Mini split for a garage: size, power and placement

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What size mini split does a garage need?

Start with the floor area — length times width — then account for what makes a garage different. We ran a few common layouts through the same model as our sizing calculator, each with a 9-foot ceiling, few windows, average sun and a temperate climate (zone 3 on our chart).

A one-car garage of 12 by 22 feet comes to about 7,800 BTU even when poorly insulated: a 9,000 BTU system. A two-car garage of 20 by 22 feet is where insulation decides the size. Well insulated, it needs about 10,100 BTU, which is a 12,000 BTU system; with an uninsulated door and walls it needs about 13,100 BTU, just past 12,000, so the calculator steps it up to 18,000.

A larger two-car garage of 24 by 24 feet lands on 18,000 BTU either way, and a three-car garage of 32 by 24 feet needs about 22,800 BTU when poorly insulated: a 24,000 BTU system. These are cooling estimates, not a load calculation. Put your own garage into the calculator, which takes your climate zone, ceiling height and insulation, and where the answer sits close to a boundary, have a qualified HVAC professional settle it with a Manual J.

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Why a garage needs more than a room of the same size

The coverage we publish for each size, and the zone chart, assume a finished room: an 8-foot ceiling, average insulation, average sun and windows. Most garages miss at least two of those. The overhead door is usually the largest opening in the building and is often uninsulated, garage walls and ceilings are frequently left unfinished, and the ceiling is often higher than a living space's.

Each of those is a line in the calculator. A 9-foot ceiling adds an eighth to the load, a 10-foot ceiling a quarter, and poor insulation another 15%. What no calculator can account for is a door that stays open while the system runs; close it, or expect the space to lag behind the thermostat.

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Heating a garage through the winter

Every DIY system we sell is a heat pump, so the same unit heats in winter. MRCOOL's installation manual for the DIY 5th Gen single-zone line lists heating with the outdoor temperature anywhere from −13°F to 75°F and the room between 32°F and 86°F, and warns that protection features can shut the unit down outside those ranges.

Two things follow for a garage. A heat pump's output falls as the outdoor temperature drops, and the heating capacity on a specification sheet is a rated figure, not what the unit delivers on the coldest night — so in a cold climate, read the heating figures for the exact model, not just its cooling size. And because the manual's heating range starts at 32°F in the room, plan to keep the garage above freezing through the winter rather than letting it freeze and starting the system on a cold morning.

NEEP, the Northeast Energy Efficiency Partnerships, publishes a list of cold-climate air-source heat pumps with each listed model's output at 5°F. MRCOOL notes the listing in a model's features where it applies, and it is worth checking before you buy for a garage you mean to heat.

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115V or 230V in a garage

A garage outlet is not a supply for a mini split. Every system needs its own dedicated circuit and a disconnect at the outdoor unit, whatever its voltage, and in most places a licensed electrician runs it.

In the DIY 5th Gen single-zone line, MRCOOL builds the 9,000 and 12,000 BTU systems for 115V with a 20 amp maximum breaker, and the 18,000 and 24,000 BTU systems for 208/230V with 20 and 25 amp maximums. A garage that sizes to 18,000 BTU therefore needs a 208/230V circuit — a double-pole breaker, the same class of supply a dryer uses — so ask the electrician about room in the panel before you settle on a size.

Other series set their voltage per model, and every product page states it.

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Where the indoor and outdoor units can go

MRCOOL's manual rules out installing the indoor unit near any source of heat, steam or combustible gas, near a doorway, or in direct sunlight, and asks for a solid wall, good air circulation and about one meter's clearance from other electrical devices. In a garage, that means keeping it well away from where gasoline or other fuels are stored and from any fuel-burning appliance, and off the wall beside the overhead door.

The outdoor unit follows the same manual: nowhere near combustible gas, not where it is exposed to large amounts of dust, and on a level pad or wall bracket with clear air around it. Keep it clear of the driveway and of where snow is piled when the drive is cleared.

A ceiling cassette needs a flat ceiling with joists on 16-inch centers, at least 8.3 feet from the floor to the unit, and a way to reach the space above — conditions an open-truss or storage-loft garage ceiling may not meet. A wall-mounted unit needs only a solid section of wall.

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The wall penetration and the condensate drain

The line set, the cable and the drain leave through one hole cut with a 3.54-inch (90 mm) core drill. MRCOOL's manual has it sloping slightly toward the outside, the indoor end about 0.2 to 0.275 inches higher, so water runs out rather than in, and it warns to avoid wiring and plumbing in the wall. Once the line set is through, the hole is packed with the supplied neoprene, or with spray foam.

A cooling unit produces water continuously, and the drain has to fall the whole way to somewhere it can discharge outside. Letting it run onto the slab leaves water where people walk and park. If the indoor unit cannot sit where its drain falls to an exterior wall, plan a condensate pump.

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If the garage is a workshop

The indoor unit's filter is reusable, and the manual asks you to inspect and clean it regularly. Sawdust and sanding dust load it faster than household dust does, so in a workshop that means more often. Outside, keep the outdoor unit away from where dust settles, since the manual rules out locations exposed to large amounts of it.

A mini split heats and cools the air it draws through that filter; it is not dust collection. Capture dust at the tool, and the system keeps doing the job it was sized for.

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Choosing the system

A garage on its own is a single-zone project: one outdoor unit and one indoor unit, sized from the estimate above. The sizes we carry, and the lowest current price at each, are below.

If the garage is one of several rooms you are treating at once, it can be one zone of a multi-zone system instead. Size it as its own room — it may need a larger indoor unit than a bedroom of the same floor area — and decide now, because adding a zone later means a different outdoor unit.

Current prices could not be shown here. Each system lists its own price; see single-zone systems.