Spa Drain Interval and Refill Volume

The rule is arithmetic on a habit rather than a measurement, and its value is that it turns a vague sense that the water is getting tired into a date. WHY A SPA AND NOT A POOL is the whole insight, and it falls straight out of the relation. Dissolved solids, body oils, cosmetics and disinfection byproducts accumulate in proportion to bather load, and nothing removes them except replacing water -- filtration and sanitiser do not. So the interval is volume over load, and a spa is a small volume carrying a heavy load. The same handful of bathers on a full-size pool gives an interval measured in years, which is exactly why pools are not drained on a schedule and spas are, and why the two get managed so differently despite being the same chemistry. THE INTERVAL IS INVERSE IN BATHER LOAD, so a spa used twice as hard needs draining twice as often. A schedule set for a quiet household is wrong for the week the family visits, and the failure mode is not dramatic -- the water goes dull, foams, resists sanitiser and starts to scale, and the owner adds more chemicals to a problem that only fresh water solves. A TDS READING BEATS THE CONVENTION where one is available, because it measures the thing the rule estimates. Rising total dissolved solids makes sanitiser less effective and pushes the water toward scaling, and comparing a current reading against the fill water shows how much of the total is accumulation rather than what came out of the tap -- which matters where the supply is already hard. And a commercial spa is a different question: health codes commonly require far more frequent draining, continuous monitoring and recorded testing, independent of any rule of thumb, and a public bather load is both higher and much less predictable. This computes an interval from an entered volume and load. It does not measure or predict TDS, evaluate water balance, saturation index, sanitiser demand or the chemistry of the refill, account for supplementary treatment such as enzymes, clarifiers or a mineral system, determine what any health code requires for a public spa, or address the draining, disposal and refill chemistry themselves -- a refill starts a new balance and needs its own testing. The NSPF CPO handbook, the spa manufacturer's instructions, and the applicable health code govern.

the one-third rule: drain interval in days = spa gallons / (3 x average daily bathers), which is exactly inverse in bather load and linear in volume.

The residential spa drain convention as the NSPF CPO handbook and manufacturers state it. A TDS measurement, where available, supersedes the convention because it measures what the rule estimates.

One published rule of thumb.

Estimate. AHJ and licensed professional govern.

Field names used by the API: spa_gallons, daily_bathers, days_since_drain, alternative_bathers, fill_tds_ppm, current_tds_ppm, tds_limit_ppm, interval_days, interval_weeks, days_remaining, alternative_interval_days, tds_rise_ppm, refill_gallons

• A convention, not a measurement a TDS reading beats it where one is availablethe NSPF CPO handbook
• Residential only health codes require far more frequent draining for public spasthe applicable health code
• Refill chemistry not addressed a refill starts a new balance and needs its own testingthe spa manufacturer's instructions