Pump Run Time from Turnover: The Worked Card

Why this matters

Pump schedules on a route are mostly inherited: eight hours because the last company set eight, or because the timer trippers were already there. On a single-speed pump that was at least roughly tied to flow. On a variable-speed pump turned down to save power it is often wrong by half, and the first sign is a pool that clouds every August while the chemistry log looks fine. Turnover is the one number that ties hours to gallons. This card is the formula, worked at two speeds on the same pool, plus what one turnover actually buys you and what changes when the speed drops.

The formula and the worked sheet

Turnover is the time it takes for a volume of water equal to the whole pool to pass through the pump and filter once.

TURNOVER WORKSHEET     pool: 20,000 gal

turnover hours = gallons / (gpm x 60)

RUN A  full speed      flow 40 gpm (meter)
  40 gpm x 60 min    = 2,400 gal per hour
  20,000 / 2,400     = 8.33 h  (8 h 20 min)

RUN B  low speed       flow 25 gpm (meter)
  25 gpm x 60 min    = 1,500 gal per hour
  20,000 / 1,500     = 13.33 h (13 h 20 min)

CHECK the other way
  2,400 x 8.33       = 19,992 gal  (about 20,000)
  1,500 x 13.33      = 19,995 gal  (about 20,000)

Units: gallons divided by gallons per hour leaves hours. The 60 converts the pump's per-minute flow to per-hour. Get the volume from 'How to Estimate Pool and Spa Gallons'; a schedule built on a guessed volume is a guessed schedule.

Sanity check without a calculator. At 40 gpm the pump moves about 2,400 gallons an hour, so a 20,000 gallon pool takes a little over 8 hours. If your answer says 500, it is still in minutes and the 60 was left out.

Where the flow number comes from

The formula is only as good as the gpm in it, and there are three ways to get one. They are not equally good.

• A flow meter. An inline meter on the return side gives a measured figure. It needs the straight run of pipe its instructions call for before and after it, or it reads with turbulence. This is the number to use when it exists.
• The pump's displayed estimate. Many variable-speed pumps show an estimated flow computed from speed and power against a built-in model of the plumbing. It is good enough to schedule from when the pump has been set up for that pad, and noticeably off when the filter is dirty, a valve is half shut, or the pump was never configured. Note on the route sheet which one the number came from.
• The pump curve. Reading flow off the manufacturer's curve requires the total head of the system, which on most pads nobody has measured. A curve read at a guessed head is a guess drawn neatly. Use it to cross-check a display, not as the source.

The flow also moves with the filter. A cartridge at 10 psi over its clean reading is passing less water than the same cartridge clean, so the schedule set on a clean filter delivers less turnover as the week goes on. That is part of why the filter gets cleaned on pressure, per 'Filter Cleaning and Media Replacement', and sized for the turnover in 'Cartridge Filter Sizing Per Turnover Rate'.

What one turnover actually buys you

One turnover does not filter all the water once. The pool is mixing the whole time, so filtered water returns and gets pulled back through while some of the original water has not yet been reached. Under an assumption of good mixing, the share of the original water that has been through the filter at least once is:

That comes from treating the pool as a well-mixed tank, where the unfiltered share falls as e to the minus number of turnovers. Real pools with dead spots (steps, benches, the corners away from the returns) do worse than this. The practical reading: the second turnover is still doing a lot of work, the third much less. A common residential starting point is at least one turnover a day, moving toward two in high bather load, heavy debris or hot weather; tune by the water, not the clock. Commercial and public pools are a different case: the turnover period is set by the state or local health code that governs them, and nothing on this card replaces it.

Why the slower run costs more hours and less power

On a given set of plumbing, flow from a centrifugal pump rises roughly in proportion to speed, and the power it draws rises roughly with the cube of speed. These are the pump affinity relationships, and they hold when the system is mostly pipe friction with little static lift, which describes most residential pads.

Apply them to the two runs above, holding the plumbing constant:

• Flow ratio, low to full: 25 over 40 is 0.625.
• Power ratio: 0.625 cubed is about 0.244, so the low speed draws about a quarter of the full-speed power.
• Hours ratio: 13.33 over 8.33 is 1.6.
• Energy for one turnover: 0.244 times 1.6 is about 0.39 of the full-speed figure.

So the same turnover on the low speed takes 1.6 times as long and uses roughly 40 percent of the energy. That is the whole case for running long and slow, and it is why the hours have to go up when the speed comes down. A tech who cuts speed and keeps the old 8 hour schedule gets 8 times 1,500, or 12,000 gallons, which is 60 percent of one turnover a day. Payback math for the pump itself belongs to 'Variable-Speed Pump Retrofit ROI'.

Worked schedule: the split day

The customer wants the skimmers pulling hard while leaves are falling in the morning, and the pump quiet the rest of the day. Target: one turnover, 20,000 gallons.

• Skim window at full speed: 2 hours at 40 gpm. 2 times 2,400 is 4,800 gallons.
• Remaining volume: 20,000 minus 4,800 is 15,200 gallons.
• Low speed for the rest: 15,200 over 1,500 is 10.13 hours, about 10 hours 8 minutes.
• Total running: 2 plus 10.13 is 12.13 hours, about 12 hours 8 minutes.
• Check: 4,800 plus 10.13 times 1,500 (15,195) is 19,995 gallons. It closes.

Compared with run B, the split day saves about 1.2 hours of running (13.33 minus 12.13) and spends 2 of its hours at full power. Under the affinity ratio above, those 2 full-speed hours cost about the same energy as 8.2 low-speed hours (2 over 0.244), so the split day uses more energy than run B for the same turnover. In units of one full-speed hour: the split day is 2 plus 10.13 times 0.244, about 4.5; run B is 13.33 times 0.244, about 3.3; run A is 8.33. The split day spends about 1.4 times run B's energy and a little over half of run A's. That is a fair trade for real skimming in leaf season and a poor one in July. Programming the windows on the pump or the automation belongs to 'Variable-Speed Pump Programming and Optimization'.

What sets a floor under the low speed

Turnover is not the only constraint. Before settling on a low speed, confirm each of these at that speed:

• The heater's minimum flow. Below it, the pressure switch opens and the heater will not fire, which looks like a heater fault and is a schedule fault ('The Pool Tech's Boundary at the Heater: Gas, Power and Refrigerant').
• The salt cell's flow switch and its hours. A cell produces chlorine only while it sees flow, so its daily output is tied to pump hours. Cutting hours without raising the cell's output setting cuts the chlorine.
• Skimmer action. If the weir stops moving and debris floats past the skimmer, the speed is too low for surface cleaning during the hours that matter.
• A pressure-side cleaner or booster pump. Most need the main pump at a higher speed while they run.
• Freeze protection. In freezing weather the pump must move enough water through every line, which is its own setting and its own check ('Freeze Protection Verification Standard').

Where a correct schedule quietly stops delivering

The hours on the timer are not the gallons through the filter. Three things shrink the turnover without anyone touching the schedule.

• The filter loads through the week. Say the low-speed display reads 25 gpm the day the cartridge is cleaned and 21 gpm by day six as it loads (illustrative; read your own pad). The same 13.33 hours then move 13.33 times 21 times 60, about 16,800 gallons, which is 84 percent of a turnover. Building the schedule with a small margin over one turnover, or cleaning on a tighter pressure rise, covers the loaded end of the week.
• The clock drifts or resets. A mechanical timer with no battery loses its time in a power outage, and the pump then runs its 13 hours across the wrong part of the day, often across the night instead of the daytime hours the schedule was built around. Check the timer's clock against your watch at each visit, and after any storm.
• Someone overrides it. A customer turning the pump off for a party or an automation app left in a manual mode runs zero hours that day. The pump's run log or the automation history, where one exists, is the evidence.

How to verify the schedule is right

Write the volume, the measured or displayed gpm (and which), the hours at each speed and the computed gallons on the route sheet. Then let the water judge it over two or three visits: clear water, a skimmer that keeps the surface clean, a filter pressure rise that fits the debris load, and a chlorine residual that matches what the dose was sized to leave. If the water clouds while the chemistry holds, add turnover before adding chemicals.

References

• See related: Cartridge Filter Sizing Per Turnover Rate; Variable-Speed Pump Programming and Optimization
• See related: How to Estimate Pool and Spa Gallons; Filter Cleaning and Media Replacement; Variable-Speed Pump Retrofit ROI
• Your state or local health code for the required turnover period on public and commercial pools
• Pump manufacturer's curve and installation instructions; flow meter installation instructions for straight-pipe requirements