| Rev | Change | Reason |
|---|---|---|
| A | First issue | installers quote what they stock |
| B | Added cold-climate derating clause | label sizes lie at -15°C |
| C | Appendix A interactive | readers kept doing the math wrong |
This bulletin covers single-room mini-split sizing for planning and quote-checking. The right unit is a number derived from the room, not a brand preference. Oversized units short-cycle and dehumidify badly; undersized units run flat out and die early; right-sized units run quiet for fifteen years.
The load estimate follows from floor area, ceiling volume, envelope quality, solar gain, and climate duty:
Round the result UP to the next commercial size: 9k, 12k, 18k, 24k, 30k, 36k BTU/h. Step up one further size only when the unit also serves an adjoining open space.
| Shape | Fits | Spec reality | Failure mode to avoid |
|---|---|---|---|
| Single-zone wall unit | One room, the classic | 9k-18k · SEER2 20+ | Oversizing. The most common and most avoidable mistake. |
| Multi-zone, 2-4 heads | Bedrooms + living | 18k-36k combined | Tiny rooms on big compressors short-cycle; each head has a minimum load. |
| Cold-climate heat pump | Primary heating | output rated AT -15°C | Buying on label size. The derating curve at your design temp is the purchase. |