Understanding your result
The headline is the number of zones suggested, with the system airflow and the number of rooms in the sub-line. The bars show each zone’s airflow. The table lists every zone with its rooms, area, airflow and share of the system; the notes flag zones too small for a single-speed system, advise on three or more zones, and explain why upper floors behave differently.
The split rule is deliberately simple: rooms are grouped by floor, then a floor of 800 sq ft or more with both daytime rooms and bedrooms is split into a day zone and a night zone if you leave the option ticked, and a large sunroom or bonus room becomes its own zone. Untick the option to see the floor-only plan.
The airflow comes from the tonnage at 400 CFM per ton, the figure the central AC size calculator also uses, divided by the same weighting of area and exposure as the CFM per room calculator, with an extra weight for upper floors. Each zone’s ducts are then sized with the duct size calculator; for a small zone that fails the airflow check, the mini-split size calculator sizes a head instead.
How we calculate this
The grouping rules and weights are our planning simplification of the zoning practice described in ACCA Manual Zr and in manufacturers’ zoning guides, set out here so they can be checked. The floor factors reflect that upper floors carry a larger cooling load per square foot, and below-grade basements a smaller one; the 25% threshold reflects the common guidance that the smallest zone should be able to accept a meaningful share of the system’s airflow without a bypass. The duct sizes behind each zone are on the duct size chart.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Airflow per ton | 400 CFM | Residential design practice |
| Exposure factors | 0.8 / 1.0 / 1.2 / 1.4 | Our judgement, consistent with the CFM per room calculator |
| Floor factors | 1.1 upper, 1.0 main, 0.6 basement | Our judgement of typical cooling load differences |
| Day / night split | Floors of 800 sq ft or more | Our planning rule |
| Smallest zone warning | Under 25% of system airflow | Common zoning guidance (25 to 35% minimum for single-speed systems) |
Assumptions last reviewed October 8, 2026.
The planner does not design ducts, size dampers, model bypass air, or calculate room loads; it groups rooms and apportions air so the zoning conversation with a contractor starts from a reasonable layout. The guide to mini-splits, zoning and sizing compares duct zones with ductless heads, and the guide on return air explains why each zone also needs a path back to the air handler.
Two worked examples
A two-storey house on a 3-ton system
Main floor: 350 sq ft living room, 220 sq ft sunny kitchen and family room, 150 sq ft office. Upper floor: a 200 sq ft sunny main bedroom and three bedrooms of 150, 140 and 120 sq ft.
- System airflow: 3 × 400 = 1,200 CFM
- Zone 1, main floor: rooms 1 to 3, 720 sq ft, weight 764 → 620 CFM (52%)
- Zone 2, upper floor: rooms 4 to 7, 610 sq ft, weight 715 → 580 CFM (48%)
The classic two-zone layout, with the upstairs getting almost half of the air for less than half of the floor area because of the floor and exposure factors. Neither zone is under 25%, so a two-stage system handles it without a bypass.
A single-storey house split by use
One floor of 1,000 sq ft on a 2-ton system: a 500 sq ft living area, and bedrooms of 300 and 200 sq ft.
- System airflow: 2 × 400 = 800 CFM
- Zone 1, day rooms: 500 sq ft → 377 CFM (47%)
- Zone 2, night rooms: 500 sq ft → 423 CFM (53%)
The bedrooms get a little more than half of the air because the larger one is sunny. With the split option off, this house is one zone; whether two are worth it depends on how differently the rooms are used, and a programmable thermostat on a single zone is often enough.
Where to find your inputs
System size. From the outdoor unit’s model number, or the central AC size calculator.
Rooms. Every room the system serves; combine small connected spaces such as a hall and a stair.
Floor and use. Which level the room is on, and whether it is mainly used by day or at night.
Exposure. Sunny or corner for rooms with large west or south windows or two outside walls; sunroom for rooms with glass on several sides or over a garage.
Common mistakes
- Too many zones on a single-speed system. Small zones get blasted with air; use variable-speed equipment or fewer zones.
- Zoning instead of fixing ducts. A starved room may need a bigger duct or a return, not a zone.
- No return path from each zone. Closed doors and zone dampers need returns or transfer grilles.
- Bypass ducts. They waste energy and are discouraged; size the zones and equipment instead.
- One thermostat for a two-storey house. Upstairs comfort needs its own control.
- Skipping the design. Zoned duct systems need Manual Zr; a planner is a starting point.
Questions people ask
- How many HVAC zones does a house need?
- Usually one per floor, and sometimes a separate zone for a room that behaves very differently, such as a sunroom, a bonus room over a garage or a large daytime living area on the same floor as bedrooms. A single-storey house of ordinary size often needs only one zone with well-balanced ducts. The planner groups rooms by floor, then splits a floor of 800 sq ft or more into day and night rooms if you ask it to.
- Why is the upstairs always hotter?
- Heat rises, the roof above an upper floor gains heat all summer, and the ducts serving it are usually the longest and often run through a hot attic. A single thermostat downstairs stops cooling while the upstairs is still warm. A separate upstairs zone with its own thermostat, a separate upstairs system, or a mini-split head upstairs are the usual fixes; balancing dampers set once help less, because the imbalance changes with the season.
- What is the minimum size for a zone?
- With a single-speed system, a zone that calls on its own receives the whole system's airflow, so a small zone gets blasted with air, the duct velocity rises, and the coil can freeze. A rule of thumb is that no zone should carry less than about 25 to 35% of the system's airflow unless the equipment is variable speed or multi-stage, or a bypass is fitted. The planner warns below 25%.
- Is zoning better than two separate systems?
- Two systems give each floor its own equipment, run independently and keep the house partly comfortable if one fails, but cost more. A zoned single system costs less, uses one set of equipment and works well with a variable-speed or two-stage unit. For a new installation in a two-storey house of more than about 2,500 sq ft, two systems are common; for a retrofit, zoning the existing system is usually cheaper.
- Can mini-splits be used instead of duct zones?
- Yes, and they are often the simplest answer for a single problem room, a sunroom, an addition or a finished attic. Each head is its own zone with its own controls, and there are no dampers to balance. For whole-house zoning, a ducted system with a mini-split for the odd room out is a common mix.
- What equipment does a zoned system need?
- Motorised dampers in the ducts for each zone, a zone control panel, a thermostat in each zone, and usually a variable-speed or multi-stage outdoor unit and air handler so the airflow can match the zones calling. Older single-speed systems used a bypass duct to dump excess air, which wastes energy and is discouraged today. The ducts must be designed for zoning (ACCA Manual Zr).