The short answer
Drain and refill a regularly used residential hot tub about every 3 to 4 months. A spa that sees heavy use, a lot of guests, or a short-term rental schedule needs it closer to every 2 months. A lightly used spa with a tight cover can sometimes stretch to 5 or 6, but the 3 to 4 month mark is where most owners should plan on it.
Why hot tub water wears out
A hot tub holds a few hundred gallons of hot water and the same handful of people get into it over and over. Every soak adds sweat, skin oils, lotion, deodorant, and detergent from swimsuits. Every dose of sanitizer, pH adjuster, or shock leaves part of itself behind as dissolved solids once it has done its job.
None of that leaves the water on its own. It accumulates as total dissolved solids, and as the number climbs the water gets harder to manage: sanitizer gets used up faster, pH will not hold where you set it, and the water looks dull even when the test strip says it is fine. At some point chemistry cannot save it and the only fix is fresh water.
Signs the water is spent
You do not need a TDS meter to know it is time. Any of these means the water is at or past the point of a refill:
- The water clouds again within a day of being balanced and shocked.
- Foam forms at the jets and does not go away with a defoamer.
- There is a persistent chemical, musty, or sour smell even with the sanitizer in range.
- You are adding more sanitizer than you used to and the reading still drops fast.
- The pH will not stay put no matter how much adjuster you add.
- It has been 4 months and you cannot remember the last refill.
What a proper drain and refill involves
Pulling the drain plug and refilling with a hose is only part of the job. The plumbing lines and jet bodies hold old water and biofilm that a gravity drain leaves behind, and if you refill on top of it the new water starts off contaminated. A proper water change runs a plumbing purge treatment before the drain when the spa needs it, so the buildup in the lines is loosened and flushed out with the old water.
After the drain, the shell, jets, and waterline get cleaned, the filter gets a deep clean (not just a rinse, because that is where most of the old gunk ended up), and then the spa is refilled, dosed with startup chemicals, balanced, and sanitized. Heating the fresh fill back to temperature takes several hours after that.
TrueBlue does all of this as a Spa Refresh, starting at $449 for a standard residential hot tub, with the purge, filter deep cleaning, and startup chemicals included. Recurring Spa Care customers get a heads-up when the water is getting close so the refresh happens before the water goes bad instead of after.