Minimum Chilled-Water System Volume and Buffer-Tank Sizing to Prevent Short Cycling
A chiller can meet the peak cooling load yet cycle repeatedly on a mild day. When its lowest stable cooling output exceeds the building load, the water cools toward the stop threshold. Too little active water volume makes that happen quickly. A buffer tank adds thermal capacity, but its usable volume and piping arrangement determine whether it helps.
Start with the chiller requirements
Obtain the selected unit’s minimum loop volume, minimum flow, allowable flow-change rate and compressor cycling limits. Check the operating stage and fluid concentration. Requirements vary by equipment and application; a litres-per-kilowatt rule cannot replace the manufacturer’s instructions. Trane’s variable-primary-flow guide explicitly requires manufacturer-specific loop time and volume checks.
Count only water that remains thermally connected during the limiting operating condition. Include the active evaporator, pipework and coils; exclude isolated branches and standby chillers behind closed valves. In a decoupled plant, the building’s total water inventory may not be available to the operating chiller.
Calculate the thermal volume
For a preliminary, well-mixed energy balance at constant low load:
Vthermal = max(0, Qc,min − Qload) × t / (ρ × cp × ΔTallow)
Vthermal is active volume in m³; Qc,min is the relevant minimum cooling output in kW; Qload is simultaneous cooling load in kW; t is desired on-time in seconds; ρ is fluid density in kg/m³; cp is specific heat in kJ/(kg·K). ΔTallow is the permitted bulk-water temperature fall during the cycle, in K. It is the usable control swing, not the design coil supply-to-return temperature difference.
This is the SI energy-balance form of Caleffi’s buffer-sizing method. Use fluid properties at operating temperature and concentration. The simple model assumes effective mixing and steady output; startup capacity, control delays and rapidly changing loads need separate review.
Worked example: 718 litres of active water
Assume 30 kW minimum cooling output, a steady 10 kW load, a 300 s on-time and a permitted 2 K fall. These are illustrative inputs, not recommended settings. For water, take ρ = 1000 kg/m³ and cp = 4.18 kJ/(kg·K).
Vthermal = (30 − 10) × 300 / (1000 × 4.18 × 2) = 0.718 m³ = 718 L
If the manufacturer requires no greater volume and 250 L is genuinely active, the shortfall is 718 − 250 = 468 L of additional effective volume. Select the actual vessel after checking usable volume, mixing and connections. A nominal 500 L tank is not automatically sufficient. If the manufacturer’s requirement is higher, use that larger requirement.
Conceptual low-load volume check. Confirm the active flow path, effective tank volume and manufacturer limits.
Make the volume work in the circuit
Locate the tank so low-load or bypass operation still uses it. A series return-line tank on the chiller side of the bypass is one common arrangement; confirm the layout with the manufacturer. Avoid connections that short-circuit the vessel. Check pressure rating, pressure loss, air removal, insulation and vapour sealing. Recheck expansion-vessel sizing when adding water volume.
Commission at low load: trend compressor starts, on-time, entering/leaving temperatures and evaporator flow. Confirm minimum flow and stable bypass operation. Added volume cannot correct faulty sensors or an unstable sequence.
Complete the hydraulic check
For plant context, read the Chiller Plant Design Guide. Use Nexora’s Pump Head Calculator Design Suite (Excel) to organise pipe, fitting and equipment pressure losses for engineering review when the tank changes the circuit.
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