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Priming, drafting and cavitation
A pump cannot suck water. Atmospheric pressure pushes it up once the primer removes the air, which is why lift is capped by physics rather than by the pump. That single fact explains almost every failed draft.
Atmosphere does the lifting
The primer evacuates air from the pump and hard suction. With the air gone, atmospheric pressure - about 14.7 psi at sea level - pushes water up the hose.
That sets a theoretical limit near 33 feet and a practical one around 20 to 25 feet once friction, altitude and an imperfect vacuum are accounted for. A pump that will not draft at 26 feet may be perfectly healthy.
A failed prime is almost always an air leak
If the primer runs and vacuum never builds, air is getting in: a loose intake cap, a bad suction gasket, an open drain or bleeder, or a dry packing gland.
The vacuum test isolates it. Cap everything, pull about 22 inches of mercury, shut the primer and watch. Losing more than 10 inches in 10 minutes means a leak you must find before blaming the primer.
Cavitation is the pump running out of water
Demand more than the intake can supply and pressure at the impeller eye falls below the vapour pressure of water, so it boils. Those bubbles then collapse violently against the impeller.
It sounds like gravel in the pump, discharge pressure turns erratic and stops responding to RPM, and the impeller erodes. The fix is always to reduce demand or improve supply - never more throttle.
How to test it
- Cap and close everythingAll intakes capped, drains and bleeders shut. One open valve defeats the whole prime.
- Run the primer until vacuum steadiesLook for it to pull and hold, rather than climbing and falling back.
- Vacuum test if it will not hold22 inHg, primer off, no more than 10 inHg lost in 10 minutes.
- Listen while flowingGravel noise plus erratic discharge that ignores the throttle means cavitation.
- Reduce demand, do not add throttleClose a line or improve the supply. More RPM worsens cavitation and erodes the impeller.
Numbers to work to
| Theoretical lift | about 33 ft | at sea level with a perfect vacuum |
| Practical lift | about 20-25 ft | less at altitude |
| Dry vacuum test | 22 inHg, lose 10 inHg or less in 10 min | NFPA 1911 |
| Lift penalty | roughly 1 ft per 1,000 ft elevation | thinner air pushes less |
| Cavitation signature | gravel noise, pressure ignores RPM | stop and reduce demand |
Common mistakes
- Adding throttle to fix cavitation, which only accelerates impeller erosion.
- Blaming the primer when the real fault is an open drain or a bad gasket.
- Expecting full lift at altitude.
- Running a dry pump long enough to cook the packing while troubleshooting.
Drill this until it's automatic
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