How should a mini-split line set be routed and shown on ADU plans?

The refrigerant line set should take the shortest practical path between the indoor and outdoor units, stay within the manufacturer's length and height limits, and be insulated and protected the way the Energy Code requires. Your mechanical plan should show that route, the wall penetrations and the condensate drain so the installer and the plan checker see the same design.

Insulation and protection rules

• Vapor line insulation. Section 150.0(j)1 of the 2025 Energy Code sends space-conditioning piping to Section 120.3. An exception there sets heat pump refrigerant vapor lines in residential buildings at a minimum of 0.75 inch thick or R-6.0 insulation, and no insulation is required on the liquid line.
• Weather protection. Under Section 120.3(b), insulation exposed to weather needs a cover suitable for outdoor service that is water retardant and shields it from sunlight. Adhesive tape cannot be used as that protection.
• Vapor retarder. Insulation on refrigerant suction piping outside conditioned space must include, or be protected by, a Class I or Class II vapor retarder, with joints and penetrations sealed.
• Below grade. Insulated piping buried below grade must be in a waterproof, non-crushable casing or sleeve.

Length, charge and installation details

• Every mini-split has limits on line length and vertical rise, and multi-zone units add total piping limits. Longer runs can require added refrigerant, which the installer documents.
• Refrigerant charge must be verified. For many ductless systems, the CEC's Compliance Manual says this is done with the weigh-in method, based on equipment and line size and length, with the ECC rater observing.
• Section 150.0(h)3B requires a liquid line filter drier where the manufacturer's instructions call for one.
• Newer systems may use A2L refrigerants such as R-32 or R-454B, which the Compliance Manual notes are classified as slightly flammable. Follow the manufacturer's installation and space requirements.

Routing choices in an ADU

• Exterior line set cover: simplest to install, but visible and must be weather protected.
• Inside walls or soffits: a cleaner look, but plan the path before insulation and drywall, and seal every penetration to keep the envelope tight.
• Attached conversions: if the line set passes through a fire-rated wall or ceiling between the ADU and the house, the penetration must be protected to keep the rating.

Don't forget condensate

Each indoor head produces condensate in cooling mode. The drain needs a continuous slope or a condensate pump and must discharge to an approved location under the California Mechanical Code and local rules, which may restrict discharge onto walkways or public ways.

What it means for your mechanical plan

• Show the line set route, wall penetration points and outdoor unit location on the floor plan.
• Add notes for insulation thickness, weather and UV protection, and sealing.
• Show the condensate route and termination point for each head.

Plan checkers often flag missing line set and condensate details, as described in common HVAC mistakes on ADU projects. For multi-head layouts, see whether several heads can share one outdoor unit.

Sources: energy.ca.gov, energy.ca.gov.