LicenseReady
0
Residential Pool/Spa Contractor/Trade Knowledge

Hydraulics & Circulation

25% of this exam

The most calculation-heavy domain on the trade exam. Three formulas — volume, turnover-to-flow, and filter area × rate — answer most of the math, and the concepts (TDH, friction loss, velocity limits, suction vs return) answer the rest. Practice the arithmetic until the only question is which formula applies.

Core concepts

Volume math comes first

Rectangular: length × width × average depth × 7.5 gal/cu ft. Circular: 3.14 × radius² × average depth × 7.5 — square the radius, not the diameter; that's the classic trap. Every dosing and flow question starts from volume.

Turnover converts to gpm in two steps

Volume ÷ turnover hours = gallons per hour, then ÷ 60 = gpm (36,000 gal ÷ 10 hr ÷ 60 = 60 gpm). Run it backward for turnover time: volume ÷ gpm = minutes. Forgetting the ÷ 60 is the built-in distractor.

The pump fights the whole system

Total dynamic head is every resistance the pump must overcome — pipe and fitting friction, equipment, elevation — and it sets where the pump lands on its curve. Friction loss rises roughly with the square of flow: double the flow, quadruple the loss. Slower is dramatically cheaper: pump power falls with the cube of speed, so half speed draws about one-eighth the power.

Each line has a job and a limit

Suction lines (skimmers, main drains → pump) are kept to about 6 ft/s; returns about 8 ft/s. The skimmer equalizer keeps the pump supplied with water if the level drops below the weir. Dual suction outlets are commonly separated at least 3 feet so one bather can't block both.

Key facts to know cold

Gallons per cubic foot7.5 — the constant in every volume problem
Flow from turnoverVolume ÷ hours ÷ 60 = gpm
Velocity limits (common practice)~6 ft/s suction, ~8 ft/s return
Friction vs flowLoss rises roughly with the square of flow rate
Pump affinityHalf speed ≈ half flow but only ~1/8 the power
Filter flow checkDesign gpm ÷ filter area must not exceed the media's rated gpm/sq ft

See it drawn out

Fig — Pool volume in gallons
  1. Step 1 — surface area
    Circular: π r² (20 ft dia → 3.14 × 10² = 314 sq ft)
  2. Step 2 — cubic feet
    × average depth (314 × 5 = 1,570 cu ft)
  3. Step 3 — gallons
    × 7.5 gal per cu ft = 11,775 gallons

    Volume drives every downstream number: turnover, flow rate, and chemical dosing.

Where it lives in your books

The real exam is open book. Knowing which book — and which tab — answers this domain is worth as much as memorizing it.

manual

Pool References

Safety and barrier questions are code lookups; hydraulics questions are formulas. Keep the formula sheet (turnover, volume, TDH) tabbed in the handbook and the barrier rules tabbed in the code.

9 recommended tabs

Lookup strategy

  • · Tab the handbook formula pages (volume, turnover, TDH) and write the formula down before plugging numbers — the wrong answers are built from skipped steps.
  • · Watch units: turnover answers come out in minutes or hours, flow in gpm. Convert before picking a choice.

Reading isn't learning — retrieval is.

40 questions in this domain, each with an explanation and source.