Garages and workshops are among the most common ductless applications, and they are the space where the standard square-foot rule is least reliable.
Why a garage is different
The door. A standard uninsulated sectional garage door is a large, thin, leaky exterior surface. On a two-car garage it can be a third of the exterior envelope area and the dominant heat path.
The slab. An uninsulated concrete floor in contact with the ground is a continuous heat sink, and it has enormous thermal mass, which means the space takes a long time to come up to temperature.
The walls. Many garages are uninsulated, or insulated but unfinished, and the ceiling frequently opens into an unconditioned attic.
Air leakage. Garage doors seal poorly by design, and the door is opened.
What to do before choosing a head
Every dollar spent on the envelope here returns more than in a finished room, because the starting point is so poor.
Insulate the door or replace it with an insulated one. This is usually the single largest improvement available.
Insulate the walls and ceiling, and air-seal the top plates and any penetrations.
Weatherstrip the door perimeter and the bottom seal.
Insulate the slab edge if the floor is being redone, which is a long shot but the right time if it happens.
An insulated garage may need half the capacity of an uninsulated one, and the insulation costs less than the difference in equipment plus the running cost.
Sizing
After insulation, the room rules apply with adjustments upward for whatever remains uninsulated and for the door.
A single-car garage, insulated, commonly takes a 9,000 to 12,000 BTU head. A two-car garage, insulated, is typically 12,000 to 18,000. Uninsulated spaces need substantially more and will still perform badly, because the load never stops.
Where the space is used for a workshop with machinery, the equipment adds heat in summer and helps in winter, and it belongs in the calculation.
The intermittent use question
A garage used for a few hours at a weekend has a different requirement from a workshop used daily.
Recovery matters more than steady-state capacity when the space is allowed to cool between uses, and the slab's thermal mass makes recovery slow. A larger head brings the space up faster, at the cost of the usual oversizing problems once it is there.
The alternative is to hold the space at a low setback temperature continuously and raise it when needed, which suits a heat pump well because it runs at low output most of the time.
Cold climate limits
A garage in a cold climate is one of the harder ductless applications: a large load, a poor envelope, and the coldest outdoor conditions at the same time.
Cold-climate rated equipment and the extended capacity table matter here more than in a bedroom, and it is worth checking the capacity at design temperature against a load calculated for the space as it will actually be, doors and all.
Combustion and ventilation
A garage used as a workshop may contain solvents, fuel and a vehicle.
Conditioning it does not change the requirement for ventilation and for keeping ignition sources away from vapours. A ductless head recirculates air within the space rather than exhausting it, so it does nothing for air quality.
Where a vehicle is run inside, that is a carbon monoxide question that the heating system does not address.
The practical arrangement
Head high on a wall, throwing down the length of the space and away from the work area rather than across it. Outdoor unit clear of the door and of where a car reverses.
And a filter that is actually cleaned, because a workshop generates far more dust than a bedroom and a clogged head filter costs both capacity and efficiency.