Line set charge calculator

How it works

A split system’s outdoor unit ships with enough refrigerant for the unit, the indoor coil and a stated length of line set. If your line set is longer, you add refrigerant for the extra length. If it is shorter, you remove some. The tool does one multiplication:

Charge adjustment (oz) = (actual length − length covered by factory charge) × oz per foot

A positive result is refrigerant to add. A negative one is refrigerant to remove. From 16 oz up the tool also shows pounds and ounces.

The inputs:

• Liquid line OD: 1/4, 5/16, 3/8 or 1/2 in, each with a default rate.
• Actual line set length in feet (default 50).
• Length covered by factory charge in feet (default 15).
• Manufacturer’s oz/ft (optional): this replaces the default whenever you enter it.

The adder is keyed to the liquid line because that line is full of liquid, while the suction line carries vapor. For R-410A our data give a saturated liquid density of 61.7 lb/ft³ at 100°F against 2.2 lb/ft³ for saturated vapor at 40°F. Foot for foot, the liquid line holds far more refrigerant.

Default values and where they come from

The defaults are the typical figures in Rheem/Ruud R-454B and Oxbox (Trane Technologies) R-410A installation instructions:

In those instructions the factory charge covers 15 ft of line set. Some manuals use 25 ft. The rate also depends on the refrigerant and the manufacturer. Treat these as typical values. The nameplate and installation manual for the unit in front of you take priority. Type its oz/ft in the override box and its covered length in the factory-charge box.

Worked example

The default case: 3/8 in liquid line, 50 ft installed, factory charge covers 15 ft.

• (50 − 15) × 0.6 = 35 × 0.6 = 21.0 oz to add, which is 1 lb 5.0 oz.

The same job on a unit whose manual says the factory charge covers 25 ft:

• (50 − 25) × 0.6 = 25 × 0.6 = 15.0 oz to add.

A short set: 3/8 in, 10 ft installed, 15 ft covered.

• (10 − 15) × 0.6 = −5 × 0.6 = 3.0 oz to remove.

A larger system: 1/2 in liquid line, 75 ft, 15 ft covered.

• (75 − 15) × 1.2 = 60 × 1.2 = 72.0 oz to add, which is 4 lb 8.0 oz.

Reading the result

The number is a starting charge, not the final word. Weigh it in with a scale after evacuation (see evacuation and microns), run the system, then verify:

• TXV or EEV: subcooling to the manufacturer’s target, within ±3°F for the installer under CEC RA3.2. Use the subcooling calculator.
• Fixed orifice or piston: superheat within ±5°F of the target. Use the superheat calculator and the target superheat calculator.

Below 55°F outdoors those tests are not valid. CEC then calls for the weigh-in procedure, which is accepted at any outdoor temperature. In that case the weighed charge, including the line set adjustment, is the whole job.

A result of zero means the installed length matches the length the factory charge covers.

Common mistakes

• Using the suction line size. The adder goes by the liquid line OD.
• Assuming 15 ft. Check whether the manual says 15 ft, 25 ft or something else.
• Applying R-410A rates to another refrigerant. Rates differ by refrigerant and manufacturer. Use the manual’s figure for R-454B, R-32 or anything else.
• Measuring the wrong length. Use the actual installed length of the line set between the units, per the manual’s definition.
• Charging blends as vapor. Weigh R-410A, R-454B and other blends in as liquid. The temperature glide calculator explains why.
• Skipping the final check. A weighed charge that is never verified can still be wrong if the factory charge, the length or the rate was off.