Pool Hydraulics Basics

Hydraulics is simply how water behaves when it moves, or sits still, in your pool's plumbing. You don't need an engineering degree to use it. A few core ideas explain why pumps are sized the way they are, why filter pressure rises, and why slowing a pump down saves so much electricity.

Flow: how much water moves

Flow is measured in gallons per minute (GPM). It decides how quickly the pool's water passes through the filter, heater and sanitizer. Too little flow leaves dead spots; too much wastes energy and strains equipment. Our Choosing the Right Size Pump guide shows how to calculate the flow your pool needs.

Head: the resistance the pump works against

Every foot of pipe, every elbow, valve and fitting, and every piece of equipment resists flow. That resistance is called head and is measured in feet. It's often called total dynamic head (TDH). Head comes from two places:

• Friction loss as water rubs along pipe walls and turns corners. It rises steeply as flow increases.
• Elevation, if water has to be lifted, for example to a raised spa, a waterfall or roof-mounted solar panels.

Pressure and your gauges

Pressure is head expressed another way: 1 psi equals about 2.31 feet of head. Your filter gauge shows the pressure on the return side of the system. A reading that slowly rises means the filter is collecting dirt. A sudden jump suggests a blockage after the filter, such as a closed valve. A sudden drop suggests a restriction before the pump, such as a full basket, a clogged impeller or a low water level.

The pump curve

Every pump has a curve showing how many GPM it delivers at each amount of head. As head goes up, flow goes down. The point where your system's resistance meets the pump's curve is where the pump actually operates. That's why two identical pumps can move very different amounts of water in two different backyards.

Why slower is cheaper: the affinity laws

For a given pump and plumbing system, changing the motor speed changes things in predictable ways:

• Flow changes in proportion to speed. Half the speed gives about half the flow.
• Head changes with the square of speed. Half the speed gives about one quarter of the head.
• Power changes with the cube of speed. Half the speed uses roughly one eighth of the power.

Running a variable-speed pump at a lower speed for more hours can move the same total amount of water for a fraction of the energy. Real-world savings vary, but the principle is why variable-speed pumps have become standard.

Suction side vs. pressure side

• Suction side (pool to pump): skimmers, main drains and the pump basket. Restrictions here starve the pump. That can cause cavitation (a gravelly noise), loss of prime and damage. Air leaks here show up as bubbles in the pump basket.
• Pressure side (pump back to pool): filter, heater, sanitizer and returns. Restrictions here raise filter pressure. Leaks here drip or spray water.

Practical ways to improve hydraulics

• Keep skimmer and pump baskets empty and the filter clean.
• Use the largest practical pipe size. Moving from 1.5-inch to 2-inch pipe cuts friction substantially at the same flow.
• Use sweep elbows or 45° fittings instead of sharp 90° turns where you can.
• Don't leave valves partly closed without a reason. Each one adds head.
• Match the pump to the system instead of simply buying more horsepower.

Find plumbing parts in Pipe Fittings & Adapters and Valves & Actuators, gauges in Pressure Gauges, and pumps in Pool & Spa Pumps. For pump trouble, see Pool Pump Won't Prime or Loses Prime.