6.3 Circulation, Turnover, and Flow

Turnover is a time, not a magic spell. It is the time required to circulate a volume of water equal to the pool volume through the recirculation system. It is not a promise that every molecule went through the filter. Some parcels short-circuit to a nearby inlet; some sit in a corner for many theoretical turnovers. Domain 2B pairs collection with circulation; this section is the flow math and the mixing picture. Pump curves and total dynamic head, and filter media ratings, remain the next chapter. You still need enough physics here to read a flow meter honestly.

Independent teaching uses the public AFO formula convention, 2024 MAHC turnover tables as a voluntary model, and your AHJ. The 2025 NRPA AFO candidate handbook does not reprint a national AFO statute of required hours by venue type. Do not memorize a fake NRPA table. Memorize the formula, run the arithmetic, then open local code or the MAHC table if your jurisdiction adopted it.

The definition, twice, because the trap is popular

Correct: If the pool holds 120,000 gallons and you move 120,000 gallons through the pump in 6 hours, that is a 6-hour turnover.

Incorrect: After 6 hours the water is fully filtered and every drop is new. Mixing is imperfect. Classic complete-mix teaching sometimes says it takes about four turnovers to exchange something like 98% of the water in a well-mixed tank — and even that is a model, not a photograph of your dead corner. Use turnover to size flow. Use inlet aiming, brushing, and clarity to judge mixing.

2024 MAHC §4.7.1.10.2: turnover time is calculated from total volume divided by total design recirculation flow rate through the filtration process. That is the same idea as the operator formula below.

The formula you will use

GPM = volume in gallons / (turnover hours × 60)

Why 60? Turnover is in hours. Pumps and meters are rated in gallons per minute. There are 60 minutes in an hour.

Rearrange when you need to:

• Turnover hours = volume gallons / (GPM × 60)
• Volume gallons = GPM × turnover hours × 60

Worked example 1 — handbook-scale pool

Volume = 120,000 gallons (also the AFO PSF base volume from the chemistry chapters). Required turnover = 6 hours.

GPM = 120,000 / (6 × 60) = 120,000 / 360 = 333 GPM.

If your flow meter reads 250 GPM, you are not a little low. Actual turnover = 120,000 / (250 × 60) = 120,000 / 15,000 = 8 hours. Chemistry and clarity will feel that extra two hours.

Worked example 2 — large municipal basin

Volume = 360,000 gallons. Turnover = 6 hours.

GPM = 360,000 / 360 = 1,000 GPM.

Same 6-hour intent, three times the volume, three times the flow. A 333 GPM pump package that was perfect on the 120,000-gallon tank is not a 360,000-gallon solution.

Worked example 3 — spa, shorter clock

Volume = 5,000 gallons. Turnover = 0.5 hours (30 minutes) — a common hot-water spa maximum in MAHC-style codes.

GPM = 5,000 / (0.5 × 60) = 5,000 / 30 = 167 GPM.

Notice: 167 GPM is also 60,000 gallons at a 6-hour turnover (60,000 / 360 = 167). A small hot spa can demand the same flow as a modest pool because the clock is twelve times faster.

Worked example 4 — wading pool

Volume = 20,000 gallons. Turnover = 1 hour (MAHC-style wading maximum).

GPM = 20,000 / 60 = 333 GPM.

Same 333 GPM as the 120,000-gallon, 6-hour pool. Shallow dirty water needs fast exchange, not a small pump because it is a small pool.

Codes specify maximum turnover times — by venue, by AHJ

Public codes usually state a maximum time (you may turn over faster; you may not be slower than the cap). They vary by venue type because a spa at 104°F with a dense bather load is not a 50-meter diving well.

2024 MAHC Table 4.7.1.10 (voluntary model, not federal law, not an NRPA AFO hours table) includes these maximum times:

Spa, therapy, and exercise pools get a second MAHC table: hot water ≥ 93–104°F (34–40°C) is 0.5 hours or less for all loads. Cooler therapy/exercise water can be 1, 2, or 4 hours depending on gallons per person. Spas are much shorter than pools — often 30 minutes in that hot-water row. Follow the AHJ. If your state still says 6 hours for public pools and 30 minutes for spas in older language, that is what the inspector has. If your state adopted MAHC tables, use those rows. If NRPA's public handbook does not list a number, do not invent an AFO required-hours chart.

New or substantially altered venues under MAHC §5.7.1.9.1 are to be operated at the designed flow to provide the required turnover 24 hours per day except as the code allows. Older venues have a separate operational paragraph. Night setback is a code question, not a money-saving default.

Dead spots, short-circuiting, and inlet aiming

Flow in gallons per minute is necessary and not sufficient.

Short-circuiting: a return inlet points at a nearby skimmer or gutter. Treated water leaves the inlet and is collected again without sweeping the basin. The flow meter looks great. The far corner does not.

Dead spots: stairs, lazy corners, under underwater benches, behind ladder rails, deep-end floors when inlets are all shallow. Algae starts there first. Combined chlorine and temperature stratify there.

Inlet aiming: 2024 MAHC §5.7.1.3 tells operators to check inlets at least weekly for rate and direction of flow and to adjust them to produce uniform circulation and a uniform disinfectant residual. That is a wrench-and-eyeball skill: look at how the water moves dust, bubbles, or a little dye where permitted. Floor inlets and wall inlets are different patterns; both can be aimed wrong.

Occupied versus night flow: bodies mix water (good) and add waste (bad). Some plants legally reduce flow overnight; mixing gets worse just when you might also cut air-handler outdoor air. If you reduce flow, know the code, watch the meter, and do not cut below the turnover cap. If you do not reduce flow, the meter should still match design — occupancy is not an excuse for a clogged strainer.

Flow meter versus design GPM

The design GPM is the number on the drawings that makes the turnover math true. The flow meter is the number the water is actually doing. 2024 MAHC §5.7.1.8: flow meters shall be provided and maintained in proper working order. A stuck meter that forever shows design flow is a decoration.

Read the meter at the same time you read filter pressure. Typical story:

1. Clean system: meter at design (say 333 GPM), filter differential at the clean baseline.
2. Filter loads: head rises, pump output falls, meter drops (say 260 GPM).
3. Actual turnover lengthens. Inlets soften. Skimmers weaken.
4. You clean or backwash (next chapter's media rules), differential drops, flow returns.

A clogged hair-and-lint strainer does the same thing from the suction side: flow falls before the filter pressure ever tells on you. That is why collection baskets are part of circulation math.

If the meter reads above design, you may be over-pumping a filter (exceeding media flow rate — next chapter) or reading a meter on a bypass. If it reads below design, do not shrug. Either restore flow or you are operating a longer turnover than the wall plaque claims.

These are arithmetic examples, not a code table. Plug your volume and your legal maximum hours into the same formula.

Trap: nameplate GPM on the pump

A pump labeled 200 GPM at some catalog head is not a flow meter. Impellers wear, baskets clog, filter cakes grow, and valves get throttled. Believe the meter, then ask why it disagrees with design. Total dynamic head math will explain the why in the next chapter; the operator action today is: restore strainers and filter cleanliness, open the valves that should be open, and aim inlets so the 333 GPM you paid for actually sweeps the tank.

Circulation is a clock (turnover), a rate (GPM), and a pattern (mixing). Run the formula on 120,000 gallons and on 360,000 gallons until 333 and 1,000 are automatic. Then walk the basin looking for the water that the formula never met.