Zone-by-Zone Sizing
Mini-split sizing: what most installers get wrong
Mini-split sizing follows the same load calculation foundation as any other HVAC system. The difference is that each indoor head serves a specific zone, so you need a load calculation for each zone separately. The common mistake is sizing by room square footage, which produces wildly oversized systems in well-insulated homes.
Single-zone vs. multi-zone
A single-zone system is a dedicated outdoor unit for one indoor head. It's more efficient and simpler than multi-zone. Use it when you need to condition just one area. Multi-zone systems (2-8 heads on one outdoor unit) reduce installation cost but are less efficient because the outdoor unit must run whenever any head calls. They're best when multiple zones will run simultaneously most of the time.
Mini-splits and humidity
Mini-split indoor heads have smaller coils running at higher evaporator temperatures than central ducted systems. This gives them SHR values of 0.85-0.92 — they're poor dehumidifiers. In humid Canadian summers (Ontario, Quebec, Atlantic), a mini-split that meets the cooling load may still leave the space too humid. Use the SHR calculator to check. Consider a dedicated dehumidifier in humid climates.
Cold-climate sizing
For heating-primary applications, size to the heating load at the design outdoor temperature — not the 47°F (8°C) rated capacity. See the heat pump sizing calculator for detailed cold-climate analysis.
Frequently Asked Questions
Probably 9,000 BTU/hr (0.75 ton) to 12,000 BTU/hr (1 ton) — but you can't know without calculating the actual load. A 500 ft² bedroom with good insulation, one window, and no west exposure might need 6,000 BTU/hr. A 500 ft² open-concept office space with large windows facing west could need 18,000 BTU/hr. Use the room load calculator to get the real number for your specific space.
Most residential multi-zone systems support 2-5 indoor heads. Some commercial-grade systems go up to 8 zones. The outdoor unit capacity must meet the sum of all indoor head capacities at design conditions. In practice, manufacturers allow some oversizing of the outdoor unit (typically 100-130% of the sum of indoor unit capacities) to account for diversity — not all zones calling simultaneously. Always size to the manufacturer's multi-zone matching tables, not just total BTU arithmetic.