineral oil, and mineral oil does not mix with R-134a. Put the two together and the oil stops circulating. It separates out where the refrigerant is coldest, lies in the evaporator as a film that steals capacity, and never returns to the compressor that needs it. The retrofit lubricant is polyol ester, and getting residual mineral oil down to the small percentage a retrofit tolerates means draining and refilling the compressor two or three times. That is routine on a system with a drain plug. A domestic hermetic compressor has none: the oil is sealed inside the shell with the motor. So the honest version of this job is a compressor change, with the correct ester oil already in it and a new drier alongside.

• Mineral oil and R-134a separate, and the oil pools where the refrigerant is coldest
• Polyol ester oil pulls moisture out of the air aggressively while the system is open
• A hermetic compressor has no drain, so the oil arrives with the compressor or not at all
• The drier has to be rated for HFC refrigerant and ester oil; the desiccant used for CFC service is not a substitute
• Joints get brazed with nitrogen flowing, or the inside of the tubing scales and that scale ends up in the compressor

Why the freezer side is where a conversion disappoints

R-12 and R-134a behave closely enough at air-conditioning temperatures that the automotive world converted millions of systems without much drama. Domestic refrigeration is not air conditioning. A freezer evaporator runs well below zero Fahrenheit, and down there R-134a gives up capacity faster than R-12: suction pressure sits lower, the compressor moves less refrigerant per revolution, and the machine makes up the difference in run time. These built-ins meter through a capillary tube rather than an expansion valve, and a capillary tube is a fixed restriction sized for one refrigerant at one charge. There is nothing to re-tune. A converted freezer usually holds temperature but takes longer to get there and recovers slowly after the door has been open.

• Capillary-tube systems are critically charged, so an ounce either way changes how the machine runs
• The R-134a charge is not the R-12 figure on the plate, and it is weighed in rather than guessed at on gauges
• Longer run times mean a hotter condenser, which matters most in a tight opening with nowhere for the heat to go
• A conversion is judged on the hottest week of the year, not on the afternoon it was finished

A conversion is not a leak repair

Refrigerant is not consumed and does not wear out. If a system is low, it leaked, and changing what goes back in does nothing about where the last charge went. On cabinets this old the leaks land in predictable places: the aluminum evaporator tubing inside the liner, the process tube and its pinch-off, the brazed steel-to-copper joints at the compressor, the condenser where it is clamped to its mount and thirty years of vibration have worked at the joint, and drier connections left by an earlier repair. Several of those are a morning's work. An evaporator buried in foam behind the liner usually ends the conversation. EPA Section 608 applies throughout: refrigerant is recovered rather than vented, and a retrofitted system is meant to be labeled with what is now inside it.

• Recover first, because what comes out tells you how much was still in there
• A standing nitrogen pressure test read over hours finds slow leaks a handheld detector walks past
• A system that ran empty has pulled in air and moisture, and both come out before it is charged
• Evacuation is measured on a micron gauge with a decay test, not by a set number of minutes on the pump

The real options, and what each one is for

Repairing the leak and recharging on reclaimed R-12 keeps the system exactly as designed: the capillary tube matches the refrigerant, the mineral oil stays where it belongs, and the lubricant question never arises. The cost is the refrigerant, and that supply only shrinks. A full conversion with a new compressor, new drier, ester oil and a weighed R-134a charge makes sense when the compressor is tired anyway or the machine is staying another decade. The third option is no sealed-system work at all, which is right more often than people expect: plenty of machines called in for suspected refrigerant loss have a condenser packed solid, a fan that has stopped, or a defrost cycle that no longer completes, and each of those warms a cabinet without an ounce having gone anywhere.

• Repair and recharge on R-12: keeps the original design, and the refrigerant is the expense
• Compressor change and full conversion: the retrofit done properly, priced accordingly
• No sealed-system work at all: the condenser, the fans and the defrost circuit are checked first because they are cheap
• Replacing the cabinet: on a built-in that becomes a cabinetry job with an appliance attached