Common Pump Diagnosis Reference
Why this matters
Pumps are everywhere in service trades - sump pumps, well pumps, booster pumps, circulator pumps, pool pumps. They share diagnostic logic + failure modes. The tech who knows pump basics diagnoses fast across many pump types. This is the field card.
Pump basics
What a pump does:
- Moves fluid from low pressure to higher pressure
- OR from one location to another
- Powered by motor (electric usually)
Key parameters:
- Flow rate (GPM)
- Head pressure (PSI OR feet of head)
- Power consumption (watts OR amps)
Centrifugal pumps (most common):
- Impeller spins; throws water outward
- Casing converts to pressure
- Quiet, durable
- Most residential applications
Positive displacement (less common residential):
- Forces specific volume each rotation
- Diaphragm, gear, screw
- For very high pressure OR specific applications
Pump types in service
Sump pump (basement):
- Removes water from sump pit
- Submerged motor + impeller
- Float switch triggers
- 1/3 to 1/2 HP typical residential
- Critical during storms
Sewage pump / ejector:
- Like sump but handles solids
- Larger impeller passages
- Outdoor OR specific basement systems
Well pump (submersible):
- Deep below ground in well
- Pressure tank stores water
- Pressure switch cycles pump
- 1/2 to 5+ HP typical
Well pump (jet):
- Above-ground; suction below
- Less efficient than submersible
- Older OR shallow wells
Booster pump (residential):
- Increases water pressure
- For homes with low city pressure
- Variable-speed often
Circulator pump (heating):
- Moves hot water through baseboards / radiators
- Small; low pressure; constant operation
- Heating system component
Pool pump:
- Circulates pool water through filter
- Variable speed (newer) OR single speed (older)
- 1 to 3 HP typical
- Heavy daily operation
Hot tub pump:
- Similar to pool but smaller
- Heated water
Sprinkler pump:
- Boosts pressure for irrigation
- Often above-ground centrifugal
Common pump failure modes
Pump won't start:
- Power loss (breaker, fuse, wire)
- Switch failure (float, pressure, motor switch)
- Motor burned out
- Capacitor failed
- Locked / seized impeller
Pump starts + immediately trips:
- Locked impeller
- Wrong voltage
- Motor failure
- Overload setting too low
Pump runs but no flow:
- Suction blockage
- Air leak (suction side)
- Impeller damage
- Foot valve failed (jet pump)
- Closed valve in line
Pump runs but low pressure:
- Worn impeller
- Air in system
- Suction restriction
- Speed reduced (variable speed)
Pump cycles too often:
- Pressure tank waterlogged
- Pressure switch problem
- System leak
- Demand exceeds capacity
Pump runs constantly:
- System leak
- Pressure switch stuck
- Demand exceeds supply
- Tank waterlogged
Pump makes unusual noise:
- Cavitation (low suction; vapor bubbles collapsing)
- Bearings worn
- Impeller damage
- Air in pump
Pump leaks:
- Seal worn
- Casing damaged
- Connection loose
Diagnostic process
For any pump complaint:
- Verify power:
- Breaker on?
- Voltage at pump terminals?
- Correct voltage for pump?
- Verify control signal:
- Float switch (sump) working?
- Pressure switch (well) working?
- Demand signal (circulator) present?
- Verify motor:
- Resistance test
- Current draw if running
- Visual inspection
- Verify mechanical:
- Impeller free-turning
- Pump body intact
- Connections tight
- Verify system:
- Pipes clear
- Valves open
- Pressure tank healthy
- No leaks
Systematic; don't replace parts without diagnosis.
Sump pump specific
Most common failures:
- Float switch stuck
- Float switch broken
- Motor burned out
- Discharge line clogged / frozen
- Check valve failure
Backup sump:
- Battery-powered secondary
- Activates if primary fails OR overloaded
- Critical for flood prevention
Pump location:
- Sump pit should be clean
- Free of debris
- Float can move freely
Well pump specific
Pressure tank:
- Stores pressurized water
- Bladder OR diaphragm separates water + air
- Waterlogged = bladder failed = pump short-cycles
- Pre-charge: typically 2 PSI below cut-in pressure
Pressure switch:
- Standard cut-in 30 PSI; cut-out 50 PSI
- Or 40/60 PSI
- Switch contacts arc + fail
- Switch can stick
Submersible well pump:
- 1 wire (2-wire) OR 3 wire (3-wire) common
- 3-wire has external control box
- Pulled from well for replacement
Circulator pump specific
Heating system circulator:
- Often Taco / Grundfos / Bell+Gossett
- Cartridge replacement often easier than pump replacement
- Quiet whine = normal
- Loud rumble = worn bearings
- No flow = airbound OR seized
Pool pump specific
Variable speed (VSP):
- Common new pools
- Lower energy than single speed
- Programmable run schedule
- More expensive
Single speed:
- Old standard
- Runs at one speed only
- Higher energy
- Less expensive
Common pool pump issues:
- Worn seals (leaks at shaft)
- Impeller worn / damaged
- Strainer basket full
- Air leak at lid (suction side)
Capacitor problems
Most single-phase motors have starting capacitor:
Start capacitor failure:
- Motor hums; won't start
- Capacitor bulged OR leaking
- Replace; cheap
Run capacitor failure:
- Motor runs but draws excessive current
- Inefficiency, heating
- Replace
Testing capacitor:
- Discharge before handling (shock + spark)
- Capacitance meter
- Visual: bulged top = failed
Electrical diagnosis
No power to pump:
- Breaker tripped (check + reset)
- Fuse blown (check)
- Wire damaged (visual)
- Disconnect switch off
Power present, motor won't run:
- Capacitor failed
- Motor windings open / shorted
- Switch failure (pressure, float)
- Wire connection loose
Motor runs but trips breaker:
- Wrong voltage
- Locked impeller / overload
- Motor short
- Wire short to ground
Mechanical diagnosis
Impeller damage:
- Open pump (per manufacturer)
- Inspect for cracks, wear, debris
- Replace if damaged
Foot valve failure (jet pump):
- Allows water to drain back
- Pump loses prime
- Repair / replace foot valve
Check valve failure:
- Water flows backward
- Pump short-cycles
- Replace check valve
Pump replacement vs repair
Replace when:
- Motor failed (most pumps)
- Impeller catastrophically damaged
- Casing cracked
- Pump 10+ years (residential)
Repair when:
- Capacitor failed
- Switch failed
- Seal worn (some pumps)
- Bearing replacement (some pumps)
Cost / benefit on each.
Replacing pumps
Sump pump:
- Disconnect power
- Remove discharge line connection
- Lift out
- Install new
- Reconnect
- Test
Submersible well:
- Specialized; well company
- Pulling pipe + wire from well
- Replace pump
- Lower back into well
- Sometimes pressure tank too
Pool pump:
- Disconnect power
- Disconnect plumbing
- Remove bolts
- Install new
- Reconnect; verify alignment
References
- HI (Hydraulic Institute) standards
- Manufacturer service manuals (Goulds, Grundfos, Pentair, Taco, etc.)
- NSF / ANSI 60, 61 (drinking water pumps)
- Manuall internal: Common Motor Types, Common Pump Types