At the heart of the hydraulic system of the pool is the pool pump, but without well-designed plumbing and flow dynamics, even the best pump can underperform.
Understanding how pressure, velocity, and flow rate interact, and how factors like head loss and pump selection affect performance, is essential for designing and maintaining an energy-efficient and effective pool system.
How Water Flows Through the Pool’s Plumbing System
The water in a swimming pool moves in a closed-loop system. The cycle typically starts when water is drawn from the pool through skimmers, main drains, and occasionally vacuum ports. It’s pulled into the suction side of the pump, then pushed through the filter, heater, salt or chlorination system, and eventually returned back to the pool through return lines.
Flow Rate (GPM)
Flow rate is the volume of water that passes through the system in a given amount of time. It’s typically measured in gallons per minute (GPM). An ideal flow rate ensures effective filtration, heating, and chemical distribution, without placing excessive strain on the system.
Flow rate is influenced by:
• The pump's horsepower and speed (especially with variable speed pumps).
• The diameter and length of plumbing pipes.
• The number and type of fittings (elbows, tees, valves).
• The height difference between water sources and equipment.
Too low a flow rate can lead to poor filtration, while too high a flow rate can cause excessive wear on the system and may exceed the maximum flow rating of equipment like filters or heaters.
Velocity
Velocity refers to the speed at which water moves through the pipes and is measured in feet per second (ft/s). As velocity increases, friction losses within the pipe system increase as well. The recommended maximum velocity (according to PHTA’s ANSI15) in pool plumbing is:
• 6 ft/s on the suction side (before the pump),
• 8 ft/s on the pressure side (after the pump).
Maintaining proper velocity prevents noise, reduces wear on plumbing, and improves system efficiency.
Pressure
Pressure, measured in psi (pounds per square inch), is the force exerted by the moving water on the plumbing and equipment. The pool pump generates pressure to overcome resistance in the plumbing system. Gauges on filters typically monitor this pressure to indicate when the system is becoming clogged or needs cleaning. Higher pressure usually means more resistance — often from dirty filters, undersized plumbing, or long pipe runs.
The Role of Head Loss in Hydraulic Calculations
Acritical concept in pool hydraulics is head loss, or total dynamic head (TDH). Head loss represents the resistance that the pump must overcome to move water through the system. It includes friction losses in pipes, fittings, and equipment, as well as vertical elevation differences.
Friction Loss
Friction occurs when water rubs against the internal surfaces of pipes and fittings. The more bends, elbows, valves, or narrow pipes in a system, the greater the friction and, therefore, the head loss.
Friction loss depends on:
• Pipe size: Smaller diameter pipes create more friction.
• Flow rate: Higher flow means more friction.
• Pipe material and condition: Rough or old pipes increase resistance.
• Number of fittings: Every 90° elbow or valve adds measurable head loss.
Friction losses are typically calculated using reference tables or formulas such as the Hazen-Williams equation. Reducing unnecessary fittings and using larger diameter pipes can significantly decrease friction loss.
Equipment Losses
Every component — filters, heaters, chlorinators, valves — adds a resistance to flow. Manufacturers usually provide head loss curves or pressure drop values that help estimate these effects under different flow conditions.
Static Head vs. Dynamic Head
To properly size a pump, it's important to understand the two primary types of head in a hydraulic system:
Static Head
Static head is the vertical distance water must travel between two points — for instance, from the water surface in the pool to the pump inlet (if the pump is above the water level), or from the pump to a raised spa or feature.
Static head does not change with flow rate. It includes:
• Suction lift: When the pump is higher than the water source.
• Discharge head: When water must be pushed up to a higher elevation.
Dynamic Head
Dynamic head (or friction head) is the resistance caused by water moving through the pipes, fittings, and equipment. Unlike static head, dynamic head increases with flow rate. Every increase in water velocity raises friction loss exponentially.
When sizing a pump, both types of head must be added together to get the Total Dynamic Head (TDH): TDH = Static Head + Dynamic Head (Friction Loss) For example, a system might have:
• 5 feet of suction lift (static head),
• 10 feet of discharge elevation (static head),
• and 35 feet of friction loss at a given flow rate (dynamic head).
In this case, the TDH is 50 feet.
The Effect of Head on Pump Selection
Pumps are selected based on their ability to overcome total dynamic head at a desired flow rate. Each pump has a performance curve that shows how much water it can move (in GPM) at various head levels (in feet).
• A pump that delivers 80 GPM at 40 feet of head may only deliver 30 GPM at 70 feet of head.
• If head loss is underestimated, the pump will not meet performance expectations, resulting in poor circulation, inadequate filtration, or strain on the motor.
• Oversizing the pump can lead to excessive velocity, wasted energy, and equipment wear.
That’s why accurate hydraulic calculations are essential. Modern software or manual calculation methods (like the Affinity Laws and Hazen-Williams formulas) can be used to model a system before choosing the pump.
