Can Mini Split Lines Run Inside a Wall? (Builder Guide)

If you're planning a ductless system install and don't want line covers running across the side of your house, you're probably wondering whether those mini split lines can just disappear inside a wall. Short answer: yes, they can. But the details matter more than the yes.

I'm a builder in Boise and I've framed wall channels, drilled plates and fixed drywall on enough mini split jobs to know where this goes smoothly and where it might get expensive. Here's what you might want to know.

Key Takeaways

• In Boise and the Treasure Valley, you also have to think about framing, insulation and code: don't make holes too big in studs or plates, and make sure any penetrations in exterior walls and top plates are air sealed.

Can Mini Split Lines Run Inside a Wall? (Direct Answer Up Front)

The "line set" is a group of parts: two copper refrigerant lines (a smaller liquid line and a larger suction line), electrical connections, communication wiring between the indoor and outdoor units, and usually a separate condensate drain line. All or some of these can be hidden in a wall cavity, whether it's an interior wall or exterior wall.

In Boise-area homes, most walls are framed with 2x4 or 2x6 studs, and exterior walls have insulation and sheathing. That limits how much space you've got inside the cavity, and how many holes you can drill before you hurt a stud. The copper refrigerant tubing should be one long piece with no hidden joints or fittings inside finished walls, because a leak in a closed cavity costs a lot to find and fix. Insulated lines are usually run in wall cavities without PVC, but they need support and protection.

Why Exterior Walls Are Usually Simpler

The typical Boise install looks like this: a wall-mounted unit on an exterior wall, an outdoor heat pump on a pad or bracket just outside, and a short line set through the siding covered by a line hide on the exterior.

Installing on an exterior wall makes refrigerant line runs simpler for a few reasons:

• On exterior walls, the line set needs just one hole - one hole that goes straight through the wall behind the head.

• The refrigerant lines, wire and communication wiring have a short run, and it's usually covered by a slim duct or line hide on the back side of the house.

• On exterior walls, condensate drains can slope straight down to daylight or an approved termination - no pump, no long runs.

• Installing on an exterior wall makes the job less complex and takes less labor. There's less drywall to cut, less patching to do and the job goes faster.

Installing on an Interior Wall

Here's what changes:

Installing a mini split on an interior wall is often more complex. Common paths from an interior wall include:

• Up into the attic, across, then down an exterior wall to the condenser.

• Down into a basement or crawl space, then out through the rim joist.

• Sideways into a closet or chase, then up or down to the exterior.

What Actually Runs Inside the Wall? (Lines, Wiring and Drain)

Refrigerant lines: Two copper tubing - liquid and suction - in factory-specified sizes with maximum lengths per manufacturer (for example, Mitsubishi small units allow about 65 ft one-way, with extra refrigerant charge needed beyond about 25 ft). Using heavy-duty insulation protects lines from moisture and improves efficiency. Insulation must be continuous and sealed around refrigerant lines to prevent condensation in the wall cavity. Proper insulation prevents condensation issues in wall runs.

Electrical connections and communication wiring: Low-voltage communication cable runs between the indoor and outdoor units, often tied with the line set. Power wiring follows the NEC and Idaho electrical code. An Idaho homeowner electrical permit only covers work the owner personally does on their primary or secondary residence and outbuildings - rentals and commercial buildings don't qualify, and Boise and Meridian run their own electrical permitting. Electrical wiring for mini splits must meet electrical codes and system requirements.

Running Lines Inside Stud Bays

According to the 2018 IRC (Section R602.6 and Table R602.3(5)), bearing and nonbearing walls have different rules:

Nonbearing 2x4 studs can run 14 ft unsupported at 24 in o.c. and 2x6 nonbearing studs 20 ft. 2x3 studs are not allowed in exterior walls - which matters in some older Boise homes with unconventional framing where running lines is harder.

Practical guidance:

Local building codes may restrict in-wall routing of refrigerant lines in unventilated cavities. Older homes may require more precautions due to existing structural conditions - always check before you drill. Installation requires following building codes and regulations.

Attic, Basement and Crawl Space Routes

Attic routes are common for second-floor rooms on interior walls. Lines go up through the ceiling, across the attic, then down an exterior wall to the outdoor unit. In Boise's hot summer attics, insulate and support refrigerant lines to prevent heat gain. Don't bury junctions under blown fiberglass insulation without marking the route.

Basement and crawl space routes work well for first floor rooms. Lines drop down an interior wall into an unfinished basement or crawl space, run horizontally along joists, then exit through the rim joist. In vented crawl spaces, strap lines to joists - don't let them rest on the ground. Protect foam insulation from critters and moisture.

Condensate Drain vs. Condensate Pump

Gravity drain (preferred): A 3/4" PVC or tubing drain line sloped at about 1/8" to 1/4" per foot all the way to daylight or an approved drain. Condensate drains rely on gravity to function properly. Condensate drainage must maintain a consistent downward slope. Condensate lines should slope downward to prevent clogs. Long horizontal runs across ceiling cavities get tricky - sagging pipe creates traps and hidden leaks.

Interior vs. Exterior Wall Moisture Control

• Holes through sheathing and siding must be sealed with foam insulation, gaskets or flashing.

• Poor air sealing leads to drafts, cold spots and ice build-up.

• Running warm condensate drains through insulated exterior walls without proper slope can cause hidden moisture.

• Top and bottom plates must be sealed after drilling to prevent air leaks between floors and into attics.

• Cutting multiple bays can disturb fireblocking. Firestops must be repaired to code.

Indoor Unit Options That Affect Line Routing

• Wall-mounted unit: Most common in Treasure Valley homes. Works well with a clear route to an exterior wall, attic or crawl space. Needs solid backing and clear air distribution around the unit.

• Ceiling cassette: Recessed in the ceiling, ideal for rooms with no good exterior wall. Lines and condensate run above the ceiling. Needs enough depth between finished ceiling and framing.

• Compact ducted air handler: Hidden in an attic, crawl space or closet with short ducts to supply grilles. No visible indoor equipment on the wall - only small grilles show. This is a duct system alternative that keeps line sets in accessible service areas.

• Floor console: Sits low on the wall, helpful where windows or sloped ceilings limit upper wall space. Lines often run down through the floor into a basement or crawl space, making gravity drain simpler for interior rooms.

When In-Wall Lines Are Worth the Cost

• Interior wall far from an exterior wall or unfinished space (longer line set, more labor)

• Multiple drywall cuts, patches and repainting

• Condensate pumps, longer electrical runs, extra insulation

• Planning during a remodel or new construction when walls are already open

• Using closets, dropped soffits or shared wall channels instead of cutting every stud bay

• Grouping multiple indoor units along shared routes to a single outdoor unit

DIY vs. Hiring a Pro in Idaho

Some homeowners can legally handle parts of a mini split install in Idaho - mounting equipment, running lines through open framing, patching drywall. But refrigerant handling must comply with EPA rules, and most homeowners hire a licensed HVAC tech to evacuate, pressure test and charge the system.

Idaho-specific rules to know:

• An Idaho homeowner electrical permit only covers work on your primary or secondary residence and outbuildings. Rentals and commercial buildings don't qualify.

• Boise and Meridian run their own electrical permitting - don't assume state rules automatically apply.

• An unregistered contractor in Idaho can't pull building permits, loses lien rights and the right to sue for payment, and faces a misdemeanor with up to a $1,000 fine or 6 months in jail. Check registration before hiring someone to cut into your walls.

Planning an Interior-Wall Mini Split

• Exact indoor unit location, which wall it mounts on, height and clearance

• Full route of the line set, electrical connections, communication wiring and condensate drain from indoor unit to outdoor unit

• Where lines will be visible: exterior slim duct, interior soffits, closets, mechanical rooms

• Will condensate drain by gravity, or will we need a pump? Where will it discharge?

• How many stud bays, top plates or fireblocks need drilling, and how will they be repaired and sealed?

• What access will we have for future service if a drain clogs or a line leaks?

FAQ