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Line set charge calculator

Extra charge is the added liquid line length times the tube's internal volume per foot times the refrigerant's liquid density; for 3/8 in R-410A line at 100°F that is about 0.52 oz per foot, while the Carrier, Payne, Lennox, Trane and Daikin manuals cited here call for 0.6 oz per foot beyond 15 ft, and the manual governs.

Liquid line charge calculator

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Physics estimate of the liquid that fills the extra tubing. The installation manual's per-foot adjustment governs.

How the calculator works

Split systems leave the factory charged for a set length of line. The DOE Building America diagnostics guide says condensing units typically carry enough charge for 15 to 25 ft of tubing, and every manual cited on this page uses 15 ft. Manufacturers and California's Title 24 weigh-in procedure adjust that charge by liquid line length and diameter. The calculator estimates the liquid that extra line holds from physics:

Extra charge (oz) = extra liquid line length (ft) x internal volume (ft3/ft) x liquid density (lb/ft3) x 16

  • Internal volume comes from the inside diameter of ACR copper tube made to ASTM B280. The Copper Development Association's Copper Tube Handbook lists 3/8 in annealed ACR tube at 0.375 in OD, 0.032 in wall and 0.311 in ID, which holds 0.00053 ft3 per foot. Drawn (hard) 3/8 in tube has a 0.030 in wall and 0.315 in ID. Annealed 1/4 in holds 0.00020 ft3/ft, 5/16 in 0.00034 and 1/2 in 0.00103.
  • Liquid density is the refrigerant's saturated liquid density at the stated liquid line temperature, computed with CoolProp. Warmer liquid is less dense: Arkema's R-410A property table gives 61.73 lb/ft3 at 100°F and 59.56 lb/ft3 at 110°F.

The estimate ignores the vapor line. The same Arkema table puts saturated R-410A vapor at 40°F at 2.20 lb/ft3, against 71.91 lb/ft3 for liquid at that temperature, so a foot of suction line holds far less refrigerant than a foot of liquid line even though the tube is larger.

The installation manual's number governs

Manufacturers publish their own per-foot adjustment, and that is the number to charge by. Title 24's weigh-in procedure tells installers to adjust charge per the manufacturer's specifications, and the manual's number often differs from the physics estimate. Examples:

  • Carrier 24ACC6 (R-410A): "Adjust charge by adding or removing 0.6 oz/ft of 3/8 liquid line above or below 15 ft. (4.57 m) respectively."
  • Payne PH5SAN5 (R-454B): factory charged for 15 ft of line set, adjusted by 0.6 oz/ft of 3/8 in liquid line above or below that.
  • Lennox SL25KCV (R-454B): 3 ounces per 5 ft of 3/8 in liquid line, adjusted from a 15 ft line set.
  • Trane 4TTR7 (R-410A): a charge multiplier of 0.6 oz per foot of tubing beyond 15 ft.
  • Daikin IOG-4036B (R-410A): add 0.6 ounces per foot of liquid line beyond 15 ft.
  • Carrier R-454B long line guideline: 0.27 oz/ft for 1/4 in, 0.40 oz/ft for 5/16 in and 0.60 oz/ft for 3/8 in liquid line, with the factory charge covering 9 oz (15 ft of 3/8 in).

Base lengths are not universal. In May 2025, during a shortage of R-454B cylinders, Rheem announced a temporary increase in factory charge for its 13.4 and 14.3 SEER2 R-454B air conditioners and 14.3 SEER2 heat pumps, enough for a 30 ft line set with a 3/8 in liquid line instead of the previous 15 ft. Read the base length on the unit or in its manual before you adjust anything.

Shorter lines work the same way in reverse: Carrier's instructions say to remove charge for a line set shorter than 15 ft.

Long line sets

Past a certain length the extra charge is not the only issue. Carrier's R-454B guideline lists refrigerant migration, oil return, capacity loss and pressure drop in the liquid line as long-line concerns. It requires a TXV indoor metering device on every long-line application, specifies accessories beyond set lengths, caps total equivalent length at 250 ft, requires at least 10°F of subcooling so no refrigerant flashes before the TXV, and makes subcooling the primary charging method. Lennox's SL25KCV instructions call for a factory-installed crankcase heater and nonbleed TXV between 51 and 150 linear feet and do not recommend more than 150 ft. Check the long-line guide for the specific equipment before running a long set.

Worked example

An R-410A condensing unit, factory charged for 15 ft, installed with 40 ft of line set and a 3/8 in annealed liquid line. Liquid line temperature is taken as 100°F.

  1. Extra length: 40 - 15 = 25 ft.
  2. Internal area of 0.311 in ID tube: 3.1416 / 4 x 0.3112 = 0.0760 in2. Per foot: 0.0760 x 12 / 1728 = 0.000528 ft3.
  3. Mass per foot: 0.000528 x 61.73 lb/ft3 = 0.0326 lb, or 0.52 oz.
  4. Physics estimate: 25 x 0.52 = about 13 oz.
  5. Manual adjustment at 0.6 oz/ft: 25 x 0.6 = 15 oz. Weigh in 15 oz, then confirm with subcooling (TXV) or superheat (piston) once the system has run, as the Carrier and Payne instructions describe.

The same line on R-454B: its liquid is less dense, about 58 lb/ft3 at 100°F by CoolProp, so the physics estimate drops to about 0.49 oz/ft. Carrier's R-454B guideline still uses 0.60 oz/ft for 3/8 in line, which shows why the manual's number is the one to use.

Weighing in

California's Title 24 weigh-in procedure describes the same job: the factory charge is expected to be correct for the manufacturer's standard liquid line length and diameter, and the installer adjusts it for a line that differs, per the manufacturer's instructions, using a refrigerant scale accurate to 0.5 oz or 0.5 percent. The DOE guide notes that weighing in is useful when it is too cold outside to charge by superheat or subcooling, and Trane recommends it below 55°F outdoors. See the subcooling and superheat calculators for the check that follows.

Last reviewed October 10, 2026.

Sources

  1. Copper Development Association, Copper Tube Handbook (A4015-26/24), Table 14.2e ACR tube dimensions
  2. Arkema, Forane 410A Thermodynamic Properties (English units)
  3. Carrier, 24ACB3/24ACC6/24APB6 R-410A Installation Instructions
  4. Payne, PH5SAN5 R-454B Heat Pump Installation Instructions
  5. Lennox, SL25KCV Installation Instructions (508447-01, R-454B)
  6. Trane, 4TTR7 Split System Cooling Installer's Guide (18-AC94D1-4-EN), weigh-in method
  7. Daikin, IOG-4036B R-410A Condensing Unit Installation and Service Reference
  8. Carrier, Single-Stage and Two-Stage AC/HP Models Using R-454B: Residential Piping and Long Line Guideline
  9. Rheem, Rheem announces temporary increase in R-454B unit pre-charge levels (May 2025)
  10. California Energy Commission, 2022 Reference Appendices, RA3.2.3 Weigh-In Charging Procedure
  11. U.S. DOE Building America, Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement (DOE/GO-102013-3766, 2013)
  12. CoolProp, open-source thermophysical property library (Bell et al., Ind. Eng. Chem. Res. 2014)
  13. ASME B36.10M, Welded and Seamless Wrought Steel Pipe (Steel pipe outside diameters and Schedule 40/80 walls (values taken from the pipe schedule table on this site, sourced from InfraBuild's B36.10 data charts).)
  14. InfraBuild Steel Centre, Pipe & Fittings Data Charts: American Standard Steel Pipe ASME B36.10 (pp. 2-3) and Stainless Steel Pipes to ASME B36.19 (p. 15) (NPS to DN and mm conversions; B36.10 walls in mm.)
  15. ASTM D1785, Poly(Vinyl Chloride) (PVC) Plastic Pipe, Schedules 40, 80, and 120 (PVC Schedule 40 and 80 pipe is made to iron pipe size (IPS) outside diameters with the minimum walls given in the standard.)
  16. Westlake Pipe & Fittings, Schedule 40 and 80 PVC Pressure Pipe (PL-PS-025, ASTM D1785) (PVC Schedule 40 (1/2 to 18) and Schedule 80 (1/2 to 12) outside diameters and minimum walls.)
  17. Charlotte Pipe, Submittal for PVC Schedule 40 Pressure Pipe (IPS, ASTM D1785) (States that PVC Schedule 40 pipe is iron pipe size (IPS) conforming to ASTM D1785.)

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