Hot Water Circulation Pump Cavitation Checker

Hydraulic Institute (ANSI/HI 9.6.1) guidance is NPSHa ≥ 1.1–1.3 × NPSHr for most services, with higher ratios for high-energy pumps. Treat 0.6 m as a floor for small pumps. Remember NPSHr is defined at 3% head drop — real damage-free operation wants clear air above it.

You're meeting the vapor-pressure curve: at 80 °C water's vapor pressure is ~47 kPa, at 95 °C it's ~85 kPa — if loop pressure at the pump suction is marginal, the hottest water finds it first. Check the expansion tank charge (pre-charge should match fill pressure, measured cold with the system isolated), verify the pump isn't pumping toward the tank connection (that lowers suction pressure dynamically), and raise the cold fill slightly if the building height allows.

Like pumping gravel — a crackling rattle loudest near the impeller eye, often with fluctuating discharge pressure and flow. Long-term evidence is sponge-like pitting on impeller vanes near the leading edge. Brief cavitation during upsets is survivable; sustained operation eats impellers in months.

In order of typical cost: cool the liquid or reduce its vapor pressure exposure, raise the liquid level / lower the pump, fatten and shorten the suction line (bigger pipe, fewer elbows, full-bore valves, clean strainer — friction is often the cheapest win), pressurise the suction vessel, or slow the pump (NPSHr falls roughly with the square of speed). A lower-NPSHr impeller or an inducer from the OEM is the last resort.