Reference

The pool glossary

Every term a pool owner runs into, defined the way a service tech would explain it — including the ones the industry uses slightly wrong.

80 terms

Water chemistry

Bather load: How many people use the pool and for how long. Every swimmer contributes organics that consume chlorine, which is why a pool party can drop free chlorine to zero in an afternoon.
Black algae: Not actually algae, despite the name — it is cyanobacteria. Grows in dark raised spots with a protective layer over a root structure that digs into the plaster. Needs mechanical brushing with a stainless brush before chemicals can reach it.
Breakpoint chlorination: Raising free chlorine high enough — roughly ten times the combined chlorine reading — to destroy chloramines completely rather than adding to them. Getting close but not reaching breakpoint makes the smell worse, not better.How to shock a pool
Calcium hardnessalso: CH: How much dissolved calcium your water holds. Too little and the water pulls calcium out of plaster, grout and even the heat exchanger. Too much and you get scale. Plaster pools want 200–400 ppm; vinyl and fiberglass can run lower.
Calcium hypochloritealso: Cal hypo: Granular chlorine at 65–73% available chlorine. Cheap and shelf-stable, but it adds about 0.7 ppm of calcium hardness for every 1 ppm of chlorine, which matters in hard-water regions.
Chlorine lock: A widely used and slightly misleading term for what happens when cyanuric acid climbs so high that normal chlorine levels stop working. Nothing is chemically "locked" — the chlorine is simply far less active, and the fix is dilution, not a product.
Clarifier: A polymer that clumps very fine particles together so the filter can catch them. Slower than a flocculant but far less work — you filter the problem out rather than vacuuming it out.
Combined chlorinealso: Chloramines, CC: Chlorine that has already reacted with sweat, urine, sunscreen or other organics and is now largely spent. It is what produces the sharp "chlorine smell" people blame on too much chlorine — which is exactly backward. Above 0.5 ppm it is a signal to shock.Free vs total chlorine
Cyanuric acidalso: CYA, Stabilizer, Conditioner: A sunscreen for chlorine. It binds loosely to chlorine and protects it from UV, dramatically extending how long it survives outdoors — but it also slows chlorine down, so your required free chlorine level rises with it. The single most under-tested number in residential pools.Cyanuric acid explained
Dichlor: Stabilized granular chlorine at about 56%. Convenient and pH-neutral, but it adds around 0.9 ppm of cyanuric acid per 1 ppm of chlorine. Fine occasionally, a slow trap if used all season.
DPDalso: Diethyl-p-phenylenediamine: The reagent that turns pool water pink in the presence of chlorine. In a standard DPD kit you match that pink against printed color blocks, which works at everyday levels and becomes guesswork above about 5 ppm, where all the pinks look alike.How to test pool water
FAS-DPD: A drop-count titration for chlorine, and the reason to own a better test kit. Instead of matching a shade of pink you add drops until the pink goes clear and count them, so the result is a number you counted rather than a color you judged. Reads to 0.2 ppm and stays accurate at shock levels where color kits saturate.Best pool test kits
Flocculantalso: Floc: Drops suspended particles to the floor as a heavy sludge, which you then vacuum to waste. Fast and effective, and a genuine mess if you skip the vacuum-to-waste step.
Free chlorinealso: FC: The chlorine still available to sanitize your water. This is the number that matters day to day, and the one your target range refers to. Everything else in a chlorine reading is either already spent or not doing useful work.How to balance pool chemistry
Green algae: The common one. Free-floating or clinging, turns water green and slimy, and almost always the result of free chlorine sitting below the level your CYA demands. Responds quickly to sustained shock levels and brushing.Cloudy and green water
Langelier Saturation Indexalso: LSI, Saturation index: A single number combining pH, temperature, calcium hardness, alkalinity and TDS to say whether your water will dissolve surfaces, scale them, or leave them alone. Between −0.3 and +0.3 is balanced. It is the number professionals actually manage to.
Muriatic acidalso: Hydrochloric acid: The standard product for lowering pH and total alkalinity, usually sold at 31.45% strength. Always dilute into water — never add water to acid — and never store it near chlorine.
Mustard algaealso: Yellow algae: A chlorine-tolerant algae that looks like sand or pollen on shaded walls and steps, and returns fast if you miss any. Requires higher shock levels, thorough brushing and disinfection of anything that touched the water.
ORPalso: Oxidation-reduction potential: A live electrical measurement, in millivolts, of how strongly the water is oxidizing — in practice, how hard the sanitizer is actually working rather than how much of it is present. Automated controllers dose against it. The probes drift and need calibrating, and an uncalibrated one confidently dosing chemicals is worse than no controller at all.Pool automation guide
pH: How acidic or basic your water is, on a scale where 7 is neutral. Pool water wants 7.4 to 7.6. Low pH corrodes metal and etches plaster; high pH causes scale, cloudy water and reduced chlorine effectiveness.Total alkalinity vs pH
Phosphates: Algae nutrient that enters pools through fill water, fertilizer runoff and some pool chemicals. High phosphates do not cause algae on their own, but they make an under-chlorinated pool far quicker to bloom.
Sequestrantalso: Metal sequestrant, Chelating agent: Holds dissolved metals such as iron and copper in solution so they cannot stain surfaces. It does not remove metals; it manages them, and it degrades, so it needs re-dosing.
Shockingalso: Superchlorination: Raising free chlorine sharply to destroy chloramines, kill algae or recover from contamination. The correct target is set by your cyanuric acid level, not by a number on a bag.How to shock a pool
Soda ashalso: Sodium carbonate: Raises pH quickly and alkalinity slightly. If alkalinity is already fine, aerating the water raises pH for free and is usually the better move.
Sodium bicarbonatealso: Baking soda, Alkalinity increaser: Raises total alkalinity with only a small effect on pH. About 1.5 lbs per 10,000 gallons raises alkalinity roughly 10 ppm. Very hard to overdose dangerously.
Sodium hypochloritealso: Liquid chlorine, Bleach: Liquid chlorine, typically 10–12.5% strength for pool grade. Adds no cyanuric acid and no calcium, which makes it the cleanest thing to dose a pool with. It loses strength on the shelf, so buy fresh and store out of sunlight.
Total alkalinityalso: TA: The water’s resistance to pH change — the buffer that stops pH bouncing every time something is added. Target 70–120 ppm. Set alkalinity before you fine-tune pH, because alkalinity is what holds pH in place.Total alkalinity vs pH
Total chlorinealso: TC: Free chlorine plus combined chlorine. Useful only as a way to calculate combined chlorine by subtraction.
Total dissolved solidsalso: TDS: Everything dissolved in your water added together — salt, calcium, chlorides, the residue of every chemical ever added. Very high TDS makes water dull and chemicals less effective, and the only real fix is partial drain and refill.
Trichlor: Stabilized chlorine tablets, around 90% available chlorine and strongly acidic. Extremely convenient, and the leading cause of runaway cyanuric acid and sinking pH in residential pools.
White water moldalso: Pink slime: Biofilm growth, usually in low-circulation areas and inside plumbing, that looks like tissue paper or pink slime. Chlorine alone struggles with it because biofilm shields the organisms underneath.

Equipment & plumbing

Actuator: The motorized head on an automated valve that turns it between positions, letting a controller switch between pool, spa or water features on a schedule.
Backwash: Reversing flow through a sand or DE filter to flush trapped debris out to waste. Cartridge filters are not backwashed; they are removed and rinsed.
BTUalso: British thermal unit: The unit heaters are sized in. One BTU raises a pound of water one degree Fahrenheit, so a gallon takes about 8.34 of them per degree — which is why a 20,000 gallon pool needs roughly 167,000 BTU for every single degree of rise. Gas heaters are quoted at a fixed input; heat pump output falls as the air gets colder, so compare those at the temperature you will actually run them.Pool heater types compared
Cartridge filter: A filter using pleated fabric elements, which Pentair rates to 20 microns on its Clean & Clear. No backwashing means no water wasted, and cleaning is a manual rinse two to four times a season.Filter types compared
Cavitation: Vapor bubbles forming and collapsing inside a starved pump, producing a distinctive gravel-in-a-bucket noise. It destroys impellers and seals, and it means the pump is being asked for more water than the suction side can deliver.
Check valve: A one-way valve that stops water flowing backward — commonly fitted before a heater or salt cell, and after a chemical feeder, so equipment is not exposed to concentrated chemical when the pump stops.
DE filteralso: Diatomaceous earth filter: A filter using grids coated in diatomaceous earth powder, filtering to 3–5 microns — the finest of the three common types. Produces the clearest water and asks the most of the owner.Filter types compared
Filter pressure: The reading on the gauge downstream of the pump. Write down the pressure when the filter is freshly cleaned; a rise of 8–10 psi above that is your cue to clean it, regardless of the calendar.
GPMalso: Gallons per minute, GPH, Flow rate: How fast water moves through the system. It is the number filters, heaters and salt cells are actually rated against, and most residential pools run somewhere around 40 to 60 gpm. Equipment sold by the hour uses gph instead — the same figure multiplied by sixty.Pump sizing guide
Heat exchanger: The component inside a heater where combustion heat or refrigerant heat transfers into the water. Cupronickel versions resist chemically aggressive water far better than standard copper.
Heat pump: A pool heater that moves heat from the air into the water rather than burning fuel. Very efficient in warm weather, progressively less effective as air temperature falls below about 50°F.Heater types compared
Impeller: The spinning vaned disc inside a pump that actually moves the water. A partially blocked impeller is one of the most common causes of low flow that looks like a failing pump.Pump not priming
Inches of mercuryalso: inHg, Suction vacuum, Vacuum gauge: The unit a vacuum gauge reads in, used on the suction side of the pump where water is being pulled rather than pushed. It measures how far below atmospheric pressure the pipe has been drawn — a perfect vacuum is about 29.9 inHg, and a healthy residential suction side sits around 5–15. Higher than usual means something is restricting the flow; low with air in the pump lid means a suction-side leak, because the pump is drawing air instead of water.Pump not priming
Main drain: The suction outlet at the deepest point, which pulls water from the bottom of the pool. Modern installations use two or a single compliant cover to prevent entrapment.
Mechanical sealalso: Shaft seal: The seal where the motor shaft enters the wet end of the pump. When it fails you get water dripping between the motor and the pump body — a cheap part, and a motor-killer if ignored.
Multiport valve: The multi-position valve on sand and DE filters that selects filter, backwash, rinse, waste, recirculate or closed. Never move it with the pump running — that is how the internal spider gasket gets destroyed.
Prime: The state of a pump being full of water and moving it. Pumps cannot move air, so if air gets into the suction side the pump loses prime and stops pulling water.Pump not priming
Salt chlorine generatoralso: SWG, Salt cell, Chlorinator: A cell that electrolyzes dissolved salt into chlorine as water passes through it. A saltwater pool is still a chlorine pool — it simply manufactures its chlorine continuously instead of having it poured in.Salt systems explained
Sand filter: A filter using a bed of graded silica sand or glass media. Pentair puts traditional sand at 50 to 100 microns, which makes it the least fine of the three and the most forgiving to own.Filter types compared
Skimmer: The wall-mounted opening that pulls surface water — and floating debris — into the filtration system. Most pools have one or two, and they only work properly when the water level sits mid-mouth.
THPalso: Total horsepower: Rated horsepower multiplied by the service factor, and the only horsepower figure worth comparing. A "1.5 HP" pump with a 1.65 service factor is doing the work of nearly 2.5 THP and will outwork a "2 HP" pump with a service factor of 1.0 — so the big number on the box routinely gets the comparison backward.Pump sizing guide
Total dynamic headalso: TDH, Feet of head: The total resistance the pump has to overcome — pipe friction, fittings, elevation, filter and heater. Sizing a pump without knowing head is how pools end up with pumps that are far too large.Pump sizing guide
Turnoveralso: Turnover rate: The time it takes to circulate a volume of water equal to the whole pool through the filter. Residential pools generally aim for one turnover per day, achieved slowly rather than quickly.
Variable-speed pumpalso: VS pump, VSP: A permanent-magnet pump whose speed can be dialed to any RPM. Because power falls much faster than speed, running long and slow typically cuts pump energy by 60–80% versus a single-speed pump.Pump running cost calculator
Weiralso: Skimmer door, Flapper: The hinged flap in the skimmer mouth. It accelerates surface flow and stops debris washing back out when the pump shuts off. A stuck or missing weir noticeably reduces skimming performance.

Construction & finishes

Bond beam: The thickened top section of a concrete pool shell that carries the structural load around the perimeter. Cracks here are structural and are not a cosmetic repair.
Coping: The capping material around the pool edge where the shell meets the deck. It protects the shell edge, sheds water away from the pool, and is the single most visible detail in most pool designs.
Expansion joint: The flexible gap between the pool shell and the deck, sealed with mastic. It absorbs ground movement; when it fails, deck load transfers into the coping and tile line and cracks them.
Fiberglass shell: A one-piece molded pool shell delivered by truck and set into an excavation. Fastest to install and the smoothest surface, at the cost of being limited to the manufacturer’s available shapes and sizes.
Freeboard: The vertical distance from the water surface to the top of the coping or tile. Too much and skimming suffers; too little and water washes over the edge with every swimmer.
Gelcoat: The pigmented outer resin layer of a fiberglass pool. It is the waterproof and cosmetic surface, and it can chalk or fade with age and aggressive water chemistry.
Gunite: A dry concrete mix hydrated at the nozzle and sprayed over a rebar cage to form a pool shell. Allows any shape, takes the longest to build, and lasts longest with periodic resurfacing.Inground pool types compared
Hydrostatic relief valve: A valve in the floor of a concrete pool that lets groundwater push in rather than lifting the shell. It is why draining a pool in a high water table without professional advice can literally float it out of the ground.
Pebble finish: An interior finish of small pebbles in a cement matrix. More durable and forgiving of chemistry swings than plain plaster, rougher underfoot, and typically 15–20 years of life.
Plasteralso: Marcite, White coat: The cement-based interior finish on a concrete pool. Standard white plaster typically lasts 7–12 years before it needs replacing; aggregate finishes last longer.
Shotcrete: The wet-mix cousin of gunite, delivered already mixed and sprayed through a hose. Similar end result, different placement method, and often more consistent because the mix is batched rather than blended at the nozzle.
Tanning ledgealso: Baja shelf, Sun shelf: A shallow flat area, usually 6–12 inches deep, at one end of the pool. The most requested design feature of the last decade and genuinely useful for small children and lounge chairs.
Vanishing edgealso: Infinity edge, Negative edge: A wall built slightly below water level so water spills over into a catch basin and is pumped back. Visually spectacular, and it adds a second pump, a basin, and a permanent maintenance obligation.
Vinyl liner: A fitted waterproof membrane held in a steel, polymer or concrete wall structure. The structure lasts decades; the liner itself is a consumable, typically replaced every 8–12 years.Inground pool types compared

Safety & standards

ASTMalso: ASTM International, ASTM F2208, ASTM F1346: The standards body whose specifications pool safety equipment is tested against — F2208 for residential pool alarms, F1346 for safety covers. The distinction that matters when shopping is between certified to a standard, meaning an independent lab tested it, and the looser compliant with, which can mean the manufacturer believes it would pass.Best pool alarms
Barrieralso: Pool fence: The physical enclosure separating the pool from the house and yard. Baseline code is a 48-inch height, no gaps over 4 inches, no external footholds, and a self-closing, self-latching gate that swings away from the pool.Pool barrier requirements
CPOalso: Certified Pool Operator: The most widely recognized operator certification in North America, covering water chemistry, circulation, filtration and code compliance. Commonly required for commercial pool operators, and useful shorthand for a professional who has been formally trained.
Entrapment: When suction at a drain traps a body, limb or hair against an outlet. Compliant covers, dual drains and vacuum release systems exist specifically to make this impossible.
Equipotential bonding: The buried copper grid tying pool shell, rebar, coping, deck, ladders and equipment to the same electrical potential, so no voltage difference can exist across a swimmer. It is not the same thing as grounding, and both are required.
GFCIalso: Ground fault circuit interrupter, RCD: A device that cuts power within milliseconds when current leaks to ground. Required on pool equipment circuits and pool lighting, and worth testing at the start of every season.
ISPSCalso: International Swimming Pool and Spa Code: The model code most US jurisdictions base their pool rules on, covering barriers, drains, circulation and safety equipment. Local amendments vary, so the enforced version is always the one your inspector holds.Pool barrier requirements
Layers of protection: The drowning-prevention model that assumes any single measure will eventually fail. Supervision, a compliant barrier, door and gate alarms, swim competence and rescue readiness each catch different failures.Drowning prevention
SVRSalso: Safety vacuum release system: A device that detects a suction blockage and immediately kills the pump or releases the vacuum. Required in some single-drain configurations under the VGB Act.
VGB Actalso: Virginia Graeme Baker Pool and Spa Safety Act: US federal law requiring anti-entrapment drain covers and, in some configurations, secondary safety systems. Covers are date-stamped and must be replaced when they expire — a check almost no homeowner knows to make.

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