Pool Turnover Time and Flow Rate: Auditing Your Pump Room

Most pools must turn over in 6 hours or less, and two details trip people up. Under the MAHC (check what your state adopted), volume includes any surge or balance tank (4.7.1.10.3.2), and only flow that actually passes through the filters counts (4.7.1.10.2, 4.7.1.10.3). Here is how to audit a pool against that from one flow-meter reading.

What are the maximum turnover times?

Table 4.7.1.10 in the 2024 MAHC Code sets these maximums:

Venue	Maximum turnover
All other pools (most pools)	6 hours
Diving pools	8 hours
Activity pools, lazy rivers, wave pools	2 hours
Plunge pools, runout slides	1 hour
Wading pools	1 hour (secondary treatment required)
Interactive water play	0.5 hour (secondary treatment required)

Spas, therapy and exercise pools use a second table by temperature and bather load, from 4 hours at 72 to 93°F with more than 2,500 gallons per person, down to 0.5 hour at 93 to 104°F. CDC's operating page also lists 30 minutes or less for splash pads. The required time is the lesser of the table value and the time the individual components need (4.7.1.10.3.1). Water pumped to a slide by a separate pump and not filtered does not count (4.7.1.10.2.1).

How do you audit turnover from a flow-meter reading?

Every filtration system must have a flow meter accurate to within plus or minus 5 percent of design flow (4.7.1.9.1). The audit is four steps. A made-up example: an apartment pool with 120,000 gallons and a 2,000 gallon surge tank. The filters have 12.6 square feet each, per the nameplate. The design flow is 360 gpm, and today's meter reads 337 gpm.

1. Volume: 120,000 + 2,000 = 122,000 gallons.
2. Turnover now: 122,000 ÷ (337 × 60) = 122,000 ÷ 20,220 = 6.03 hours. That is over 6.
3. Flow needed for 6 hours: 122,000 ÷ (6 × 60) = 338.9 gpm.
4. Filter loading: 337 ÷ (2 × 12.6) = 337 ÷ 25.2 = 13.4 gpm per square foot. The MAHC allows up to 15 with a 15 inch bed (5.7.2.2.1), so loading passes while turnover fails.

Two cautions. The meter tolerance is 5 percent of design flow, or 18 gpm here, so 337 could really be 319 to 355. And margin matters more than the pass-fail: (360 − 338.9) ÷ 360 = 5.9 percent. This pool has less room for flow loss than the meter's own error. If a filter is out of service, the other carries the whole flow: 360 ÷ 12.6 = 28.6 gpm per square foot, far over the limit. The Code lets you backwash while swimmers are in only if you have multiple filters, can isolate one, keep the system running as required, inject chemicals downstream of the isolated filter, and divert one filter volume to waste (5.7.2.2.4.1.2).

What are the operating rules around flow?

• Venues built or substantially altered after adoption run at design flow 24 hours a day, except as 4.7.1.10 allows; older venues also run 24 hours a day at design flow (5.7.1.9.1, 5.7.1.9.2). All components stay in continuous operation (5.7.1.1.1).
• Failure to continuously operate filtration and disinfection equipment is on the imminent health hazard list (6.6.3.1 item 7). The owner or operator must close the pool (6.6.3.2), and the health department can also order it.
• Backwash at the differential pressure rise the filter maker recommends, or when the system can no longer reach design flow (5.7.2.2.4). The Code asks you to schedule it when the pool is closed to swimmers (5.7.2.2.4.1). After a backwash with no swimmers, the pump and chemical feeders must run at least 5 minutes before anyone re-enters (5.7.2.2.4.1.1).

When is a turndown at night allowed?

Under 5.7.1.1.1.1 and 4.7.1.10.5, a turndown system may cut flow no more than 25 percent below the minimum design requirement, and only while the pool is unoccupied during posted closed hours. Clarity must meet 5.7.6, turbidity must stay under 0.5 NTU, and disinfectant and pH must stay in range the whole time.

Example arithmetic: the minimum flow for 6 hours is 338.9 gpm, so the floor is 0.75 × 338.9 = 254 gpm, which is a turnover of 122,000 ÷ (254 × 60) = 8.0 hours overnight. That reads "minimum design requirements" as the flow the 6-hour limit needs; if your health department reads it as the pool's 360 gpm design flow, the floor is 270 gpm (7.5 hours). Ask which reading it uses. Your local code may not allow turndown at all.

What does the bather-load method add?

Annex 4.7.1.10.2 describes the maximum sustainable bather load (MSBL) method. The Annex calls it a proposed method for the future. It does not replace Table 4.7.1.10, which stays required. The Annex says it can flag risk factors that call for more or less treatment, and it gives a recommended minimum design flow you can hold against your own. The steps:

1. Zone volume (ft³) = surface area × average depth.
2. Surface area per bather by depth: under 3 ft, 25 ft²; 3 to 6 ft, 30 ft²; 6.1 to 10 ft, 22 ft²; over 10 ft, 16 ft².
3. Bathers = volume ÷ (area per bather × average depth).
4. Raw flow = bathers × 5.34 gpm.
5. Turnover (h) = gallons ÷ (flow × 60), then multiply by the Annex factors that apply.

The Annex hotel example is an indoor pool, 20 by 30 ft, depth 4 to 6 ft. Volume: 20 × 30 × 5 = 3,000 ft³. Bathers: 3,000 ÷ (30 × 5) = 20. Flow: 20 × 5.34 = 106.8 gpm. Turnover: 3,000 × 7.48 ÷ (106.8 × 60) = 3.5 h. Factors for indoor (1.15) and limited-use hotel, apartment or condo (1.33): 3.5 × 1.15 × 1.33 = 5.35 h. The Annex compares this to a table value of 5 hours and keeps the lower. The Code table we read lists 6 hours for all other pools, so the lower number there is the 5.35 hours.

Now our made-up apartment pool from above. Say its main body is 3,200 ft² at 5 ft average depth (119,680 gallons, without the surge tank), outdoors and limited-use. Bathers: 16,000 ÷ (30 × 5) = 106.7. Flow: 106.7 × 5.34 = 569.6 gpm. Turnover: 119,680 ÷ (569.6 × 60) = 3.50 h. Factors: limited-use 1.33 and edge loading 1.1 (for pools over 100,000 gallons): 3.50 × 1.33 × 1.1 = 5.12 h. That needs about 119,680 ÷ (5.12 × 60) = 390 gpm. The pool's 360 gpm design flow meets Table 4.7.1.10, but its design flow is 30 gpm under what MSBL suggests. That is a reason to guard its flow closely, not a violation.

How do you catch falling flow early?

Record the flow reading right after a backwash as the pool's baseline, on the same meter each week. Meter error repeats, so a drop against the baseline is meaningful even when absolute accuracy is 5 percent. Example readings for the pool above:

Week	Flow (gpm)	Drop from 360	Turnover (h)
1	360	0%	5.65
2	357	0.8%	5.70
3	352	2.2%	5.78
4	347	3.6%	5.86
5	342	5.0%	5.95
6	337	6.4%	6.03

A 5 percent rule sends someone to the pump room at week 5, a week before turnover passes 6 hours. Our suggested trend rules (they are ours, not the MAHC's):

Signal	Action
Margin under 10% (baseline versus flow needed for the limit)	Review pump, strainer and meter calibration now, even with no drop
Drop 3 to 5% from baseline after a fresh backwash	Check strainer baskets, valve positions and the filter pressure trend
Drop 5% or more, or two weeks falling	Pump-room visit this week; log the cause
Reading at or below the flow needed for the limit	Same-day call; check clarity (see cloudy water)

Across 30 pools, keep one sheet with volume including surge, limit, flow needed, baseline, last reading and margin, sorted by smallest margin. Compare similar pools: one that drops faster than its neighbors points to a hydraulic or equipment issue, not to bather load.

What do the records need?

The daily record (6.4.1.6) includes pump and filter pressures or the flow-meter reading (item 4), the time of each backwash (item 9), and any equipment failure or power outage that interrupted circulation, filtration or disinfection for more than 1 hour (item 14). Item 17 adds turbidity meter readings. Records are kept for at least 3 years (6.4.1.2.1), enough to compare this season's baseline with last year's. See also preventive maintenance and the supervisor site visit checklist.

Poolean tracks scheduled work and repair orders for each pool, and managers can assign tasks and check progress. See the maintenance page or book a 30-minute call.

The short version

• Turnover (h) = (pool gallons + surge tank gallons) ÷ (filtered gpm × 60). Limits start at 6 hours for most pools.
• Check filter loading too: gpm ÷ filter area, at most 15 gpm per square foot with a 15 inch bed.
• Compute each pool's margin: the flow needed for its limit versus its baseline. A thin margin matters more than a pass.
• Turndown is capped at 25 percent below minimum design flow, only when unoccupied, and only where your code allows it.
• Log flow after each backwash and visit the pump room on a 5 percent drop, before turnover passes the limit.