Formula
Each room uses the same estimate as the room BTU page: the ENERGY STAR chart, times ceiling and insulation, times the sun factor, plus 600 BTU/h per person past two, plus 4,000 for a kitchen. The window-unit shopping round-down is not applied. A head rounds up to 6,000, 9,000, 12,000, 15,000, 18,000, or 24,000. Past 24,000 the head stays at 24,000 and is flagged, because one head does not cover it.
outdoor = the unrounded sum, rounded up to 9 / 12 / 18 / 24 / 30 / 36 / 42 / 48 thousand
connected ratio = sum of heads ÷ outdoor size
Sizing the outdoor unit to the sum of the rounded heads would stack every round-up. The ratio is shown against a 50–130% band that shows up on multi-zone submittals. It is not a code limit, and your manufacturer’s range wins. This is not a Manual J. Get a load calculation before you buy the equipment.
Worked examples
400 ft² and 200 ft², 8 ft, average, two people each, no kitchen. The rooms estimate 9,000 and 6,000, so the heads stay there. The outdoor unit rounds the 15,000 sum up to 18,000. Connected ratio is 15,000 / 18,000 = 83%, inside a typical 50–130% band.
One 400 ft² sunny kitchen with four people and a 10 ft ceiling. The room estimate is 17,575 BTU/h, so the head rounds up to 18,000. That is the opposite of the window-unit round-down on the BTU page, which would shop 17,000.
Reference
Catalog steps used for the round-up. They are common nominal sizes, not a single manufacturer’s list.
| Heads | Outdoor units |
|---|---|
| 6, 9, 12, 15, 18, 24 thousand BTU/h | 9, 12, 18, 24, 30, 36, 42, 48 thousand BTU/h |
Questions
Why do the heads round up when a window unit rounds down?
ENERGY STAR tells you to round a window-unit total down to the nearest 1,000. Mini-split heads come in catalog steps, and a head smaller than the room estimate cannot meet it. This page rounds each head up to 6, 9, 12, 15, 18, or 24 thousand BTU/h, and it says that is the opposite direction from the window-unit rule.
What is the outdoor unit sized to?
The sum of the unrounded room estimates, then up to 9, 12, 18, 24, 30, 36, 42, or 48 thousand. It is not the sum of the rounded heads. Adding the oversized heads double-counts the rounding.
What is the 50 to 130 percent band?
Many multi-zone outdoor units want the connected heads between about half and 130 percent of the outdoor rating. That is a manufacturer pattern, not a code rule. The page shows the ratio. It does not claim your model allows it.
Sources
- Room loads reuse the ENERGY STAR chart and adjustments. See the BTU calculator.
- Head and outdoor steps are nominal catalog sizes. The 50–130% connected-capacity band is a manufacturer pattern, labeled as such, not a standard.
- Not an ACCA Manual J. Get a load calculation before you buy equipment.