What should I check when choosing a mini-split to heat a detached garage?

Short answer

Choose a mini‑split that’s sized for your garage’s heat loss, maintains strong heating capacity at your coldest outdoor temperatures, has low‑ambient features (base pan heater, crankcase heater), fits your electrical service and line‑set run, and allows proper placement and drainage. Insulate/air‑seal the garage first, then pick a cold‑climate model with verified capacity at 5°F (or colder) and plan for a dedicated 240V circuit with an outdoor disconnect.

What to check (and why it matters)

• Heating capacity at low temps: Don’t size off the “rated” BTU at 47°F alone. Look for the unit’s output at 17°F and 5°F (or lower). Many “12k BTU” units only deliver 7–9k at 5°F, while cold‑climate models can still deliver near‑full output.
• Right size for your space: Do a quick heat‑loss calculation considering square footage, ceiling height, insulation, air leakage, and garage door U‑value. As a rough starting point, a moderately insulated 400–500 sq ft garage often lands in the 9k–18k BTU range. Oversizing hurts dehumidification and short‑cycles; undersizing won’t keep up in cold snaps.
• Garage envelope: Insulation and air sealing can cut required BTUs dramatically. Weatherstrip doors, add a good bottom seal, insulate walls/ceiling/door, and seal penetrations. Heat pumps like tight envelopes.
• Low‑ambient features: In freezing climates, prioritize models with a base pan heater, crankcase heater, wind baffles, and a heating operating range down to your lows (e.g., −5°F or −13°F).
• Electrical and code: Most mini‑splits are hard‑wired 240V with a dedicated breaker and outdoor disconnect. Confirm breaker size and wire gauge from the nameplate/installer manual. Consult your AHJ on GFCI/AFCI requirements for HVAC circuits.
• Placement and condensate management: Mount the outdoor unit above snow height and out from roof drip lines. Ensure defrost water drains to a freeze‑safe spot. Indoors, slope the condensate drain or use a rated condensate pump.
• Line‑set and install constraints: Check allowed line‑set length, vertical separation, and required flare torque. Plan penetrations away from studs, electrical, and gas lines; protect line‑sets with covers/conduit in a shop environment.
• Noise, dust, and filtration: For dusty workshops, choose indoor heads with washable pre‑filters; consider a ceiling‑mounted cassette only if you can protect it from sawdust. Verify sound ratings and keep the outdoor unit away from bedrooms and neighbors.
• Brand support and warranty: Many brands require licensed installation to keep the full warranty. Availability of parts and local service matters.
• Budget: Expect roughly $900–$2,500 for a quality single‑zone unit (9–18k BTU) plus $1,500–$3,500 for professional install and $300–$800 for electrical, depending on run length and panel capacity.

Quick spec checklist

Capacity (heating):  ____ BTU at 47°F  |  ____ BTU at 17°F  |  ____ BTU at 5°F
Operating range (heat):  down to ____ °F (look for -5°F or -13°F if needed)
HSPF/HSPF2:  ____ / ____   |  COP @ 17°F:  ____
Electrical:  ____ VAC, ____ A MCA, ____ A MOCP (breaker), wire gauge: ____
Low-ambient features: base pan heater [Y/N], crankcase heater [Y/N]
Line-set limits: total ____ ft, vertical rise ____ ft, factory charge covers ____ ft
Sound: indoor ____ dB(A) / outdoor ____ dB(A)
Warranty: parts ____ yrs, compressor ____ yrs, labor ____ yrs (if pro installed)

Step-by-step planning

1. Tighten the envelope (1–2 half-days):
   • Weatherstrip the perimeter and bottom of the garage door.
   • Add insulation to walls/ceiling and consider an insulated door.
   • Seal penetrations with foam/caulk; fire‑block as required.
2. Estimate your load:
   • Use a simple online heat‑loss calculator or a Manual J‑lite approach. If your winter design temp is 10°F and the calc says ~12k BTU, pick a unit that still delivers ~12k at 10°F.
3. Choose your unit:
   • For cold climates, look for “hyper heat” or cold‑climate models with strong 5°F capacity and a base pan heater.
4. Plan layout:
   • Indoor head on an exterior wall simplifies the line‑set/condensate run; keep it away from heavy dust sources.
   • Outdoor unit: on a wall bracket or raised pad 12–18" above grade, not under roof drip or where snow piles.
   • Confirm line‑set path, length, and bend radius.
5. Electrical and permits:
   • Dedicated 240V circuit per the unit MCA/MOCP, outdoor disconnect within sight.
   • Pull permits where required. Many jurisdictions require them for HVAC and electrical.
6. Install or hire:
   • DIY is feasible with pre‑charged systems, but achieving a proper vacuum and leak‑free flares is critical. Warranty and refrigerant laws may steer you to a licensed pro.

Tools and materials you may need

• Hole saw (2.5–3") or core bit, right‑angle drill
• Line‑set (refrigerant copper), insulation, and line‑set covers
• Flaring kit with clutch‑type flaring tool; flare gauge and deburring tool
• Torque wrench with flare crowfoot adapters (tighten to spec)
• Vacuum pump (4–6 CFM) and a digital micron gauge (target ≤500 microns, hold test)
• Nitrogen tank with regulator for pressure test and line purging
• Manifold gauge set or core‑removal tool for faster evacuation
• Outdoor wall bracket or pad, vibration isolators
• Condensate tubing and trap; condensate pump and heat tape if needed
• Electrical disconnect, weatherproof whip, conduit/fittings, correct wire gauge per MCA
• Sealants: polyurethane caulk, fire‑rated foam where required

Safety considerations

• Shut off power at the panel, lockout/tagout, and use a non‑contact voltage tester.
• Wear eye/hand protection; use a respirator when drilling masonry or handling fiberglass.
• Pressure test with nitrogen only (never oxygen). Respect pressure ratings.
• Evacuate to a deep vacuum before opening service valves; avoid refrigerant exposure.
• Seal penetrations to maintain the garage’s fire separation from the house.
• Manage defrost water to avoid ice hazards.

Tips for best results

• Aim the indoor head to wash air across the garage, not directly at a bench.
• Use line‑set covers and grommets to protect copper in a working garage.
• Add a Wi‑Fi thermostat/adapter if you plan to pre‑heat the space remotely.
• In very dusty shops, add a washable prefilter screen to the indoor head and clean monthly.

Common mistakes to avoid

• Sizing by square footage only; verify capacity at 5–17°F.
• Skipping a base pan heater in freezing climates—units ice up and trip out.
• Undersized electrical circuits or no outdoor disconnect.
• Poor condensate routing—frozen drains can back up into the unit.
• Flares not torqued to spec or lines not evacuated properly, leading to leaks.
• Mounting the outdoor unit under a roof edge or where snow blowers will bury it.

When to call a professional

• You need a formal Manual J, have sub‑zero design temps, or limited electrical capacity.
• Running a new 240V circuit, upgrading the panel, or unclear on code requirements.
• Performing refrigerant work (pressure test, evacuation) if you lack the tools/experience.
• Preserving manufacturer warranty that requires licensed installation.

With a tightened envelope and a cold‑climate mini‑split sized to your actual heat loss and local temps, a detached garage can stay comfortably warm all winter with predictable operating costs and minimal maintenance.