1. The system and the house
The simulation models a 16 kW ducted reverse-cycle system in a single-storey home with six zones. Each room is given a weighting for how much heat it picks up on a hot afternoon. Big rooms with more glass and afternoon sun pick up more, so they cost more to cool.
| Room | Heat-gain weight | Share of cooling | Outlets | Sensor |
|---|---|---|---|---|
| Whole house | 100% | 10 |
What the weightings mean
Heat-gain weight is how much heat a room picks up compared with an average room. The Family room (1.35) picks up 50% more heat than a small bedroom (0.90), so leaving it off saves the most.
Outlets decide how the airflow is split between the rooms that are on. Living areas have two outlets, the kids' bedrooms one.
Sensors. Rooms with a temperature sensor hold their own setting. Rooms without one are set by airflow percentage instead.
2. The running-cost formula
Cooling the whole house is the baseline. A zoned home only pays for the share of the house that's switched on.
Setting a room warmer or cooler
The whole-house baseline holds every room at 22°C. Setting a room warmer (in cooling) reduces how much that room costs to cool, and setting it cooler adds to it. How much depends on the weather: on a 34°C afternoon each degree is worth about 4% of that room's cooling, close to the common rule of thumb. On a mild 27°C evening the gap to outside is much smaller, so each degree counts for more, about 12% of that room's cooling.
Worked example: summer afternoon
- Lounge, Dining and Family are on at 22°C. Their heat-gain weights add up to 1.25 + 1.00 + 1.35 = 3.60.
- The whole house adds up to 6.35, so the zoned home is cooling 3.60 ÷ 6.35 = 57% of the house.
- Zoned running cost is $1.00 × 57% = $0.57/hr, about 43% less.
- Over a summer: 43% × $1.00 × 6 hours × 90 days ≈ $230. Over 15 summers, about $3,450.
Worked example: bedtime
At 10pm (27°C outside) only the three bedrooms are on. Their weights add up to 0.95 + 0.90 + 0.90 = 2.75 of 6.35, which is 43% of the house, or about 57% less. The main bedroom is set one degree warmer at 23°C, and on a mild night that degree takes off another 1–2%, so the simulation shows about $0.41/hr, roughly 58% less than cooling every room.
3. What you see moving in the simulation
- Room temperatures. Each room warms towards the outside temperature and cools when it gets air. Rooms with more heat gain warm up faster than others.
- Airflow. Air is shared between the rooms that are on, by their number of outlets. Once a room reaches its setting, its damper closes down so the air goes where it's still needed.
- The unit rests. When every room that's on reaches its temperature, the unit switches itself off until one drifts 1°C away, then starts again.
- Spill air. If only one or two small rooms are on, a ducted unit can't push all its air into them, so some is spilled into the hallway to protect the system. The simulation shows this as the hallway spill zone.
- Compressor gauge. Shows how hard the unit is working right now: full output while rooms are cooling down, easing off as they reach temperature, and zero while it rests.
- "Right now" cost is the instant cost at the current compressor output (5.2 kW × output × 32c). The headline costs use the 60% average instead, because a real system spends most of its time part-loaded.
4. Try your own numbers
Switch rooms on and off, change their settings, or use your own tariff and hours. This uses the same formula as the simulation on a 34°C summer afternoon.
Rooms
5. Where the assumptions come from
- 16 kW system, 5.2 kW electrical. A common size for a three-to-four bedroom Melbourne home. At full output a modern inverter ducted unit draws roughly a third of its cooling capacity in electricity.
- 60% average load. Inverter systems spend most of the time well below full output once the house is cool, so we average 60%.
- 32c per kWh. A middle-of-the-road Victorian usage rate. Check your bill and use your own above.
- 6 hours a day, 90 days. A family running cooling on summer afternoons and evenings, December to February.
- Room weightings. Set from typical room sizes, window areas and which way they face, as we would on a heat-load check for a real home.
- 22°C baseline. A common comfortable cooling setting.
6. What it doesn't do
- It's an illustration with a simplified house, not a heat-load calculation for your home. The app on screen is an example of how smart zoning apps generally work, not a specific product.
- Heat moving between rooms through walls and doors is simplified, so real savings with doors open can be a little lower.
- It assumes the whole-house home runs the same hours. Many zoned homes also run fewer hours, which saves more.
- Your result depends on your insulation, windows, ductwork, tariff and habits. We work it out properly at a free in-home visit.