Mini-Split for a Garage Woodshop: BTU Sizing, Install Cost, and Runtime vs. a Space Heater

A two-car garage in Texas in August is a bad place to keep wood. It's a worse place to keep a finish schedule. I've watched a stack of flatsawn red oak that looked flat in spring cup hard enough to rock on the bench by the time football season starts, and I've watched a water-based topcoat refuse to level because the thermometer in the shop read 58 and falling. So when a reader asks whether a mini-split is worth it, I don't answer with a product. I answer with a load number, a cost number, and a hard look at what the machine actually controls.

The garage is a load problem, not a square-footage problem

The number you see online — "a two-car garage needs 12,000 BTU" — comes from living-space tables, and it's low for an unconditioned garage. Total Home Supply's general cooling table puts 450–500 sq ft at 12,000 BTU, then tells you to add 20% for an old or drafty garage, 20% for poor insulation, and 30% if your climate runs over 90°F. Stack those adders and you're not at 12,000 anymore.

The garage-specific sources run hotter. Bryant gives preliminary bands by garage size: a one-car garage (200–300 sq ft) "typically requires 9,000 to 12,000 BTUs"; a two-car (400–600 sq ft) "typically requires 12,000 to 18,000 BTUs"; a three-car may need 18,000–24,000. Bryant is careful to call these "preliminary ranges, not a substitute for a load calculation." hvaccompared splits it by construction: an insulated garage with a weatherstripped door needs "roughly 20-25 BTU per square foot," while an uninsulated garage with a thin metal door and no wall insulation needs "30-40 BTU per square foot — sometimes more in extreme climates."

Run the arithmetic on a 24 ft × 24 ft bay. That's 576 sq ft. At hvaccompared's 30–40 BTU/sq ft for uninsulated space, you get 17,280–23,040 BTU, which rounds to a 24,000 BTU unit. At Bryant's two-car band it lands at the top, 18,000. At Total Home Supply's living-space table it looks like about 14,000–18,000. Same garage, three answers, and the spread is the whole story.

Then apply hvaccompared's adjustment rules: add 10–15% for ceilings above 10 ft, add 10–20% for climates that regularly exceed 100°F or drop below 10°F, subtract 10% if the garage shares an insulated wall with the house, and add 20–30% if the door opens frequently. A Texas two-car with a 12-foot ceiling, a hot afternoon, and a door that goes up every time you drag out the mower is not a 12,000 BTU job.

None of this is a load calculation. The industry standard is ACCA Manual J, and ENERGY STAR describes it as "a calculation of the amount of heating and cooling your home requires." Filterbuy says a Manual J load calculation "costs $100 to $300, takes an afternoon." If you're spending two grand on equipment, buy the afternoon.

Don't oversize to be safe

The instinct is to round up. Don't. Bryant's expert, Brian Redmond of Redmond's Complete Comfort, is quoted saying an oversized unit "will short-cycle — cooling the space too quickly without removing enough humidity," and that "your operating costs will be much higher if you install a unit that's too large." Total Home Supply makes the same point. You get a cold, clammy shop and a bigger bill.

DIY versus pro: the real numbers

Bryant puts professional garage mini-split installation at "typically costs between $2,000 and $7,000," depending on capacity, efficiency, electrical requirements, and local labor rates.

HomeComforts Depot publishes a single-zone breakdown that's more useful because it separates the line items. Equipment for 12k–18k BTU runs $700–$1,200 on the DIY route versus $600–$1,500+ for a standard unit bought for a contractor to install. HVAC labor is $0 DIY versus $300–$800+. Line-set cover is $50–$150 DIY versus $75–$200 contractor-supplied. Electrical outlet or circuit upgrade runs $150–$400 either way. Estimated total: $900–$1,750 DIY versus $1,200–$2,900+ professional.

That page contradicts itself. Its "Quick Answer" claims DIY saves only "$200–$600 in labor," while its own table implies total savings up to about $1,150. Trust the table, not the headline.

For actual equipment prices, MRCOOL's own retail listings show a Select 12,000 BTU 115V at $1,399.00, an 18,000 BTU 230V at $1,690.00, and a 24,000 BTU 230V at $1,899.00. The 5th Gen E Star 18,000 BTU with a 25 ft line set starts at $2,910.00, and the 5th Gen Hyper Heat 12K, rated to −22°F heating at 23.5 SEER2, starts at $2,557.00. hvaccompared's own estimates put a Mr. Cool DIY 4th Gen 24K around $1,400 and a Pioneer Diamante 18K around $1,050, but flag those as estimates, not fetched listings.

What makes the MRCOOL DIY line homeowner-installable is the pre-charged, pre-flared quick-connect line set that "eliminate[s] the need for an HVAC technician to handle refrigerant." HomeComforts Depot states DIY installation doesn't void the MRCOOL DIY warranty, but says the electrical work should still be done by a licensed electrician. Believe that.

Electrical is where the budget sneaks up. hvaccompared says a 12K unit typically needs a 20-amp, 240V dedicated circuit, while 18K–24K units "usually need a 30-amp, 240V dedicated circuit with a disconnect box." A panel upgrade, if you need one, runs $500–$1,200.

One warranty note worth your attention: Mitsubishi's 12-year compressor warranty applies "when professionally installed." Budget brands run 5–7 years. That gap is part of what you're buying with a pro install.

Runtime: heat pump versus resistive heat

Here's the physics that decides the argument. Electricity is 3,412 BTU per kilowatt-hour. A 1,500-watt resistance space heater therefore delivers about 5,118 BTU/hr. Electric resistance heat has a coefficient of performance of 1 — every watt in becomes a watt of heat, no more. A heat pump has a COP of about 3, per building scientist Allison Bailes III, because it moves heat instead of making it. Wikipedia's electric-heating page puts heat pumps at COP 1.5–6.0.

The DOE says heat pumps "transfer — rather than generate — heat," and modern air-source units "can reduce your electricity use by 50% compared to furnaces and baseboard heaters." ENERGY STAR says certified mini splits "use up to 60% less energy than standard home electric radiators." Those are different framings of the same physics, so don't stack them. Treat the savings as roughly half to 60% of resistance energy, which implies a real seasonal COP nearer 2–2.5 than the ideal-case 3.

Now put dollars on it. Filterbuy's method: wattage = BTU ÷ SEER2; monthly cost = (wattage ÷ 1,000) × hours/day × 30 × rate. Its benchmark rate is the EIA national average residential rate of 18.05¢/kWh as of April 2026. At 8 hours a day, Filterbuy prices an 18,000 BTU unit at 20 SEER2 at about $39/month in cooling. Filterbuy also states heating mode "generally runs 20 to 40% higher per hour" than cooling, yet "a modern heat pump mini split beats electric baseboard heat two to three times over."

Compare that to a space heater. A 1,500-watt unit pulls 1.5 kWh every hour it runs, about 27 cents an hour at Filterbuy's rate.

Three things move the bill in a garage: each degree closer to outdoor temperature adds 3–5%; dirty filters cut efficiency 5–30%; and poor insulation and leaky windows push operating cost "50 to 100% higher." That last one dominates. If your garage door is a thin metal panel with a gap at the bottom, no mini-split will save you from it.

What the swing does to your wood

This is the part that matters more than the electric bill. The wood-movement formula from Popular Woodworking is width × annual change in moisture content × the species' dimensional change coefficient, valid between 6% and 14% MC. sensorahome, citing the USDA Wood Handbook, gives red oak a tangential coefficient of 0.00369 per 1% MC per inch of width, and computes a 24-inch flat-sawn red oak top with a 4-point MC drop moving 0.355 inch — over 3/8".

Popular Woodworking notes that most climate-controlled houses change 3–4% MC during a year. WoodPursuits puts the indoor seasonal swing at "often 4–8%" and says flatsawn stock moves "often more than twice as much" as quartersawn. An unconditioned garage sits at or above the high end of that.

Here's the trap: a mini-split holds temperature. It does not necessarily hold humidity. Wikipedia, drawing on the Hailwood–Horrobin model, states that at constant relative humidity the EMC drops only about 0.5% for every 10°C rise in air temperature. In plain terms, EMC tracks relative humidity far more than temperature. sensorahome puts EMC at about 7% at 70°F and 35% RH, and about 12% at 70°F and 65% RH. A machine that cools the air without managing moisture does not stop the MC swing that moves your panels. Don't let anyone tell you a mini-split keeps wood at a fixed moisture content.

What the swing does to your finish

General Finishes, the manufacturer, states "Ideal Conditions are 70 degrees F and 50% humidity." Finishing outside 65–75°F "can lead to problems," and "a space below 55 degrees F is definitely too cold." On water-based topcoats, cold slows dry time and leveling, and the rule of thumb is "if it is cold enough to wear a sweater it is too cold to apply a water-based finish." Oil-based finishes "are not as affected by cooler temperatures," and "you can apply oil-based finishes in as low as 60 degrees if necessary." Cure time is seven days, then move the piece somewhere warmer. General Finishes also warns flatly: "Garages are not a good place to store any finish."

Today's Homeowner adds that polyurethane dries in a few hours but "can take 3 to 30 days to fully cure," wants 60–80°F and 30–60% humidity, and that cold leaves it "tacky for days."

A space heater that swings the garage from 35°F to 70°F and back doesn't give you the stable 65–75°F, moderate-humidity window either source specifies. A mini-split held at a steady setpoint gets closer. Neither fixes a wet slab or a leaky door.

My call

If your garage is insulated and the door is weatherstripped, size from hvaccompared's 20–25 BTU/sq ft, then apply the adders — you're probably in the 18,000 BTU range for a 24×24, not 12,000. If it's bare studs and a thin metal door, you're looking at 24,000, and you should spend a weekend air-sealing before you spend a dime on equipment. Get the Manual J if you're near a boundary. Buy from the MRCOOL DIY line if you're comfortable with the electrical, and hire the electrician regardless.