How Often to Drain and Refill a Hot Tub (The TDS Rule)

Spa water doesn't last forever. In a small volume of hot water with concentrated bather waste, TDS (total dissolved solids) climbs fast. No amount of sanitization or chemistry adjustment compensates for water that's simply exhausted. Regular draining and refilling is the single most important spa maintenance step many owners skip.

Why spas need more frequent water changes than pools

A 400-gallon spa vs. a 15,000-gallon pool: a bather introduces roughly the same amount of body oils, sweat, and organic waste regardless of vessel size. In the pool, that waste is diluted by 37× compared to the spa. The spa's water becomes saturated with dissolved organics and TDS far faster.

The standard formula: divide by 3

Days between changes = Spa gallons ÷ 3 ÷ Average daily bathers

Example: 400-gallon spa, 2 bathers per day: 400 ÷ 3 ÷ 2 = 67 days. Regardless of use, quarterly is the practical minimum.

Signs your water needs changing

• Persistent foam that won't clear with defoamer
• Strong chloramine/bromine odor despite adequate sanitizer levels
• Cloudy water that doesn't clear with shock and clarifier
• Skin irritation or eye redness in regular bathers
• TDS above 1,500 ppm over your source water baseline
• Difficulty maintaining pH or alkalinity — high TDS creates buffering that fights adjustments

The drain and clean procedure

1. Line flush: Add a spa system flush product 24 hours before draining. Run jets on high to flush biofilm from plumbing lines.
2. Drain completely — use the drain valve or a submersible pump.
3. Clean shell and jets — wipe down with a spa surface cleaner. Remove and clean the filter cartridge with a filter cleaning spray.
4. Refill and balance — start chemistry fresh: TA first, then pH, then sanitizer. Let water equilibrate 30 minutes before testing.

The line flush step is the most skipped and most important. Biofilm in jet plumbing re-contaminates fresh water within days if not flushed out during the drain cycle.