Refrigerant Line Sizing Calculator - VRF & Split AC

ℹ️ About This Calculator

Refrigerant line sizing sets the liquid line and suction line diameters that connect an outdoor unit to its indoor units. Undersized lines starve the evaporator and raise pressure drop; oversized suction lines drop refrigerant velocity below what is needed to carry oil back to the compressor. This calculator selects line sizes for VRF and split systems from capacity, equivalent length, height difference and refrigerant type.

Suction line sizing is a balance: big enough to keep pressure drop low, small enough to keep refrigerant velocity high enough to drag compressor oil back up risers. That is why suction risers on tall installations are sometimes reduced a size, or fitted with double risers. Liquid lines are sized for low velocity so the refrigerant does not flash into vapour before the expansion valve - subcooling and vertical lift both matter here, since every metre of lift costs liquid pressure. Equivalent length (not straight length) drives the calculation: elbows, traps and Y-joints add significant allowance. Always cross-check the result against the VRF manufacturer pipe table, as each brand publishes its own permitted lengths, lifts and branch limits.

📐 Refrigerant Line Sizing Basis (ASHRAE / JIS B8607)

ASHRAE / JIS B8607

Line selection is by capacity band and velocity limits:

  Liquid line velocity   : ~0.5 - 0.7 m/s (avoid flashing)
  Suction line velocity  : ~8 - 12 m/s (oil return minimum)

Capacity bands (typical OD, R32 / R410A):
  up to  7 kW  ->  Liquid 6.35 mm  | Suction  9.52 mm
  up to 14 kW  ->  Liquid 9.52 mm  | Suction 12.70 mm
  up to 22 kW  ->  Liquid 12.70 mm | Suction 15.88 mm
  above 22 kW  ->  Liquid 12.70 mm | Suction 19.05 mm

Corrections:
  Equivalent length  = actual length + fitting allowances
  Height difference  = adds or subtracts liquid static head
  Long runs or high lift -> upsize suction, check manufacturer table

❓ Frequently Asked Questions

How do you size refrigerant lines for a VRF system? +

Size from system capacity, equivalent pipe length and height difference. Liquid lines are chosen for about 0.5-0.7 m/s velocity to avoid flashing, and suction lines for roughly 8-12 m/s to maintain oil return. Always verify against the VRF manufacturer pipe table for permitted lengths and lifts.

What is the difference between liquid line and suction line sizing? +

The liquid line carries high-pressure liquid and is sized for low velocity to prevent flashing before the expansion valve. The suction line carries low-pressure vapour and must keep velocity high enough (about 8-12 m/s) to return compressor oil, so it is always larger than the liquid line.

Why does suction line velocity matter for oil return? +

Compressor oil circulates with the refrigerant. If suction velocity drops too low, especially in vertical risers, oil collects in the pipe instead of returning to the compressor, which eventually causes compressor failure. That is why oversizing a suction line is as harmful as undersizing it.

Does pipe length affect refrigerant line size? +

Yes. Longer equivalent lengths increase pressure drop, so long runs may need an upsized suction line. Height difference also matters because vertical lift costs liquid pressure and can cause flashing. Use equivalent length, which includes fitting allowances, not just straight length.

Which refrigerants does this calculator support? +

It covers R32, R410A, R22 and R134a. Each has different velocity limits and standard tube ODs, so the tool applies the correct band for the selected refrigerant.

Free Refrigerant Line Sizing Calculator (VRF & Split AC)

ℹ️ About This Calculator

📐 Refrigerant Line Sizing Basis (ASHRAE / JIS B8607)

ASHRAE / JIS B8607

❓ Frequently Asked Questions

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