Cooling
Mini-splits: sizing each zone and the outdoor unit
A mini-split is sized twice, once for each room and once for the outdoor unit that serves them all. Most mistakes happen in the first step, with a 12k head in every bedroom.
How a mini-split system is put together
A ductless mini-split has an outdoor unit, with the compressor and outdoor coil, connected by a pair of refrigerant pipes and a control cable to one or more indoor units, called heads. Each head sits in one room or open area, which becomes its zone, and has its own thermostat. A single-zone system has one head and one outdoor unit; a multi-zone system has two to eight heads on one larger outdoor unit. Most mini-splits sold in the US are heat pumps as well as air conditioners, so they heat in winter too.
Because each head serves only its room, sizing happens twice: each head for its room’s load, and the outdoor unit for the heads together.
Step one: size each head for its room
The mini-split size calculator applies the same climate-zone figures used for central systems to each room’s floor area, then adjusts for the room’s use and its sun:
The head is the smallest common size at or above that load. Heads come in 6,000, 9,000, 12,000, 18,000, 24,000 and occasionally 36,000 BTU per hour; the 6k is the smallest that most manufacturers make, and some multi-zone outdoor units cannot take it, starting at 9k.
The 12k-head trap
The commonest mistake with multi-zone systems is to put a 12k head in every bedroom because 12k is the size most often shown in adverts. A 150 sq ft bedroom in a mixed climate needs about 2,600 BTU per hour; even the 6k head is more than twice that. A 12k head in that room will run at its lowest output and still cycle on and off, never long enough to dehumidify, which is the room-sized version of the problem described in the guide to oversized systems.
Inverter heads modulate, typically down to about a third of their rated output, which helps but does not fix a head that is four times too big. Where a room’s load is below what the smallest head can modulate to, the alternatives are to accept some cycling with a 6k head, to serve two small rooms from one ducted mini-split air handler, or to leave a door open to a neighbouring zone.
Step two: size the outdoor unit
The outdoor unit has to supply the heads, but not all of them at full output at once: on the hottest afternoon the bedrooms are empty and the living room is busy, and in the evening the reverse. Manufacturers allow the heads’ combined rating to exceed the outdoor unit’s, often to 130% or more, and the calculator applies a diversity factor of 0.9 to the heads’ total before choosing the outdoor unit.
A living room and a bedroom in climate zone 4
A 320 sq ft living room with west windows and a 180 sq ft bedroom, both with 8 ft ceilings.
- Living room: 320 × 18 × 1.1 sun = 6,336 BTU/hr, so a 9k head
- Bedroom: 180 × 18 × 0.95 use = 3,078 BTU/hr, so a 6k head
- Heads total: 15,000 BTU/hr; with 0.9 diversity, 13,500 BTU/hr
- Outdoor unit: 18k, with the heads at 83% of its capacity
The bedroom head will modulate to its floor and cycle a little on mild days, which is acceptable. The two rooms’ actual load is only 9,414 BTU per hour, so the system has plenty in hand.
In a hot climate with every room in use at once, the outdoor unit should cover the heads at full output. The calculator’s second example, three rooms in zone 2 with a kitchen, needs 9k, 9k and 6k heads, 24,000 BTU per hour in total, and a 24k outdoor unit at 100% connected load.
Planning the zones
Zones on a mini-split are rooms, not floors. Rooms used at the same time, open to one another, can share one head; rooms used differently should have their own. The HVAC zoning planner groups rooms into zones and splits a system’s airflow between them, which is useful for a ducted system with zone dampers and for deciding which rooms deserve their own mini-split head.
Heating with a mini-split
In heating-dominated climates, size the heads and outdoor unit for the heating load as well. A head sized for summer cooling will usually be big enough for heating in climate zones 1 to 4. North of that, the heating load is larger and the outdoor unit loses capacity as the temperature falls, so a cold-climate model matters; the heat pump size calculator and the guide to balance point and backup heat cover the arithmetic.
What multi-zone systems cost
Installed cost rises with each zone, but not in proportion, because the outdoor unit, the electrical circuit and the labour of setting up are shared. The mini-split installation cost calculator uses a first-zone range of $3,000 to $6,000 and $2,000 to $4,000 for each additional zone, with adjustments for head type, line-set length and a new circuit.
| System | Range | Midpoint | Per zone |
|---|---|---|---|
| Two 12k wall heads, new circuit | $5,400 to $11,500 | $8,450 | $4,225 |
| Three 9k cold-climate cassettes, 30 ft extra line set, new circuit | $9,900 to $20,550 | $15,225 | $5,075 |
Ceiling cassettes and cold-climate outdoor units both raise the price; wall heads are the cheapest and easiest to service. Two single-zone systems instead of one dual-zone system cost a few hundred dollars more, run a little more efficiently, and keep one room cooling if the other system fails. The guide to what HVAC installations cost compares mini-splits with ducted systems.
Compared with a room air conditioner
For a single room, the comparison is with a window unit, which the guide to window and portable AC sizing covers. A mini-split head costs several times as much to put in but runs at roughly twice the efficiency, is far quieter, heats in winter, and leaves the window free. For a room used every day of the summer, the running cost difference narrows the gap over the years.