Mini-splits come up a lot when someone has a garage they want to make usable in winter, a detached shop that gets unbearably hot in July, or a room addition that the main system just does not reach. They are a good solution for all of these, but the specifics matter more than most people realize before they start getting quotes.
Here is what I want homeowners to understand before pulling the trigger on a purchase.
Sizing Is Not Straightforward in Unconditioned Spaces
For a standard room in a house, there are rules of thumb that get you close enough: roughly 20 BTU per square foot, adjusted for windows and exposure. For a garage or uninsulated shop, those rules do not apply.
The correct approach is a Manual J load calculation. An unconditioned space has a completely different thermal profile than living space: uninsulated concrete floors, thin garage doors with significant air gaps, no vapor barrier, ceiling heights that may be 10 to 14 feet, and in a shop, equipment that generates its own heat load.
Insulation is the single biggest variable. An uninsulated garage can require 30 to 50 percent more capacity than the same square footage in an insulated one. Ceiling insulation matters most because heat loss through an uninsulated roof is often the dominant factor. If you are planning to insulate the space, do that before anyone sizes a system for it.
Garage door type also matters. A 16-foot-wide uninsulated metal door is a large, poorly sealed surface that sees significant heat transfer. Gaps around the door frame add to infiltration. An insulated door with a proper bottom seal makes a real difference.
As a starting point for planning purposes, a two-car garage typically lands in the 12,000 to 18,000 BTU range when insulated. An uninsulated three-car garage or large workshop often needs 24,000 BTU or more. But these are planning numbers, not engineering. I size every job with a load calculation.
The Electrical Side
Every mini-split requires a dedicated 240-volt circuit. This is not optional; it is a code requirement. The circuit cannot be shared with any other appliance.
Each unit has two electrical specs you need to know before sizing the circuit:
MCA (Minimum Circuit Ampacity): This is the minimum wire and circuit size the installation requires. The circuit must meet or exceed this number.
MOP (Maximum Overcurrent Protection): This is the maximum breaker size allowed. Your breaker must not exceed it.
These numbers are on the unit’s data plate and in the installation manual. A licensed electrician will know to check them. Anyone who just installs a breaker without checking the MCA and MOP specs is guessing.
As a general reference: a 12,000 BTU unit on 240 volts typically needs a 15 to 20 amp circuit. An 18,000 BTU unit typically needs 20 to 25 amps. A 24,000 BTU unit is usually 25 to 30 amps. Always go by the spec sheet for the specific unit being installed.
The outdoor unit also requires a lockable disconnect switch installed nearby. This is a code requirement, not just a best practice. It allows service work to be performed safely without needing to go to the panel.
For a detached garage, if there is no existing 240-volt circuit, someone needs to run one from the main panel. For a detached structure, that often means running conduit or underground cable, which adds to the cost and may require its own permit. I work with licensed electricians when a job requires new line voltage runs.
What a Line Set Is
The line set is the pair of insulated copper tubes that connects the outdoor unit to the indoor air handler. One line carries refrigerant from the outdoor condenser to the indoor coil; the other carries it back. Both lines are insulated, and the run also includes a communication wire and a condensate drain.
The lines pass through a hole in the wall, typically around three inches in diameter, and are usually covered by a plastic line hide raceway on the exterior.
Most residential mini-splits come factory pre-charged with refrigerant for a line set of 16 to 25 feet. If your installation requires a longer run, refrigerant must be added per the manufacturer’s specifications. There is also a minimum length, usually 10 to 15 feet depending on the unit; a run that is too short creates pressure problems.
Every unit has a maximum allowable line set length. For many residential units, this is 50 to 65 feet. Exceeding it causes high head pressure, compressor problems, and efficiency loss. For a detached garage with a long routing path, this is worth calculating before settling on equipment.
Low-Temperature Performance
Washington state is mild by national standards, but we do see nights in the 20s in Kelso and Longview, and a garage or shop that is supposed to be usable in January needs to actually work in January.
Standard mini-splits begin losing meaningful capacity as outdoor temperatures drop below 20 to 30 degrees Fahrenheit. Some older or lower-tier units shut off entirely below 15 to 20 degrees.
The Bosch Climate 5000 line I install is rated to operate down to -13°F for heating. The Max Performance tier within the Climate 5000 line goes lower than that. For the Pacific Northwest, even the standard rating is well beyond what we typically see, so cold-weather operation is not a concern for most of my customers here.
When you are comparing units, look for the minimum operating temperature for heating mode. If a unit is not rated below 20°F, it is not the right choice for year-round use in this climate.
Room Additions vs. Garages
A room addition is usually a better-defined project than a garage install. The space is designed to residential energy code: insulated walls, proper windows, vapor barrier. The load calculation is more predictable and closer to standard residential assumptions.
The reason the main system does not serve additions is almost always that the ductwork was never extended, or extending it is not practical. A mini-split solves this cleanly: one line set penetration, one indoor unit, no ductwork.
Sunrooms are a different case. A sunroom with large glass panels is dominated by solar heat gain in the summer and heat loss in the winter. The glass type, SHGC rating, and orientation all change the sizing significantly. South-facing sunrooms with uncoated glass can have cooling loads that are two to three times what you would expect for the square footage. Sizing a sunroom incorrectly produces an oversized unit that short-cycles and does not dehumidify properly, which is a real problem in Western Washington’s climate.
Permits
Washington state requires a mechanical permit for mini-split installations. An electrical permit is required when a new circuit is run, which is the case on almost every job. These are standard requirements; any licensed contractor should pull them as a matter of course.
If a contractor tells you permits are not needed or suggests you pull them yourself, that is a problem. Unpermitted work can create issues when you sell the home or file an insurance claim.
One Thing That Surprises Some People: Rebates
Cowlitz PUD offers a $900 rebate for qualifying ductless heat pump installations. But this rebate applies to primary living space only. Garages, workshops, and detached shops do not qualify, regardless of what equipment is installed.
The exception is a space that has been fully converted to livable use and received a Certificate of Occupancy from Cowlitz County. Once it is permitted and inspected as living space, it qualifies.
If the system is going into a room addition that is permitted and attached to the home, that typically qualifies. If it is going into an unfinished garage that you want to work in on weekends, it does not.
Worth knowing before you budget.
If You Are In the Planning Stage
The right time to talk to a contractor is before you start buying equipment. BTU size, line set routing, electrical requirements, and rebate eligibility are all easier to plan upfront than to sort out after the fact.
If you are in Kelso, Longview, or the surrounding area, I am happy to walk through the specifics of your project. Call me and describe the space. No charge for the conversation.