BTUFit › Multi-zone mini split
Multi-zone mini split calculator
One outdoor condenser, several indoor heads, one per room that closes off. Add your rooms below to size each head and count the zones. A three-room setup (a 300 sq ft bedroom, a 500 sq ft living/kitchen, a 150 sq ft office) comes to 27,000 BTU of nominal head capacity across 3 heads.
Your zones
| Zone | Room (sq ft) | Load (BTU/h) | Head size |
|---|
Why the condenser is not the sum of the heads
Add up the heads above and you get the nominal capacity, not the outdoor unit you buy. Multi-zone condensers are rated below the total of their connected heads, because the rooms almost never peak together: the west bedroom loads up in the evening while the east one has cooled off. Manufacturers spell out two limits per model, a maximum connected capacity (often around 130% of the condenser's nominal rating) and a maximum zone count. A three-head setup that sums to 27,000 BTU of heads typically runs on a smaller condenser than that figure, and the installer picks it from the manufacturer's pairing table, not from a spreadsheet.
One head per closed room
A head cools and heats the air it can reach. A shut door stops it cold, so plan a head for every space that closes off and one shared head for anything genuinely open. Count doorways, not rooms: a living/dining/kitchen that flows together is one zone; a bonus room over the garage is its own.
Worked example: three zones
| Zone | Room (sq ft) | Load (BTU/h) | Head |
|---|---|---|---|
| Bedroom | 300 | 7,000 | 9,000 BTU |
| Living / kitchen | 500 | 12,000 | 12,000 BTU |
| Home office | 150 | 5,000 | 6,000 BTU |
| Combined heads | 27,000 BTU |
Each head is sized the same way the single-head calculator does it: square footage through the ENERGY STAR brackets, adjusted for sun and heating duty, rounded up to the next head. The heating-duty setting adds 15% for mild winters and 40% for cold ones, a stand-in for sizing to the winter load. For primary heat in a cold climate, check each model's rated capacity at 5°F, not 47°F.
Where DIY stops
Single-zone pre-charged kits are a weekend project. Multi-zone is not. The line sets are cut and flared on site, pressure-tested, vacuumed and charged, and the 230V circuit is a licensed electrician's work. Treat everything below as a budgeting number and a way to walk into quotes already knowing your zone count, then let a pro run the Manual J and match the condenser.
Frequently asked questions
How many rooms can one mini split condenser handle?
Two to five heads on most residential outdoor units, and up to about eight on the largest models. Each head is one indoor unit serving one open space, so count the rooms that sit behind a door: a bedroom, an office and a living room that all close off need three heads, not one big one.
Is the condenser size the sum of the head sizes?
No, and this is the number people get wrong. Multi-zone outdoor units are rated below the total of the heads bolted to them, because the rooms rarely hit peak load at the same moment. Manufacturers publish a maximum connected capacity (often around 130% of the condenser's nominal rating) and a maximum zone count per model. Matching heads to a condenser inside those limits is the installer's job.
Can I install a multi-zone mini split myself?
No. DIY territory stops at pre-charged single-zone kits with sealed line sets. A multi-zone system has field-cut line sets that must be flared, pressure-tested, vacuumed and charged, plus 230V wiring. That is licensed HVAC and electrical work; the warranty usually requires it too.
Do I need a head in every room?
Every room that closes off, yes. One head only conditions the air it can reach, so a shut bedroom door gets nothing. Open-plan areas (a living/dining/kitchen that flows together) share a single head sized to the combined square footage.