Multi-Zone Mini-Split System Builder
Work out each room’s heating and cooling load, pick an indoor-unit size for every zone, size the outdoor unit — and check a specific outdoor unit against the limits in its manufacturer’s data.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Total cooling load
- 7,341BTU/h
- Total heating load at design
- 10,139BTU/h
- Smallest common outdoor size covering the cooling load
- 18,000BTU/h
3 zones: 7,341 BTU/h cooling and 10,139 BTU/h heating at design; indoor units total 18,000 BTU/h, outdoor unit about 18,000 BTU/h.
- Which indoor units an outdoor unit accepts, and in what combination, is set by the manufacturer. This tool checks only the limits you enter from its documents; it does not judge compatibility.
- Total of the indoor-unit sizes
- 18,000BTU/h
Cooling load by zone
- Zone 1
- 4,036BTU/h
- Zone 2
- 1,652BTU/h
- Zone 3
- 1,652BTU/h
Heating load by zone
- Zone 1
- 5,664BTU/h
- Zone 2
- 2,237BTU/h
- Zone 3
- 2,237BTU/h
Indoor unit size (nominal cooling)
- Zone 1
- 6,000BTU/h
- Zone 2
- 6,000BTU/h
- Zone 3
- 6,000BTU/h
Show the working
- Design conditions16 °F heating, 91 °F cooling; walls R-11, ceiling R-30, windows U-0.49; no duct losses (ductless)
- Zone 1300 ft² → cooling 4,036, heating 5,664 BTU/h → 6,000 BTU/h indoor unit
- Zone 2150 ft² → cooling 1,652, heating 2,237 BTU/h → 6,000 BTU/h indoor unit
- Zone 3150 ft² → cooling 1,652, heating 2,237 BTU/h → 6,000 BTU/h indoor unit
- Totals (no diversity applied: every room at its design load at once)cooling 7,341 BTU/h, heating 10,139 BTU/h, indoor units 18,000 BTU/h
How the builder works
Each zone’s load comes from the same room-by-room method as our load estimator: walls, windows, ceiling, floor, air leakage, people and kitchen gains at your design temperatures. Ductless indoor units have no duct losses, so none are added.
Each zone then gets the next common indoor-unit size above its cooling load, and the zone loads are added up for the outdoor unit. No diversity is applied: the total assumes every room is at its design load at the same time, which is the cautious assumption for a single outdoor unit.
Checking a specific outdoor unit
If you have an outdoor unit in mind, enter four numbers from its manufacturer’s data and the builder checks them against your rooms:
- Connected capacity: the indoor units’ nominal sizes added up, as a percentage of the outdoor unit. Every manufacturer limits this in its combination tables.
- Cooling sizing: the outdoor unit’s capacity against the total load, compared with the 90–130 % range the ENERGY STAR design guidance gives for variable-speed equipment.
- Heating at your design temperature: what the outdoor unit can deliver in the cold, from its low-temperature performance data, against the heating load. Any shortfall is backup heat you need to plan for.
- Minimum capacity: if the outdoor unit cannot turn down below the smallest zone’s load, that zone alone will make it cycle on and off.
What the builder does not do
It does not decide which indoor and outdoor units work together — only the manufacturer’s documents can. It does not check line-set lengths, piping limits or electrical requirements. To see real certified combinations, use the link to the ENERGY STAR heat pump comparison, which lists mini- and multi-split systems as they were certified.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Climate
- Zone 4 — mixed (e.g. Baltimore)
- Insulation & air sealing
- Average (code-built 1990s–2000s)
- Window type
- Double pane, clear
- Ceiling height
- 8 ft
- Number of zones (indoor units)
- 3 zones
- Zone 1: floor area
- 300 ft²
- Zone 1: exterior wall length
- 35 ft
- Zone 1: window area
- 45 ft²
- Zone 1: roof or attic above?
- Yes (top floor)
- Zone 1: floor
- Slab on grade
- Zone 1: people
- 2
- Zone 1: kitchen appliances?
- No
- Zone 2: floor area
- 150 ft²
- Zone 2: exterior wall length
- 12 ft
- Zone 2: window area
- 15 ft²
- Zone 2: roof or attic above?
- Yes (top floor)
- Zone 2: floor
- Slab on grade
- Zone 2: people
- 1
- Zone 2: kitchen appliances?
- No
- Zone 3: floor area
- 150 ft²
- Zone 3: exterior wall length
- 12 ft
- Zone 3: window area
- 15 ft²
- Zone 3: roof or attic above?
- Yes (top floor)
- Zone 3: floor
- Slab on grade
- Zone 3: people
- 1
- Zone 3: kitchen appliances?
- No
- Check a specific outdoor unit?
- No — just size the system
Result
- Total cooling load
- 7,341 BTU/h
- Total heating load at design
- 10,139 BTU/h
- Smallest common outdoor size covering the cooling load
- 18,000 BTU/h
- Total of the indoor-unit sizes
- 18,000 BTU/h
3 zones: 7,341 BTU/h cooling and 10,139 BTU/h heating at design; indoor units total 18,000 BTU/h, outdoor unit about 18,000 BTU/h.
Assumptions
- Each zone’s load uses the same component method and constants as our Room-by-Room Load Estimator, without duct losses (ductless indoor units).
- Indoor units are the next generic size up from each zone’s cooling load (6k, 9k, 12k, 15k, 18k, 24k BTU/h); outdoor sizes 18k, 24k, 30k, 36k, 42k, 48k BTU/h. No diversity is applied.
- Cooling sizing is compared with the variable-speed range in the ENERGY STAR National HVAC Design Report (from ACCA Manual S): 90–130 % of the load.
Limitations
- Compatibility, allowed combinations, piping limits and minimum indoor-unit sizes come from the manufacturer. The checks here use only the numbers you enter from its documents.
- An estimate, not a Manual J. Window orientation, shading and heat moving between rooms are simplified.
Frequently asked questions
Can I connect more indoor capacity than the outdoor unit’s size?
Only if the manufacturer allows it. Each manufacturer sets the maximum total indoor capacity for an outdoor unit in its combination tables — sometimes above the outdoor unit’s own rating. Enter that limit in the builder to check your plan.
Why is the indoor unit bigger than the room needs?
Indoor units come in steps (6,000, 9,000, 12,000 BTU/h and so on), so each zone gets the next size up. A head much larger than a small room needs can cycle often; some manufacturers offer smaller heads or ducted units that serve two rooms.
Does cooling or heating decide the size?
In cold climates heating usually decides. The builder shows both, and the outdoor-unit check compares heating capacity at your design temperature with the heating load.
Sources
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
- National HVAC Design Report, Version 3/3.1/3.2 (Rev 13, historical) — ENERGY STAR (U.S. EPA). Equipment sizing limits from ACCA Manual S: cooling 90–115 % (single-speed), 90–120 % (two-speed), 90–130 % (variable-speed) recommended; furnace 100–140 % recommended.
- Guide to Sizing & Selecting Air-Source Heat Pumps in Cold Climates (rev. 04/08/20) — NEEP (hosted by U.S. DOE Building Science Education Solution Center). Size from the manufacturer’s extended performance (maximum capacity) data; ccASHP maximum capacities at 5 °F can corroborate it; avoid outdoor units whose minimum capacity exceeds the smallest indoor unit’s need.
- ENERGY STAR Certified Heat Pumps (dataset 83eb-xbyy) — U.S. EPA (public domain). Certified systems with SEER2, EER2, HSPF2, heating capacity at 47/17/5 °F and COP at 5 °F; 47 °F and 5 °F values come from different tests.
Spotted an error? Report a problem with this calculator — see our corrections policy.