Every pool pump lives on a curve. The curve says: at this flow rate, the pump produces this much head. Total dynamic head (TDH) is the resistance of your plumbing and filter. Where the two curves cross is your actual operating point. Operators who understand this don't get fooled by “I put in a bigger pump and it didn't help.”
What TDH actually is
TDH = Suction head + Discharge head + Friction losses
- Suction head: Vertical distance the pump has to lift water from the pool surface up to the pump
- Discharge head: Vertical distance and back-pressure from filter, heater, chlorinator, returns
- Friction losses: Resistance of the pipes, fittings, and valves — a function of flow rate (higher flow = much higher friction)
Measured in feet of water. A good residential pool system has 40–60 ft TDH. 80+ ft and something is wrong — undersized plumbing, dirty filter, or a closed valve.
Reading a pump curve
Every pump manufacturer publishes a curve showing flow (gpm) on the X-axis and TDH (ft) on the Y-axis. As TDH goes up, flow goes down — that's the shape. Your operating point is where your system's TDH intersects the pump's curve.
When you change RPM on a variable-speed pump, the curve scales — halving RPM drops TDH by about 4× and flow by 2×. That's why VS pumps at low RPM save so much energy.
Cavitation: the pump killer
Cavitation is when water in the pump becomes a partial vacuum and boils at ambient temperature. Tiny steam bubbles form, collapse violently on the impeller, and over time pit and destroy it. Signs:
- Rattling sound (“gravel in the pump”)
- Fluctuating vacuum gauge reading
- Lower than expected flow
- Pitting on impeller (visible when disassembled)
What causes cavitation
- Suction-side blockage: clogged skimmer, full pump basket, clogged main drain cover
- Air leak: loose pump lid, bad o-ring, cracked suction fitting
- Undersized suction piping: 1.5" where 2" is needed
- Pump too powerful for plumbing: 2 HP single-speed on 1.5" pipes
- Partially closed valve upstream of the pump
Field diagnostic kit
The two gauges every service tech should carry:
- Suction gauge (vacuum, reads in inches of mercury): Normal residential = 5–10 in Hg. Over 15 = suction-side restriction.
- Discharge gauge (pressure, reads in psi): Compare to the clean-filter baseline. 8–10 psi above = time to clean filter.
The 6-ft rule
For every foot of vertical rise on the suction side, you lose roughly 1 psi of capacity. Pumps located far above pool water level or far from the pool can suffer from net positive suction head (NPSH)starvation — the water can't keep up with what the pump wants. Keep pumps close and low where possible.
Quick cavitation troubleshooting
- Stop the pump. Open the lid. Inspect pump basket (full? cracked?).
- Look at the suction-side valve. Fully open?
- Check the skimmer — full? weir stuck?
- Clean the main drain if accessible.
- Restart the pump with the lid loose briefly to bleed air, then tighten.
- Check suction gauge. Over 15 in Hg? Still restricted somewhere.
- Check all suction-side unions and fittings for air ingress.
- If nothing helps: suspect NPSH mismatch. Call an engineer.