A split system ships pre-charged for a set length of pipe. Run the line set longer than that and you have to weigh in the extra — because every additional foot of liquid line holds refrigerant the factory never accounted for. Get it wrong and the whole system runs undercharged.
Why longer runs need more
The outdoor unit is charged for the system plus a standard length of pipe — commonly 15 or 25 ft (5 or 7.5 m). Every foot beyond that holds refrigerant, mostly as dense liquid in the liquid line. Without topping it up, a long run leaves the system starved: low subcooling, reduced capacity and a hungry evaporator.
How much per foot
It depends almost entirely on the liquid line diameter, because that pipe is full of liquid — the big suction line holds comparatively little vapour.
| Liquid line | Add per foot |
|---|---|
| ¼″ (6.35 mm) | ~0.2 oz/ft (20 g/m) |
| ⅜″ (9.52 mm) | ~0.6 oz/ft (50–57 g/m) |
| ½″ (12.7 mm) | ~1.2 oz/ft (110 g/m) |
These are typical figures — if the installation manual states one, use that instead.
Refrigerant Line Set Charge Calculator
Enter liquid line size and run length for the extra refrigerant beyond the pre-charged length.
What length does the factory charge cover?
Most splits are pre-charged for 15 or 25 ft (5 or 7.5 m); bigger ducted and VRF units differ. It is printed on the outdoor unit’s nameplate or in the installation manual, along with the maximum allowed line length and elevation difference — stay inside those limits, because no amount of extra gas fixes an out-of-spec run.
How to add it
Weigh it in. After pressure testing and evacuating the line set, charge the calculated extra amount through the service port using a charging scale — liquid charging as the manufacturer directs. Record the added ounces on the unit’s label; the next technician (and the refrigerant log, where required) needs that number.
Shorter than the pre-charged length?
Usually nothing to do — most manufacturers say the factory charge is fine down to a minimum run of around 10 ft (3 m). Some manuals specify an amount to remove for very short runs; if yours does, follow it. Never fall below the manual’s minimum line length, which protects the compressor from oil and refrigerant surges.