How to Size AC Pipework: A Guide for HVAC Engineers
Correct pipework sizing is one of the most critical factors in air conditioning system performance and longevity. Undersized pipe increases refrigerant velocity and pressure drop, reducing efficiency and potentially damaging the compressor. Oversized pipe can cause oil return issues on long runs.
This guide covers the key principles for sizing refrigerant pipework on split system and light commercial VRF/VRV installations.
Understanding Pipe Size Conventions
Refrigerant pipework is specified by outside diameter (OD) in imperial inches — not by bore or metric dimensions. Common sizes are:
- 1/4" OD — liquid line on small systems
- 3/8" OD — liquid line on most domestic split systems
- 1/2" OD — suction line on small to medium systems
- 5/8" OD — suction line on medium to large systems
- 3/4" OD — suction line on larger commercial systems
- 7/8" OD — suction line on VRF/VRV main headers
Always use refrigeration-grade copper pipe — dehydrated, capped, and nitrogen-purged. Standard plumbing copper is not suitable for refrigerant circuits.
Following Manufacturer Pipe Sizing Tables
The most reliable method for sizing pipework is to follow the outdoor unit manufacturer's installation manual. This will specify:
- Liquid line OD by system capacity (kW)
- Suction/gas line OD by system capacity
- Maximum pipe length (equivalent metres)
- Maximum height difference between indoor and outdoor units
- Additional refrigerant charge per metre of pipe beyond a set length
Never deviate from manufacturer pipe sizing tables without engineering justification — doing so may void the warranty and affect system performance.
Maximum Pipe Lengths and Height Differences
Most domestic split systems specify a maximum equivalent pipe length of 15 to 30 metres and a maximum height difference of 10 to 15 metres between indoor and outdoor units. VRF/VRV systems allow significantly longer runs — up to 150 metres equivalent on some systems — but require careful pipe sizing and additional refrigerant charge calculations.
Where the outdoor unit is above the indoor unit, oil traps may be required on the suction line at specified intervals to ensure oil return to the compressor.
Condensate Pipework Sizing
Condensate drain pipework is sized separately from refrigerant lines. Standard practice is:
- Minimum 21mm OD (16mm ID) for most indoor units
- Fall of at least 1:50 (2%) where gravity drainage is used
- Where gravity drainage is not possible, a condensate pump is required
Insulation Requirements
All refrigerant pipework must be insulated to prevent heat gain on the liquid line and condensation on the suction line. Standard specification is 13mm wall elastomeric foam insulation for most domestic installations. See our guide to pipe insulation for wall thickness selection by application.
Pressure Testing and Evacuation
Once pipework is complete, the system must be pressure tested with dry nitrogen before evacuation. Use a vacuum pump and manifold gauges to evacuate to 500 microns or below before charging with refrigerant.
Shop Refrigerant Pipework
We stock refrigeration-grade copper pipe in all standard sizes from 1/4" to 7/8" OD, with same-day dispatch on most lines. View our full pipework and fittings range or call 0161 775 1190 for technical advice.
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