How to Read a Pool Pump Performance Curve

What a pool pump performance curve shows

A performance curve is a graph that a pump maker publishes for each model. It appears in the product manual, the spec sheet, or the listing's image set. The curve pairs the two central measurements of a pump: flow and head. For a pump owner, the curve is the maker's statement of how the pump behaves in general, before the plumbing of a specific pool is added.

The curve is the maker's stated performance for that model at a given speed. A variable speed pump has a separate curve for each speed setting, so the graph may show several lines. A dual speed pump shows 2 curves, one for each speed.

The two axes: flow and head

Flow is the volume of water the pump moves, stated in gallons per minute (GPM) or gallons per hour (GPH). Head is the resistance the pump must overcome, stated in feet. Both terms have their own articles on this site (flow rate and head).

The graph places one axis on flow and the other on head. The curve passes through the two figures that may appear in a listing, such as the maximum flow and the maximum head. The highest point on the curve, where flow is zero, is the maximum head the pump can produce against a closed line.

How the curve slopes

The curve slopes downward from left to right. At one end, with no flow, the pump produces its maximum head, sometimes called shut-off head. At the other end, with no resistance, the pump reaches its maximum flow. Every real pool sits between these two extremes.

The shape of the curve tells you how the pump responds as resistance changes. A flatter curve means the pump holds its flow better as head rises. A steeper curve means flow drops more quickly when resistance increases. Both shapes are valid; the question is whether the curve sits high enough across the flow range a pool needs.

The system curve: plumbing as resistance

A pool's plumbing creates its own curve, called the system curve. Pipe length, pipe diameter, fittings, the filter, a heater, and a cleaner all add resistance. As flow increases, the resistance in the plumbing rises.

The system curve is not published in a pump listing. An installer or a maker's sizing tool can calculate it for a specific pool, usually by adding up the resistance of each part of the plumbing into a total called total dynamic head (TDH). Hayward, for example, offers a total dynamic head calculator on its site, and Pentair lists pool calculators among its resources.

• Common parts of a pool's plumbing that add resistance:
• Pipe length and pipe diameter
• Fittings, elbows, and valves
• The filter tank and the filter media
• A heater's heat exchanger
• A cleaner connected to the pump line

Where the curves meet: the operating point

A pump delivers the flow where the pump curve crosses the system curve. This crossing point is the operating point. A pump whose curve sits higher at the resistance that a pool's plumbing creates delivers more flow at that resistance.

Comparing two pump curves means comparing where each curve sits across the head range that the plumbing will produce. The pump with the higher curve at that head is the one that moves more water through the pool.

The operating point is not stated in a listing. It is the result of pairing the maker's curve with the plumbing's system curve. A listing may state a maximum flow figure, but the delivered flow in a real pool is the operating point, not the maximum.

Reading a listing with a performance curve

Some listings include a curve in the image set; others state only one or two figures, such as maximum flow and maximum head. The curve shows the full range between these figures, which a single number cannot.

A stated maximum flow alone does not tell how the pump performs in a pool, because the pump never runs at zero resistance. The curve, read together with the plumbing's system curve, is what matters. A pump with a higher maximum flow may still deliver less than a pump with a lower maximum flow if its curve drops more steeply at the head a pool's plumbing creates.

What this site cannot say about sizing

No guide on this site can match a pump curve to a pool's gallons, pipe size, or filter. Sizing requires the plumbing details of a specific pool, and the maker's sizing page plus a professional installer are the place to work that out.

This article explains the curve as a term. The inground pump buying guide and the above ground pump buying guide compare pumps on stated details, such as horsepower, speed type, and flow figures, and point to the maker's tools for the final match.

What to pick for your situation

Questions

What is a pool pump performance curve?

A graph from the pump maker that shows how much flow a pump delivers at different levels of resistance. It is published in the product manual or spec sheet, and sometimes in the listing's image set.

Why does the performance curve slope downward?

The pump has a fixed amount of energy. When resistance (head) rises, less of that energy is left to move water, so flow drops. The curve is the visual statement of that trade-off. The same pump will move more water through short, wide pipes than through long, narrow ones.

What is the system curve?

The resistance curve of a pool's plumbing. It rises as flow increases, because pipes, fittings, the filter, and a heater all resist water movement. It is not published in a pump listing; an installer or the maker's sizing tools calculate it.

Can the curve alone size a pump for a pool?

No. The curve must be read together with the system curve of the plumbing. The maker's sizing page and a professional installer handle that match. The curve alone cannot account for the pipe length, the filter, or a heater in a specific pool.

Where is the performance curve for a pump published?

In the pump's manual or spec sheet, and sometimes in the listing's image set. Makers such as Hayward and Pentair also publish sizing tools and calculators on their sites.

How are two pumps compared using their curves?

Compare where each curve sits across the head range that the plumbing creates. The pump whose curve sits higher at that head delivers more flow. The variable speed pump guide and other buying guides on this site list the stated figures side by side.