Turnover rate and the corners your water never reaches

What is turnover and how do you work yours out?

Turnover is pool volume divided by pump flow rate. A 20,000 gallon pool with a pump moving 40 gallons per minute is moving 2,400 gallons an hour, so one turnover takes a little over eight hours. That is the whole calculation.

The number people get wrong is flow rate, especially on a variable speed pump. The rated flow on the label is at a specific speed against a specific resistance. Running that same pump at a quiet overnight speed might be moving half of what the sticker says, and against a dirty filter, less again.

So work out turnover at the speed you actually run, not the speed the pump is capable of. A flow meter on the pad is the honest way to know. Failing that, use the pump curve and be conservative.

And remember that turnover changes when the filter loads. The same schedule that gave you two turnovers with a clean filter may be giving you one by the time the pressure has risen eight psi.

Why does one turnover not clean the whole pool?

Because water in a pool mixes continuously. Filtered water coming back through the returns immediately blends with unfiltered water, so the pool does not process itself in an orderly queue. After one full turnover, only around two thirds of the water has actually been through the filter.

Run a second turnover and you are around ninety percent. A third gets you into the high nineties. This is why recovery work calls for continuous running rather than a schedule, and why a single turnover a day is a maintenance minimum rather than a target.

For a normal residential pool in mild weather, one to one and a half turnovers a day is workable. In the heat of summer, with swimmers and heavy chlorine demand, two is a better aim. During a green recovery, the pump simply runs.

Where are the dead spots, and how do you find them?

The usual suspects: the steps and the area under them, behind and beneath a ladder, the corner furthest from any return, the shallow shelf or tanning ledge, the space behind a raised spa wall, and any corner with a bench or a bench seat.

Find them with your eyes rather than a gauge. Drop a small amount of dye or even just watch the surface debris. Where the debris collects and stays rather than drifting toward the skimmer, you have found a dead spot. On a windy day the entire surface pattern is wind driven and tells you nothing, so pick a still morning.

The other detector is the algae itself. If the same eighteen inches of wall goes green first every single time, that is not coincidence and it is not a chemical problem. That is a spot the water does not reach at anything like the concentration the rest of the pool gets.

How do you fix circulation without new equipment?

Aim the return eyes. This is free and it is the most underused fix in pool care. Point them so the pool rotates as a whole, generally all in the same rotational direction, angled slightly down rather than at the surface. Down keeps the deep water moving. At the surface it makes ripples and gasses off carbon dioxide, which raises pH.

Set one eye to push toward the worst dead spot. If your steps are the problem, aim the nearest return so the current wraps into them.

Balance the skimmer and main drain valves so both are pulling. A pool running entirely off the skimmer leaves the bottom stagnant, and a pool running entirely off the main drain does not skim.

Run the pump longer at a moderate speed rather than shorter at a high one. Longer run time at a lower speed filters more water per unit of electricity and keeps the whole pool moving for more hours of the day, which is exactly what dead spots need. And brush the spots you cannot solve. Some geometry is just bad, and thirty seconds of brushing per week beats a decade of arguing with it.