Coincident Cooling Loads: Why the Block Peak Is Not the Sum of Zone Peaks
Choose the equipment boundary before choosing the peak
A load report can legitimately contain several different cooling maxima. One room peaks in the morning, another in the afternoon, and the central coil may peak when outdoor-air treatment is most demanding. Choosing the largest number in the report does not resolve which number belongs on an equipment schedule.
The decision is whether a shared component must meet simultaneous demand or whether an individual component must meet its own local maximum. Keep the room terminal, central air handler and cooling plant as separate calculation boundaries. Their capacities cannot be interchanged merely because every result is expressed in kilowatts.
Sum each hour first, then find the maximum
For zone i at time t, call the cooling load Qi(t). The sum of zone peaks is Σ max[Qi(t)]. The coincident block peak is max[Σ Qi(t)]. These operations occur in the opposite order. ASHRAE’s nonresidential load chapter explains that central equipment serving several spaces is evaluated using simultaneous loads, with system effects considered in addition to room loads.
For identical zones, time intervals and load boundaries, the block peak cannot exceed the sum of individual peaks. If it does, investigate inconsistent time ranges, missing zones, multipliers or added system loads. The comparison only works when both quantities represent the same physical scope.
Do not introduce an unexplained diversity multiplier after the hourly profiles have already produced a coincident peak. That would count the same timing benefit twice. If additional occupancy scenarios are justified, represent them explicitly in the inputs and retain a clear record of which scenario governs.
Worked example: three zones and four displayed hours
Consider an illustrative variable-air-volume system serving east, west and core zones. The following values are already calculated zone total cooling loads, including each zone’s sensible and latent components. They exclude centrally treated outdoor air and fan heat. They are invented teaching inputs, not a weather simulation or design allowance.
At 09:00, east/west/core loads are 42/12/20 kW, giving 74 kW. At 12:00 they are 34/24/26 kW, giving 84 kW. At 15:00 they are 22/46/28 kW, giving 96 kW. At 18:00 they are 12/40/18 kW, giving 70 kW.
The separate zone maxima within this sample are 42, 46 and 28 kW. Their sum is 116 kW. The largest simultaneous total is 96 kW at 15:00. The coincidence ratio, defined here as block divided by sum of peaks, is 96/116 = 0.828. The reduction relative to the summed peaks is 17.2%. Definitions of “diversity factor” vary, so state the equation.
This four-point illustration teaches the arithmetic. A real calculation must examine the relevant hours and design days, including seasonal solar exposure and operating schedules. It cannot establish a project peak from four selected snapshots. Neither 0.828 nor 17.2% is a default factor for another building.
The coil can peak at a different hour
Now extend the same example to a simple central-coil balance. Assume no heat recovery, reheat, duct heat gain or leakage, and a constant 3 kW fan-heat contribution that the cooling coil must offset. Assume incremental outdoor-air treatment loads of 9, 27, 11 and 5 kW at the four respective hours. These outdoor-air values are also illustrative inputs and are not already counted in the zone loads.
The resulting coil duties are 74 + 9 + 3 = 86 kW at 09:00; 84 + 27 + 3 = 114 kW at 12:00; 96 + 11 + 3 = 110 kW at 15:00; and 70 + 5 + 3 = 78 kW at 18:00. The zone block peaks at 15:00, while the coil peaks at 12:00 in the displayed sample.
Trane’s checksums documentation distinguishes coil-peak and fan-peak reports and associates each with its corresponding time and outdoor conditions. The practical lesson is to add the components at the same hour. Adding maximum zone load to independently maximum outdoor-air load would produce 96 + 27 + 3 = 126 kW, a condition absent from this example.
For actual coil selection, use the full psychrometric process and the manufacturer’s performance at the selected entering air, leaving air, airflow and fluid conditions. The simplified arithmetic above demonstrates a boundary reconciliation; it does not calculate dehumidification performance or select a coil.
Original four-hour teaching example. Zone and coil peaks occur at different times; a real design requires the complete relevant time series.
Keep local capacity and airflow requirements visible
Trane’s block-versus-peak sizing explanation relates block airflow sizing to systems able to vary airflow, while sum-of-peaks methods commonly apply to constant-volume configurations. Confirm the software’s selected system model. A central block result alone does not establish the airflow each room needs or the installed terminal sizes.
The east terminal still needs to satisfy its own governing condition even though the central zone total peaks later. Do not multiply every terminal’s duty by the building coincidence ratio. Similarly, total cooling load cannot be inserted into a sensible-only airflow equation: separate sensible duty, supply condition and ventilation constraints before calculating air quantity.
At plant level, aggregate the actual system cooling demands at matching times and include the applicable distribution and equipment effects. Do not substitute summed catalog capacities for coincident calculated duties. Then document capacity corrections, operating limits, staging and any separately required redundancy. A standby requirement is a different decision from load coincidence.
Leave a review trail that explains the selected duty
Carrier’s HAP reporting guide lists separate zone-sizing, air-system-sizing, hourly load and psychrometric reports. Export the relevant reports together rather than circulating a single capacity figure. Record the governing timestamp, included zones, schedule assumptions, system additions and software sizing settings.
As a final check, reconstruct the selected peak from its components and compare several nearby hours. Investigate a change in governing time after a zoning or ventilation revision. Retain both the coincident central result and the individual terminal requirements. This makes the equipment schedule defensible without hiding the timing assumptions that produced it.
Sources and further reading
Continue your engineering workflow
The paid 157-page Cooling Load Design Handbook PDF covers room and coincident building peaks, airflow, coil duty and equipment selection, with three complete worked projects.
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