Mini Split Calculator

Mini Split formula

Base load = Area (sq ft) x BTU per sq ft  (20 mild, 25 moderate, 30 hot, 35 very hot)
Adjusted load = Base load x sun factor x insulation factor x Ceiling height / 8
Cooling BTU = Adjusted load + 600 x (people - 2) + 4,000 if kitchen
Head size = next standard head at or above the load (9k, 12k, 18k, 24k, 30k, 36k); for heating, size to 1.25 x Cooling BTU

Worked example

A 24 x 16 ft family room with 10 ft ceilings in a hot climate, very sunny, poorly insulated, with 4 people and an open kitchen: 24 x 16 = 384 sq ft. 384 x 30 = 11,520 BTU. Times 1.1 for sun, 1.15 for insulation and 10 / 8 for the ceiling: 11,520 x 1.1 x 1.15 x 1.25 = 18,216 BTU. Add 2 x 600 = 1,200 for the extra people and 4,000 for the kitchen: 23,416 BTU per hour.

The smallest head that covers it is 24,000 BTU, a 2 ton unit. If the same head will heat the room on 15 F nights, size for 23,416 x 1.25 = 29,270 BTU and pick a 30,000 BTU head, or pair a 24,000 BTU head with a cold-climate outdoor unit that holds its rated output at 5 F.

Quick reference

Sizing a ductless head

A mini split head serves one room or one open area, so size it to that space alone: area times a climate factor, adjusted for sun, insulation, ceiling height, people and cooking. Then step up to the next standard head. Do not skip two sizes. An inverter compressor can throttle down, but a 24,000 BTU head in a bedroom that needs 6,000 will still cycle, run humid and cost more than a 9,000 BTU head.

If the head will be the main heat source, size it for heating instead. A heat pump's rated capacity is measured at 47 F outdoors and falls as the temperature drops, so for winters that regularly go below 20 F pick a head 20 to 30 percent above the cooling load, or choose a cold-climate model whose published capacity at 5 F still meets the room's heat loss. Check the manufacturer's capacity table, not just the nameplate.

Multi-zone systems

For more than one room, run the calculator for each and add the head sizes. Outdoor units are rated in total BTU for 2, 3, 4 or 5 heads, and the manufacturer publishes which head combinations each one supports. Because all rooms rarely peak at once, most brands allow the heads to total up to about 130 percent of the outdoor unit's rating, but going that high leaves no reserve for heating.

Every zone needs a line set run and a condensate drain, and most brands limit total line length and the rise between heads and the outdoor unit. A single-zone unit per room is often cheaper to run and more reliable than one large multi-zone unit, because each compressor matches its own load. Have a licensed HVAC technician pull the vacuum, charge the lines and wire the disconnect; the sizing here is a planning estimate.

Key facts

• Ductless indoor heads come in 9,000, 12,000, 18,000, 24,000, 30,000 and 36,000 BTU; one ton of cooling is 12,000 BTU per hour.
• Multi-zone outdoor units run 2 to 5 heads, occasionally 8, and are rated from about 18,000 to 48,000 BTU; the connected heads can usually exceed the outdoor rating by 30% or so.
• Energy Star's cold-climate heat pump specification requires at least 70% of rated heating capacity at 5 F, and many models are listed to operate down to -13 F or lower.
• A standard heat pump loses a large share of its heating capacity between 20 F and 0 F, which is why a head that is fine for summer can fall short in a northern winter.
• Inverter mini splits modulate down to about 25 to 30% of rated capacity, so a slightly oversized head still runs long, quiet cycles instead of short-cycling like a window unit.

Frequently asked questions

Sources and references

• •Manual J Residential Load Calculation, 8th Edition (ACCA)
• •Ductless Heating and Cooling: sizing and cold-climate performance (ENERGY STAR)
• •Air-Source Heat Pumps: cold-climate certification tested to 5 F (ENERGY STAR)
• •All-Electric Heat Pumps: Hyper-Heating H2i capacity at 5 F and multi-zone systems (Mitsubishi Electric Trane HVAC US)