Swimming Pool (C-53) · Lesson 2 of 8

Sizing a Filter: Flow Rate per Square Foot

"Sizing a Filter: Flow Rate per Square Foot" is lesson 2 of 8 in the Swimming Pool (C-53) Learning Hub. It opens with "The Filter Sees the Whole Pump Flow" and works through 6 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. The Filter Sees the Whole Pump Flow
  2. The Printed Maximum Filtration Rates
  3. Filter Area = Flow / Rate
  4. Backwash Rate and Backwash Piping
  5. Where the Backwash Waste Goes
  6. Pump Head Goes With the Filter Type

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Recirculation flow" through "Pump head".

What you'll be able to do

  • Explain the main idea of "Sizing a Filter: Flow Rate per Square Foot" in plain language you can use on the exam.
  • Apply this step with confidence: "Filter Area = Flow / Rate".
  • Apply this step with confidence: "Backwash Rate and Backwash Piping".
  • Apply this step with confidence: "Where the Backwash Waste Goes".
  • Apply this step with confidence: "Pump Head Goes With the Filter Type".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Filter Area = Flow / RateSame pool, same 93.5 gpm: three filter typesMINIMUM FILTER AREA AT 93.5 GPMRapid sand: 3 gpm/sf31.2 sq ftDiatomaceous earth: 2 gpm/sf46.8 sq ftCartridge: 0.375 gpm/sf249.3 sq ftBar length is proportional to area (same flow)BACKWASH AND PUMP HEADRapid sand backwash (min 15 gpm/sf)15 x 31.2 sq ft = 468 gpmPipe check at 6 ft/s (3125B.1)468 / (448.831 x 6) = 0.174 sq ftinside diameter 5.65 in. or largerMinimum pump head (3126B.1)Rapid sand: 45 ftDE (pressure) and cartridge: 60 ftVacuum DE: 20 in. vacuum, 40 ftArea = Flow / Rate | 93.5 gpm needs 31.2, 46.8 or 249.3 sq ftBackwash and filter drain waste is an indirect waste and never returns to the pool2025 CPC §813.1; 2025 CBC §3141B2025 CBC §§3126B.1, 3129B.1, 3130B.1, 3132B.1; 2025 CPC Useful TablesCC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: The Filter Sees the Whole Pump Flow

Every gallon the pump moves goes through the filter, so sizing starts from the recirculation flow you found from the turnover time. 2025 CBC §3128B.1 requires every filter, of any type, to be designed and built to the NSF/ANSI 50 performance standard, and §3128B.2 requires each filter vessel to be piped and valved so it can be isolated for repairs and backwashing. The 2025 Energy Code §150.0(p)3 carries a filter-size rule into single-family pools: filters must be at least the size specified in NSF/ANSI 50 for public pool intended applications.

Step 2: The Printed Maximum Filtration Rates

The CBC prints a ceiling on how fast water may pass through each square foot of filter area. Rapid sand pressure filter: 3 gpm per sq ft (§3129B.1). Diatomaceous earth filter, pressure or vacuum: 2 gpm per sq ft (§3130B.1). Cartridge filter: 0.375 gpm per sq ft (§3132B.1). For a high-rate sand filter, §3131B.1 sends you to the NSF/ANSI 50 flow rates and prints no number, so take that rate from the filter's NSF/ANSI 50 listing, not from memory.

The remaining 4 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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