Why Hot Tub Water Must Be Drained and Refilled

Sanitizer kills microorganisms and oxidizers burn off some contaminants — but a lot of what enters hot tub water never leaves it except through the drain. Every dose of every chemical adds dissolved solids. Dichlor adds cyanuric acid that accumulates dose after dose. Sweat, urea, and cosmetics leave byproducts behind even after oxidation. Evaporation removes pure water and leaves everything dissolved behind, concentrating it. That one-way accumulation is why every manufacturer publishes a drain interval — and why old water gets harder to manage no matter how good your routine is.

Fair statement of the chemistry: it’s not that sanitizer “can’t destroy anything” — oxidation genuinely removes many organics. It’s that the leftovers (salts, stabilizer, oxidation byproducts, minerals) are dissolved solids that only dilution or replacement removes.

How Often? What the Manufacturers Actually Say

FROG systems: drain and refill at least every 4 months (the mineral cartridge is timed to it).
Softub: the Owner’s Guide says 2–3 times per year; the current Softub Water Treatment Guide says 3–4 times per year — both add “or whenever proper water chemistry becomes difficult to manage.” We’d call that 3–4 months for a typical household, sooner with heavy use.
Rule of thumb used across the industry for usage-adjusted timing: divide your spa’s gallons by three, divide by average daily bathers, and that’s roughly your days between drains (a 300-gallon tub, 2 bathers a day → ~50 days). Treat it as an estimator, not a law — and let the water’s behavior overrule the calendar in either direction.

Two absolutes you can ignore: “three months is the maximum any tub can go” and “heavy use means 6–8 weeks, period.” Neither is a manufacturer rule. Light-use tubs legitimately run longer; party tubs die sooner. The symptoms below are the real clock.

Your water is telling you when

Chemistry stops responding — pH won’t hold, sanitizer demand climbs, doses that worked in month one do nothing in month four.
Persistent dullness, foam, or odor that shrugs off shocking and filter cleaning.
Water feels “thick” or leaves skin sticky — concentrated dissolved solids.
On dichlor systems: everything reads normal but chlorine acts sluggish — accumulated cyanuric acid slowing it down.

Partial replacement: the honest middle step

Draining a third to half and refilling fresh genuinely dilutes solids and stabilizer, and can buy weeks on a tub that’s borderline — it’s especially worth it on swim spas and large tubs where a full change is a project. But dilution only stretches the interval; when even partial changes stop helping, do the full reset.

Do the reset properly

Purge the plumbing with a pipe & jet cleanser in the old water, drain with a utility pump, refill through a pre-filter, and rebalance from your system’s guide. A fresh fill on dirty plumbing inherits the old water’s problems.

One claim we’re retiring: that old water “releases trapped chemical gases that attack the liner.” We found no manufacturer or technical source for it. Old water’s real offenses — accumulated solids, stabilizer, unresponsive chemistry, odor, and the pH swings that genuinely do threaten liners — are documented, and they’re plenty of reason to change it.

Drain and Refill FAQs

Because every chemical addition is itself dissolved solids, so “correcting” old water concentrates it further. Dosing treats what sanitizer and oxidizer can affect; it cannot remove salts, stabilizer, or byproducts. Past a point, each fix makes the underlying problem incrementally worse – fresh water is the only subtraction in the system.

Total dissolved solids – the sum of everything dissolved in the water: minerals from the fill, salts left by every chemical dose, and byproducts of oxidized contaminants. It’s not literally “everything that ever entered the tub” (filtration and oxidation do remove things), but it rises steadily with time and dosing, and high TDS water behaves unpredictably.

Softub’s own documents say 2-3 times per year (Owner’s Guide) to 3-4 times per year (current Water Treatment Guide), or whenever chemistry becomes difficult to manage. In practice that’s every 3-4 months for typical use – sooner for a heavily used 140-gallon Sportster, where the small volume concentrates everything faster.

As an estimator, yes – it’s a long-standing industry rule of thumb that scales drain frequency to bather load, and it’s useful for setting expectations (300 gallons ÷ 3 ÷ 2 daily bathers ≈ 50 days). It is not a manufacturer specification. Follow your manufacturer’s published interval and let water behavior fine-tune it.

Yes, in the right spot: when water is getting difficult but isn’t fully spent, draining a third to half and refilling dilutes dissolved solids and stabilizer and often restores easy chemistry for a while. It’s a stretch, not a substitute – when partial changes stop helping, purge, drain fully, and refill.

Additional Resources

Continue with the guide that best matches what you are seeing or what you need to do next.

Softub Plumbing Flush Guide — The purge step that makes the fresh fill count.
How to Drain and Store a Softub — If the tub is coming down rather than refilling.
Very High Total Alkalinity in a Hot Tub — When stubborn chemistry is fill-water, not age.
EZ Hot Tub Water Care Hub — The refill, balance, and startup routine for your system.