25.1 Refrigerant Piping, Copper Tubing & Pipe Sizing

25.1 Refrigerant Piping, Copper Tubing & Pipe Sizing

Trade Area D — Equipment Component Installation — is 14 percent of both the Class A and Class B trade exams. After condensers, compressors, air handlers, and ducts (Chapters 23–24), the CILB outline asks you to install refrigerant piping, hang horizontally supported piping, work copper tubing and other tubing, and treat refrigerant tubing under 2 inches differently from tubing over 2 inches. The 2026 Air Conditioning CBT books that carry this cluster are Florida Building Code — Mechanical, 2023 Chapter 11, Pipefitters Handbook, 3rd Edition (1967) (R82), and Refrigeration & Air Conditioning Technology, 9th Edition (2021). Chapter 20 already hung a residential line set. This chapter is the piping system: materials, size, oil return, and the 2-inch split.

Quick Answer: Field refrigerant pipe is Type ACR copper (ASTM B280) or another row on Table 1107.4, not plumbing Type M off the hardware rack. Size liquid, suction, and discharge from equivalent length and oil velocity. Under 2 inches is typical line-set ACR; over 2 inches is hard-drawn ACR or steel. Pitch horizontal suction about 1/2 inch per 10 feet toward the compressor and trap a riser that must lift oil. Class B still caps at 25 tons / 500,000 Btu per system.

Why the pipe is part of the machine

A split air conditioner or heat pump is one listed circuit only after the field pipe is the right diameter, the right metal, and sloped so oil that left the compressor returns. Undersize the suction line and you eat capacity as saturation temperature drops, starve the compressor of mass flow, and drop velocity until oil sits in the evaporator. Oversize a suction riser at part load and the same oil never climbs. Undersize the liquid line and the pressure drop flashes liquid into the TXV, which then hunts. The exam will hand you a tonnage, a developed length, and a vertical offset and ask which tube you pull — not “whatever is on the truck.”

F.S. 489.105(3)(f) and (g) both include piping that completes the air-conditioning or refrigeration system. Class A is unlimited capacity. Class B is the same trade skill through 25 tons of cooling and 500,000 Btu of heating in any one system. A 16-ton supermarket rack with 2-1/8-inch suction is still Class B piping if each independent system stays under 25 tons. A single 30-ton circuit is Class A even if the copper looks like a line set. Class B candidates still need the theory of large pipe; the Class B outline drops 25–100 ton and centrifugal install clusters, not Trade D piping.

Ammonia (R-717) is not a copper circuit. FBC Mechanical 1101.1.2 sends ammonia to IIAR 2 through 5 and out of Chapter 11. Ammonia attacks copper and copper alloys. Steel pipe is the ammonia metal. Do not braze ACR onto an ammonia job and call it “the same refrigeration piping.”

Copper tubing versus other tubing — Table 1107.4

Air-Conditioning and Refrigeration (ACR) copper is seamless tube made for field refrigerant service. It is cleaned, dehydrated, nitrogen-charged, and plugged so the bore is not a plumbing store’s water pipe. ASTM B280 is the ACR specification. FBC Mechanical Table 1107.4 also lists copper tube to ASTM B68, B75, B88, B280, and B819; copper linesets to B280 and B1003; copper pipe to B42 / B302; steel pipe to ASTM A53 / A106; steel tube to A254 / A334; aluminum tube; and brass pipe. That table is the allowed-metal list. PVC, CPVC, and PEX are not on it — those plastics belong on condensate or hydronic work (Section 25.3), not in the refrigerant circuit.

Note a under Table 1107.4: soft annealed copper tubing larger than 1-3/8 inch OD shall not be used for field-assembled refrigerant piping unless it is protected from mechanical damage. Annealed (soft) coil is what you flare and bend by hand. Hard-drawn sticks are what you run for long, large, or exposed mains. The CILB under 2 inches / over 2 inches split is this shop change: a 7/8-inch suction is still a line-set problem; a 2-1/8-inch suction is a hard-drawn or steel main with different hangers and joints.

Do not confuse ACR OD sizes with plumbing nominal sizes. ACR is sold by outside diameter: 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, 1-1/8, 1-3/8, 1-5/8, 2-1/8, 2-5/8, 3-1/8 inch. A 3/4-inch ACR tube is 3/4 inch outside. A 3/4-inch Type L plumbing tube is a different OD/ID pair. Reading a plumbing friction table as if it were an ACR suction chart is a failed size.

Type L (ASTM B88) shows up on some Division 23 specs as an allowed copper tube. Type M is the thin plumbing wall. Even though B88 is listed, the exam and the shop still want ACR or Type L wall, nitrogen-clean, not a wet Type M water coil that was never dehydrated. ASTM B819 is medical-gas cleanliness; it is listed, but you still size and trap it as refrigerant pipe.

Sizing liquid, suction, and discharge

A split line set is sized from the manufacturer’s equivalent-length table, not from a statewide “3/8 and 3/4 for 3-ton” habit. Equivalent length is measured pipe plus the handbook/manufacturer allowance for each elbow, valve, and fitting. Four 90° elbows on a 40-foot run are not 40 feet.

Suction (vapor) line. Size for two jobs at once: keep pressure drop from stealing capacity (a common design target is on the order of 2°F of saturation drop, using the manufacturer’s table), and keep velocity high enough that oil stays in the vapor stream, especially in upflow risers. Horizontal suction can run slower than a riser; a riser that is one size too large at 30 percent compressor capacity is an oil-logged evaporator waiting for a warranty call.

Liquid line. Size so the pressure drop does not flash liquid before the metering device. Subcooling at the condenser is the margin. A long, hot Florida attic run on an undersized liquid line eats that subcooling and the TXV hunts. Heat-pump liquid lines see reversed roles (Chapter 20); size both legs to the manufacturer.

Discharge (hot-gas) line. Keep velocity high enough to carry oil to the condenser, but not so high that you hammer the discharge valve. Slope a horizontal discharge toward the condenser. A discharge loop (inverted trap) at the compressor outlet on some systems keeps liquid from draining back into the head during the off cycle.

Worked example. A Class B 5-ton R-410A split in Pasco County has 45 feet of measured pipe, four 90° elbows, and a 12-foot suction riser because the air handler is in a first-floor closet and the condenser is on the roof. The manufacturer’s table at that equivalent length calls 3/8-inch liquid and 7/8-inch suction. The helper wants to “upgrade” the suction to 1-1/8 inch “for less pressure drop.” That larger riser drops oil-return velocity on a mild Tampa evening when the compressor unloads. Stay on the 7/8-inch riser, trap the base, and insulate the suction the entire attic run. Substituting leftover 3/8 and 5/8 because that is what the van holds is the opposite error — capacity and flash gas.

Over 2 inches. A 40-ton Class A rack with a 2-5/8-inch suction is not a line-set problem. You run hard-drawn ACR or steel, braze or weld (Section 25.2–25.3), support on the Table 305.4 row (Section 25.4), and size from the rack manufacturer or an engineered pressure-drop calc. Class B still studies the diameter; Class B does not install that 40-ton system.

Oil return — pitch, traps, double risers

Oil leaves the compressor as a mist. It must come home. Industry practice (Refrigeration & Air Conditioning Technology; not a numbered FBC slope for refrigerant suction) is to pitch horizontal suction in the direction of flow about 1/2 inch per 10 feet toward the compressor so oil does not pond in a belly. Do not confuse this with condensate slope (1/8 inch per foot on FBC Mechanical 307 drains).

A P-trap (oil trap) at the bottom of a suction riser collects oil until the slug is big enough for vapor to push it up the riser. Put the trap at the base of the riser, not randomly in the middle of a horizontal run. When the evaporator sits above the compressor, pitch down to the compressor and use an inverted trap (inverted loop) at the evaporator outlet so liquid refrigerant does not drain into the compressor during the off cycle.

A double suction riser is the part-load tool on a capacity-controlled compressor: a small riser carries oil at low load; a trapped larger riser comes in when load (and velocity) rises. Oversizing a single riser “to be safe” is how a supermarket case starves the compressor of oil at night.

Insulation. FBC Mechanical 1109.7: refrigerant piping that will run below the dew point of the surrounding air, where condensate would damage occupants, structure, or equipment, shall be insulated or protected. In a Florida attic that is the suction line on a cool-only split and both legs on a heat pump (Chapter 20). UV-jacket the outdoor run. Bare suction over a drywall ceiling is a stain, a mold call, and a 1109.7 miss.

Location. FBC Mechanical 1109.2.1: exposed refrigerant piping in open spaces that afford passage shall be not less than 7 feet 3 inches above the finished floor. 1109.2.3 prohibits refrigerant pipe exposed in a fire-resistance-rated exit-access corridor, in an interior exit stairway or ramp, in an exit passageway, or in an elevator/dumbwaiter shaft. 1109.2.7: identify pipe in areas other than the equipment room at intervals not exceeding 20 feet, letters not less than 1/2 inch, with the refrigerant designation and safety group.

Florida HVAC scenario

Suncoast Mechanical, a certified Class B shop in Pinellas County, change-outs a 4-ton split. The old 3/8-and-3/4 set is undersized for the new R-410A indoor coil’s equivalent-length table, which wants 3/8 liquid and 7/8 suction. The roof is 14 feet above the air handler. The mechanic runs 3/4 suction because “we always do,” skips the riser trap, leaves a belly in the attic with no pitch, and uses Type M plumbing copper from a supply house because it was cheaper than ACR. Four independent failures: wrong OD, no oil-return trap, no 1/2-in-per-10-ft pitch, and wrong product (not dehydrated ACR / Table 1107.4 field tube). Class B is legal on 4 tons. The license does not make Type M and a flat belly legal. If the same owner had pointed at a 30-ton split on the strip center, Class B would have been the first no, before any ACR was unrolled.

Traps: (1) Using plumbing Type M as a line set. (2) Upsizing a suction riser for “less drop” and losing oil. (3) Treating 2-1/8-inch pipe like 7/8-inch line set. (4) Pitching suction like a condensate drain, or not pitching it at all. (5) Copper on ammonia.