Hot tub water should be completely changed every 3–4 months depending on use. A proper refill includes a line flush before draining, a thorough clean of the shell and filter bay, and careful chemistry setup before the first soak.
Key Facts
- Most hot tubs need a complete water change every 3–4 months, or when TDS exceeds 1,500 ppm above fill baseline.
- Always run a pipe flush product 1–2 hours before draining to remove biofilm from plumbing lines.
- Fresh water chemistry must be tested and balanced before the first soak — do not enter immediately after refilling.
- The correct setup sequence for fresh water: TA first, then pH, then hardness, then sanitiser.
Before the Drain
One to two hours before draining, add a spa line flush product to the existing water and run the jets at high speed for 30–60 minutes. Pipe flush removes biofilm — the protective coating that bacteria form on the interior surfaces of the plumbing lines. Without flushing, this biofilm survives the drain and re-establishes itself in the fresh water. The flush water will often turn brown or produce foam as it dislodges biofilm — this is expected. Drain the flush water away from lawn areas (it is highly treated), then rinse the spa shell.
Clean While Empty
With the spa drained, clean the shell with a spa-surface cleaner (not a household cleaner — these leave residue). Pay particular attention to the waterline area, jet housings, and the filter housing. Remove the filter cartridge and clean or replace it. Wipe down the underside of the cover. Rinse all surfaces thoroughly before refilling — any cleaner residue will cause foaming. If the jets have hard mineral scale deposits, apply a diluted citric acid solution, wait 10 minutes, and rinse.
Setting Up Fresh Water
Fill the spa using the hose inserted through the filter housing (not over the shell) to minimise air entrainment in the plumbing. Heat to operating temperature (100–104°F). Test the fresh water before adding any chemicals to establish the baseline. Add chemicals in order: 1) Sodium bicarbonate to raise TA to 80–100 ppm. Wait 4 hours. 2) pH increaser or acid to bring pH to 7.4. Wait 2 hours. 3) Calcium chloride if hardness is below 150 ppm. 4) Add chlorine to reach 3–5 ppm. Wait 30 minutes. Test all parameters and confirm they are in range before the first soak.
Examples
Thursday 6pm: add pipe flush to running spa. Thursday 8pm: drain spa. Thursday 9pm: clean shell and filter. Friday 7am: refill with hose through filter bay. Friday 9am: heat to 102°F, run full test on fresh water. Tap water: TA 60 ppm, pH 7.1, Ca 80 ppm, FC 0. Friday 10am: add sodium bicarb to raise TA to 90 ppm. Friday 2pm: pH reads 7.4 on its own after TA adjustment. Add calcium chloride to raise Ca hardness to 200 ppm. Friday 6pm: add chlorine granules to reach 5 ppm. Saturday morning: all levels in range. Safe to soak.
Common Mistakes to Avoid
- Skipping the line flush before draining — biofilm in the plumbing lines will immediately contaminate the fresh water and cause persistent foam and cloudiness.
- Filling the spa over the shell edge (not through the filter housing) — this traps air in the plumbing lines and can cause air locks in the pump.
- Soaking in the freshly filled water the same day without testing and balancing chemistry first.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference