22 Circulation & Filtration Practice Questions & Answers

1. 1. Water leaving the pool travels through the recirculation system in a fixed order. Which sequence is correct?

   • A.Pool → skimmers and main drain → pump strainer basket → pump → heater → filter → chemical feeder → return inlets
   • B.Pool → skimmers and main drain → pump strainer basket → pump → filter → heater → chemical feeder → return inlets✓ Answer
   • C.Pool → skimmers and main drain → pump → pump strainer basket → heater → filter → chemical feeder → return inlets
   • D.Pool → skimmers and main drain → chemical feeder → pump strainer basket → pump → heater → filter → return inlets

   Each position in the loop protects the component behind it: the hair-and-lint strainer sits ahead of the pump so debris never reaches the impeller, the filter precedes the heater so the heat exchanger receives clean water, and chemical feed is the last step before the returns so concentrated product never sits in the filter or heater.

   Source: PHTA Pool & Spa Operator Handbook, circulation system chapterReport a problem with this question

2. 2. Why is the chemical feed point placed downstream of both the filter and the heater rather than ahead of them?

   • A.The filter would remove the chemical before it reached the pool, so nothing would be delivered
   • B.Feeders are designed to operate under vacuum and cannot function on the pressure side
   • C.Chemicals dissolve only in heated water, so they must contact the water after it leaves the heater
   • D.Concentrated feed solution would corrode the heat exchanger and degrade filter media, so feeding after them protects the equipment✓ Answer

   Undiluted acid or sanitizer solution is far more aggressive than pool water; injected upstream it would attack copper or cupro-nickel heat exchanger tubes and shorten the life of filter media, so the injection point always sits after the filter and heater on the line heading back to the pool.

   Source: PHTA Pool & Spa Operator Handbook, chemical feeder installation guidanceReport a problem with this question

3. 3. A check valve is required in the line between a chemical feeder and its injection point. What does it accomplish?

   • A.It stops chemical solution or system water from flowing backward into the heater and other equipment when the pump shuts down✓ Answer
   • B.It boosts the feeder's output so the correct dose is delivered at high flow
   • C.It bleeds trapped air out of the feed line automatically
   • D.It keeps the feeder tank from emptying faster than the pump can circulate

   When circulation stops, the pressure that kept the feed moving forward disappears and gravity or siphon action can pull concentrated solution back up the line; a check valve or anti-siphon device makes that flow one-way, which is what keeps aggressive chemical out of the heater and off the filter.

   Source: PHTA Pool & Spa Operator Handbook, chemical feed equipment and backflow protectionReport a problem with this question

4. 4. What is the function of the hinged weir in a surface skimmer?

   • A.It regulates how much water is drawn from the main drain
   • B.It supports the skimmer basket so the basket cannot float free
   • C.It accelerates the thin surface layer into the skimmer throat and holds collected floating debris from washing back into the pool✓ Answer
   • D.It supplies make-up water when the pool level falls below the skimmer opening

   The weir floats at the water line and forces flow over a narrow lip, which speeds up the surface film where oils and floatables collect; when the pump stops the weir falls closed so the debris already captured stays in the skimmer instead of drifting back out.

   Source: PHTA Pool & Spa Operator Handbook, skimmer components and operationReport a problem with this question

5. 5. An operator finds the skimmer equalizer line plugged. What operating problem does an open, properly functioning equalizer line prevent?

   • A.Excessive pressure building on the filter's influent gauge
   • B.Debris from the skimmer basket passing into the pump strainer
   • C.The pump drawing air and losing prime if the water level drops below the skimmer weir✓ Answer
   • D.Surface oils collecting on the far side of the pool from the skimmer

   The equalizer opens through the pool wall below the water line, so if evaporation or splash-out drops the level under the weir the pump keeps drawing water from that submerged opening rather than sucking air, which is what protects prime and the water-lubricated shaft seal.

   Source: PHTA Pool & Spa Operator Handbook, skimmer equalizer line functionReport a problem with this question

6. 6. A large commercial pool uses a perimeter overflow gutter with a surge tank instead of wall skimmers. What is the surge tank there to do?

   • A.Hold the water displaced by bathers and returned by the gutter so the pump always has a flooded suction supply✓ Answer
   • B.Settle out fine dirt so the filter has less work to do
   • C.Provide contact time for the sanitizer before water returns to the pool
   • D.Store backwash water until it can be discharged

   A gutter system skims the entire perimeter and sends that water to a balance or surge tank, whose volume absorbs the level swing caused by bathers entering and leaving; without that reservoir the pump would alternately flood and starve as the pool level surged.

   Source: PHTA Pool & Spa Operator Handbook, gutter and surge tank systemsReport a problem with this question

7. 7. How does the centrifugal pump used on pool circulation systems move water?

   • A.The spinning impeller throws water outward into the volute, creating a low-pressure area at the impeller eye that pool water is pushed into✓ Answer
   • B.A separate vacuum pump evacuates the suction line and pulls water up to the impeller
   • C.A piston displaces a fixed volume of water on every stroke, giving constant flow regardless of resistance
   • D.Heat from the motor expands the water and drives it toward the filter

   Centrifugal force flings water from the impeller vanes into the volute, and the pressure drop this leaves at the eye lets atmospheric pressure on the pool surface push more water in; because it depends on that pressure difference, output falls as system resistance rises, unlike a positive-displacement pump.

   Source: PHTA Pool & Spa Operator Handbook, centrifugal pump principlesReport a problem with this question

8. 8. A pump repeatedly loses prime. The strainer pot is only part full and swirling with air, and the pool water level has fallen to the bottom of the skimmer opening. What should the operator check first?

   • A.The return inlet fittings, because small eyeballs pull air backward through the pump
   • B.The pool water level and the skimmer, because a level below the weir lets the skimmer pull air into the suction line✓ Answer
   • C.The filter's effluent gauge, because a dirty filter starves the pump of water
   • D.The chemical feeder setting, because underfeeding thins the water

   Air in the strainer pot always points to the suction side, and the most common suction-side source is a pool level low enough for the skimmer to gulp air; a dirty filter sits downstream of the impeller and cannot introduce air into the suction line.

   Source: PHTA Pool & Spa Operator Handbook, pump priming and troubleshootingReport a problem with this question

9. 9. A pump makes a sound like gravel rattling inside the housing and flow is unsteady. Which condition best explains cavitation?

   • A.Sand has carried over from a broken lateral and is circulating through the pump
   • B.Water on the discharge side is boiling because the heater is set too high
   • C.Suction-side pressure has dropped low enough for vapor bubbles to form, and they collapse violently as they reach the higher pressure in the impeller✓ Answer
   • D.The filter is dirty, which raises discharge pressure and rattles the impeller vanes

   Cavitation is a suction-side phenomenon: restricted or undersized suction piping, a clogged basket, or excessive suction lift drops the local pressure below the water's vapor pressure, and the bubbles that form implode on the impeller, eroding metal and destabilizing flow.

   Source: PHTA Pool & Spa Operator Handbook, cavitation and suction-side hydraulicsReport a problem with this question

10. 10. Which observation points to an air leak on the suction side of the circulation system rather than a leak on the pressure side?

    • A.A steady drip from the filter's clamp band that leaves a wet spot on the equipment pad
    • B.Air collecting in the pump strainer pot and bubbles streaming from the return inlets with no water on the deck✓ Answer
    • C.A puddle beneath the multiport valve that grows while the pump runs
    • D.Water spraying from the union between the filter and the heater

    Everything ahead of the impeller is under vacuum, so a defect there draws air in and shows up as air in the pot and bubbles at the returns, never as spilled water; anything after the impeller is pressurized and therefore leaks water outward.

    Source: PHTA Pool & Spa Operator Handbook, suction versus pressure side troubleshootingReport a problem with this question

11. 11. Total dynamic head is determined from the vacuum gauge on the pump suction and the pressure gauge on the discharge. What is the correct way to combine those two readings?

    • A.Subtract the vacuum reading from the pressure reading and report the difference in psi
    • B.Take whichever reading is larger, since the smaller one is already included in it
    • C.Convert each reading to feet of head with its own conversion factor, then add the two results✓ Answer
    • D.Add the pressure reading in psi to the vacuum reading in inches of mercury and report the sum

    Pounds per square inch and inches of mercury are different units, so each must first be expressed in feet of head — the common unit of the pump curve — before the suction and discharge resistances can be summed into total dynamic head; adding the raw gauge numbers together is meaningless.

    Source: PHTA Pool & Spa Operator Handbook, total dynamic head measurementReport a problem with this question

12. 12. An operator partially closes a valve on the return line of an operating system. What happens to the flow rate, and why?

    • A.Flow decreases, because the added restriction raises total dynamic head and a centrifugal pump delivers less water as head rises✓ Answer
    • B.Flow decreases only if the filter is also dirty; restriction alone has no effect
    • C.Flow stays the same, because the pump automatically compensates for any restriction
    • D.Flow increases, because the higher pressure downstream forces more water through the returns

    A centrifugal pump follows its performance curve: every point of added resistance moves the operating point up the head axis and left along the flow axis, so any throttled valve, undersized pipe, elbow or loaded filter buys higher head at the cost of gallons per minute.

    Source: PHTA Pool & Spa Operator Handbook, pump performance curves and system headReport a problem with this question

13. 13. Over two weeks the filter influent pressure has climbed steadily while the flow meter reading has fallen. What does this pattern indicate?

    • A.Resistance downstream of the pump has increased — typically a filter loaded with captured debris✓ Answer
    • B.Air is leaking into the suction line ahead of the pump
    • C.The flow meter has failed, since pressure and flow always move together
    • D.The pump basket is clogged and starving the pump

    Rising pressure with falling flow is the signature of a restriction on the pressure side: the pump is working against more head, so it pushes fewer gallons per minute, and a filter accumulating dirt is by far the most common cause.

    Source: PHTA Pool & Spa Operator Handbook, filter pressure and flow meter diagnosticsReport a problem with this question

14. 14. The filter pressure gauge reading has dropped AND the flow meter reading has dropped. Where should the operator look?

    • A.At the chemical feeder, which is over-injecting into the return line
    • B.At the filter media, which must be replaced
    • C.On the suction side — a clogged skimmer or pump basket, a throttled suction valve, or low water starving the pump✓ Answer
    • D.At the return inlets, which are aimed incorrectly

    When less water reaches the impeller, the pump has less to push, so both the flow meter and the downstream pressure fall together; a plugged filter would do the opposite by driving pressure up while flow falls.

    Source: PHTA Pool & Spa Operator Handbook, circulation troubleshooting sequenceReport a problem with this question

15. 15. A high-rate sand filter shows almost no pressure rise between cleanings, yet the water stays hazy and dirt returns to the pool. What is the most likely condition inside the tank?

    • A.The sand bed is too deep for the tank and is compacting
    • B.The pump is oversized, which forces sand to filter finer than intended
    • C.The filter is being backwashed too thoroughly, leaving the sand too clean to work
    • D.Channeling — water has cut low-resistance paths through the bed and short-circuits past most of the sand✓ Answer

    Water follows the path of least resistance, so once oils and inadequate backwashing let channels or mudballs form, most of the flow bypasses the media; pressure stays deceptively low because the bed is no longer resisting flow, and clarity suffers because little water is actually being strained.

    Source: PHTA Pool & Spa Operator Handbook, sand filter channeling and mudballingReport a problem with this question

16. 16. What should determine when a sand filter is backwashed?

    • A.Any visible cloudiness in the pool, backwashing immediately whenever water looks off
    • B.Whenever the pool has had a busy weekend, since bather load is the real measure
    • C.A fixed weekly schedule, regardless of gauge readings
    • D.A rise above the clean, start-up pressure recorded for that filter, of the amount the manufacturer specifies, usually with a matching drop in flow✓ Answer

    Backwashing is triggered by differential pressure measured against the baseline recorded when that filter was clean, because a partially loaded bed actually traps finer particles than fresh media; cleaning on a calendar wastes water and gives up filtration quality just when the filter is working best.

    Source: PHTA Pool & Spa Operator Handbook, filter backwashing and pressure differentialReport a problem with this question

17. 17. Right after a commercial cartridge filter's elements were hosed off and reinstalled, the pressure is still high and flow is still low. What is the most likely explanation and correct response?

    • A.The pump is undersized and should be replaced with a larger one
    • B.The cartridges need a fresh precoat of filter powder added through the skimmer
    • C.The pleats are blinded by body oils and lotions that hosing cannot remove, so the elements need a degreaser soak, rinsed thoroughly, with a clean spare set installed meanwhile✓ Answer
    • D.The filter simply needs to be backwashed, since hosing is only a supplement to backwashing

    Cartridge filters have no reverse-flow path and are never backwashed; oils bond to the polyester fibers and hold fine dirt in place, so only a chemical soak restores permeability, and keeping a second set of elements lets the facility stay open while the dirty set is treated.

    Source: PHTA Pool & Spa Operator Handbook, cartridge filter cleaning and maintenanceReport a problem with this question

18. 18. Immediately after a diatomaceous earth filter is backwashed, what must the operator do before the system returns to normal service?

    • A.Add a fresh charge of filter powder slowly through the skimmer with the pump running, so the grids are recoated✓ Answer
    • B.Nothing further; the fabric grids themselves are the filter medium
    • C.Run the system in the recirculate position for an hour to let the grids dry
    • D.Add powder directly into the pool so it settles onto the grids overnight

    In a precoat filter the powder — not the septum fabric — is the medium that traps particles, so backwashing removes the medium entirely; running without recoating drives dirt directly into the cloth, permanently blinding grids that would otherwise last for years.

    Source: PHTA Pool & Spa Operator Handbook, precoat (DE) filter operationReport a problem with this question

19. 19. Which statement about handling and disposing of spent filter powder from a precoat filter is correct?

    • A.The spent slurry is a solid waste that generally must go to a separation tank and be disposed of as the wastewater authority having jurisdiction directs, and dry powder should be handled with respiratory protection✓ Answer
    • B.It can be dumped into the pool, where the filter will recapture it
    • C.It may be rinsed into the nearest storm drain because it is a natural mineral
    • D.It should be dried and returned to the filter to save material

    Precoat powder leaves the filter as a solid-laden slurry that clogs drainage systems, and the dry material contains crystalline silica that is hazardous to inhale, so the durable practice is to capture it in a separation tank, follow the wastewater authority's requirements, and wear appropriate respiratory protection when charging the filter.

    Source: PHTA Pool & Spa Operator Handbook, filter waste handling and safety data sheet guidanceReport a problem with this question

20. 20. After exactly one turnover period has elapsed, how much of the pool's water has actually passed through the filter?

    • A.Exactly half of it, since flow is split between skimmers and the main drain
    • B.Roughly two-thirds of it — about 63 percent — because filtered water immediately remixes with unfiltered water✓ Answer
    • C.None of it can be estimated, because filtration is unrelated to circulation time
    • D.All of it; that is what one turnover means

    Turnover describes a volume of water equal to the pool's volume passing through the filter, not each individual gallon being filtered once; because treated water returning through the inlets blends continuously with untreated water, only about 63 percent has actually been through the filter after one turnover period, which is why several turnovers are needed to clear a pool.

    Source: PHTA Pool & Spa Operator Handbook, turnover rate and dilution principleReport a problem with this question

21. 21. Which statement correctly describes what filtration does for pool water?

    • A.It works only on particles larger than a bather can see, and has no effect on turbidity
    • B.It removes dissolved minerals, which is why filtered water is softer
    • C.It physically removes suspended particles to reduce turbidity; killing pathogens is the job of disinfection, not the filter✓ Answer
    • D.It sterilizes the water as it passes through the media, so a well-run filter can substitute for a sanitizer residual

    Filtration is a mechanical process — media strain and adhere particles out of the stream, which is measured as clarity or turbidity — while inactivating bacteria, viruses and protozoa depends on a chemical disinfectant residual carried in the water itself; the two functions cannot substitute for each other.

    Source: PHTA Pool & Spa Operator Handbook, filtration versus disinfectionReport a problem with this question

22. 22. Algae keeps reappearing in one far corner of a large pool even though the sanitizer residual measured elsewhere is satisfactory and the flow meter shows the designed flow. What is the best first corrective step?

    • A.Adjust the directional return fittings so treated water is distributed into that corner, eliminating the dead spot✓ Answer
    • B.Aim the nearby returns straight at the skimmer to speed up surface flow
    • C.Close the main drain valve entirely so all flow comes from the skimmers
    • D.Run the pump at a higher speed than the system was designed for

    Adequate total flow does not guarantee good distribution; if inlets are not aimed to sweep every area, water stagnates in corners and steps where residual is consumed and algae takes hold, so redirecting the eyeball fittings restores circulation to the dead spot without changing the pump or the chemistry.

    Source: PHTA Pool & Spa Operator Handbook, return inlet placement and water distributionReport a problem with this question

Practice questions based on the Pool & Hot Tub Alliance (PHTA) Pool & Spa Operator Handbook and public CDC healthy-swimming guidance. CPO and Certified Pool/Spa Operator are marks of the Pool & Hot Tub Alliance; this site is not affiliated with or endorsed by PHTA. Water-quality limits, turnover requirements and bather loads are set by the health code where your facility operates — always follow that code and your local health department, and confirm current course requirements before testing. About the CPO certification →