💧 Required Flow Rate
⚙ Total Pump Head
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
|---|
How to Use the Chilled Water System Calculator
Enter the building cooling load the loop must carry and the design supply/return temperature difference.
Enter the longest run's equivalent length and any additional static head from elevation, coils, and valves.
Choose a target pipe velocity appropriate for pipe material and noise sensitivity.
Click Calculate CHW System to see flow rate, pipe diameter, friction loss, and total pump head.
Understanding Chilled Water Distribution Sizing
Chilled water distribution sizing is a separate design step from chiller plant selection, covered by the chiller plant calculator. Once total plant capacity is set, the distribution system needs its own pipe and pump sizing to actually deliver that cooling to every coil in the building at the right flow rate and pressure.
Flow Rate Follows Directly From Load and Delta-T
Chilled water flow rate scales directly with cooling load and inversely with delta-T: a wider design temperature difference between supply and return moves the same load with less water. A standard 10 degree Fahrenheit delta-T needs about 2.4 GPM per ton, while a wider 16 to 20 degree delta-T needs meaningfully less, directly shrinking pipe size and pump energy.
Pipe Diameter Balances Velocity and Friction Loss
Selecting pipe diameter from target velocity keeps friction loss and pump energy in a reasonable range while avoiding flow noise. Most commercial CHW mains target 4 to 8 feet per second; going narrower than needed pushes velocity and friction loss up, while oversizing pipe adds material cost without a meaningful efficiency benefit.
Pump Head Combines Friction Loss and Static Components
Total pump head is friction loss through the longest pipe run plus any additional static head from elevation change, coil pressure drops, and control valve losses. Undersizing this figure results in a pump that can't deliver design flow to the most remote coil in the system, showing up as comfort complaints at the far end of the distribution loop even when the chiller plant itself is correctly sized.
Pumping Power and Variable Speed Operation
Pumping power scales with the product of flow and head, which is why many modern CHW systems use variable-speed pumps with differential pressure reset rather than constant-speed pumps sized for worst-case design flow. This calculator estimates design-condition pumping power as a baseline; actual annual energy use depends heavily on how much of the year the system runs below full design flow.
Frequently Asked Questions
A standard chilled water system designed for a 10 degree Fahrenheit (5.5 Celsius) delta-T needs approximately 2.4 US gallons per minute per ton of cooling. Systems designed for a wider delta-T, such as 16 to 20 degrees Fahrenheit, need less flow per ton, which reduces pipe size and pumping energy but requires coils and chillers selected for that wider temperature spread.
Most chilled water distribution piping targets 4 to 8 feet per second in mains and branch lines, balancing pipe cost against friction loss and pump energy. Velocities above roughly 8 feet per second in occupied-adjacent spaces risk audible flow noise, while velocities much below 4 feet per second increase pipe size and material cost without a corresponding benefit.
A wider delta-T moves the same total cooling load with less water flow, which directly reduces required pipe diameter, friction loss, and pump energy. This is why many modern chilled water designs push toward a 16 to 20 degree Fahrenheit delta-T instead of the traditional 10 degrees, provided the chiller and coils selected can actually achieve that wider spread at part load as well as design conditions.
Flow rate determines how much water moves through the system to carry the cooling load, while pump head determines how much pressure the pump must add to overcome pipe friction, fittings, and any elevation change to actually deliver that flow. A correctly sized pipe with an undersized pump still fails to deliver design flow, so both calculations are necessary before selecting pump equipment. See the chiller plant calculator for plant-level sizing.
Related HVAC Tools
Plant equipment sizing
Heat rejection sizing
General hydronic pump sizing
Detailed friction loss reference
Free cooling on the same loop
Coil served by this loop