Installation
Additional refrigerant charge for pipe length
- where
- additional charge (g)
- liquid pipe length, size A (m)
- additional charge per metre, size A (g/m)
- liquid pipe length, size B (m)
- additional charge per metre, size B (g/m)
- allowance already in the factory charge (g)
Explain this equation
Works out how much extra refrigerant to weigh into a split or VRF system for its pipe run. Use it at commissioning, after pressure testing and evacuation.
Most of the refrigerant in the pipework sits in the liquid line, so manufacturers give a charge per metre for each liquid line size. Multiplying each length by its rate and adding gives what the pipework needs. The factory charge already covers a certain length, so that allowance D is taken off. If the result would be negative the answer is zero: you add nothing.
Charge scales with the internal volume of liquid line, which is proportional to length for a given size.
Sense check: 37 m at 50 g/m is 1850 g; less a 1500 g allowance leaves 350 g to add.
Watch out: use the manufacturer's rates and pre-charged length for that exact model. Never remove refrigerant just because the run is short, unless the installation manual says so.
Works out how much refrigerant to add on site when the liquid line is longer than the length covered by the factory charge.
Enter your values to see the result.
Good to know
Take the g/m rates and the deduction (or the pre-charged length × rate) from the outdoor unit’s installation manual: they depend on the model and liquid pipe size. If the manual gives a pre-charged length instead of a deduction, enter D = pre-charged length × rate. Record the added charge on the unit label and in the F-gas records, and recalculate CO₂e with the total.
Common mistake
Measuring the gas line instead of the liquid line, or charging by a generic rule of thumb instead of the unit’s own figures.
Limits
Two liquid pipe sizes; extend by running twice for more. Multi-split and VRF systems often have extra per-indoor-unit charges: follow the manual.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.