Pumps and flow

An impeller spins, adding energy to water. Flow depends on system resistance. Close a valve (more head) and flow falls along the pump curve. Open everything wide and you may run off the right side of the curve — high amps, possible cavitation risk depending on design.

A useful memory hook:

Tons ≈ gpm × ΔT / 24 (for water, common HVAC rule-of-thumb form)

If ΔT collapses because flow is too high or coils bypass, the plant looks “low capacity” while pumping lots of water nowhere useful.

A clogged strainer mimics a failing pump: noise, low flow, hot motor. Always check suction conditions. Cavitation sounds like gravel — low NPSHA from air, clogged suction, or wrong level.

Towers and coils need designed flows. Balancing valves and circuit setters are adjusted so each path gets its share. “Wide open everything” is not balancing. After construction, rebalance when strainers fill or when valves are replaced.

Standby pumps need exercise and check valves that actually seat. A failed check can dead-head or recirculate between pumps.

The pump is the heart; strainers are kidneys; balancing valves are traffic cops. Strong heart + clogged kidneys = weak circulation.

Suction and discharge gauges tell head. A pump that once made 40 psi differential and now makes 15 with the same valve lineup is worn, air-bound, or spinning backward after a motor swap — verify rotation with the arrow.

Slowing pumps saves energy until you undershoot coil or chiller minimums. Coordinate VFD minimum speeds with equipment protections. Bypass the VFD only with a plan.

Different loops, different chemistries, different design heads. Tag pumps clearly. Service the wrong strainer while the building cooks is an avoidable comedy.

Chillers interlock on flow proofs. Nuisance trips need clean paddles/sensors and real flow — not a jumper. Jumpers are how evaporators freeze.

Replacement pumps must match design head/flow. A “same flange” pump with a smaller impeller underperforms quietly for years. Check curves, not only bolt patterns.

### Field memory hooks

Write the key temperatures, flows, or mode checks for this topic on your job notes. Teach the helper beside you to repeat the layered order aloud before tools come out. Most callbacks shrink when the first visit follows a written order instead of a parts guess.

When OEM data exists — capacity tables, wiring diagrams, water-flow charts — photograph the rating plate and the page you used. Future you (or the next tech) should not have to rediscover the same facts under pressure.

Situation. Chiller trips on low flow. Flow switch is good. Tech finds a construction rag in the suction strainer after a coil retrofit upstream. Pump amps are low; discharge pressure high relative to flow.

What to think. Obstruction. Clear strainer; flush; verify switch and ΔP.

Conclusion. Hydraulics before transducer replacement sprees.

### Teach-back for the helper

Before leaving the topic, have the helper explain in their own words: what is normal, what is measured first, and what must never be skipped for safety. If they cannot teach it back, review the main “How it works” blocks again with a sketch on paper.

Connect this lesson to the previous one and the next one as a chain. Field skill is sequence memory — not isolated trivia. Keep OEM charts and job photos with the work order so the next visit starts smarter.

### Common measurement habits

Use the same meter locations every time so readings compare day to day. Steady the system before judging pressures or temperatures — mid-shift and mid-defrost lie. Write ambient conditions on every note page.

If a reading disagrees with the control display, believe the calibrated handheld until sensors are proven. Controls cannot fix what they cannot see accurately.

### Safety and professionalism

Apply LOTO where energy can hurt you. Wear eye protection around refrigerants, chemicals, and coil fins. On roofs and at window units, respect fall hazards. Explain findings to the owner without jargon soup — clear next steps beat impressive vocabulary.

Dispose of oil, filters, and refrigerant legally. Cutting corners on recovery or tower chemicals is not “saving the customer money”; it is transferring risk.