How often should you change hot tub water?

Unlike pools, which can run for years without a full drain, hot tubs require periodic water changes. No amount of shocking, balancing, or filtration can fix the underlying problem — total dissolved solids (TDS) that accumulate until the water is simply exhausted. Here's how to calculate your water change interval and recognize when it's time to drain regardless of the schedule.

Why hot tub water needs replacing

Every time you soak, you introduce dissolved compounds into the water: sweat, body oils, cosmetics, sunscreen, urine traces. Every chemical you add — sanitizer, pH adjusters, alkalinity increaser, calcium, shock — adds to the dissolved load. Over time, these compounds and their reaction byproducts accumulate as total dissolved solids (TDS).

High TDS causes several problems that you cannot fix with chemistry:

• Water becomes harder to balance. pH and alkalinity resist adjustment; you add chemicals that seem to do nothing, then the numbers swing in unpredictable ways.
• Sanitizer efficiency drops. High TDS creates chemical competition — other dissolved substances consume sanitizer before it can do its job.
• Foaming increases. Organic compounds act as surfactants, and the aeration from jets creates persistent foam that defoamer only temporarily masks.
• Water looks and smells off. Even with correct chemistry readings, heavily used water can look dull or produce a musty or chemical odor.

There is no chemical solution to high TDS. Drain, clean, and refill.

The standard guideline: 3–4 months

For a typical residential hot tub used by a household of two to four people several times per week, the standard recommendation is a full water change every 3–4 months. This accounts for normal chemical accumulation and typical bather load.

Think of it as maintenance on a schedule, not a reaction to a problem. Set a reminder and drain before the water starts showing signs of exhaustion — it's much easier to manage fresh water from the start than to fight deteriorating water chemistry in the final weeks of a long interval.

The bather-load formula

If you want a more precise estimate based on your actual usage, use this formula — a common industry guideline:

Examples for a 400-gallon spa:

Use your actual daily average, not maximum capacity. If two people soak three times a week, your daily average is roughly 2 × (3/7) ≈ 0.85 people per day — giving an interval closer to 4 months for a 400-gallon spa. The formula rewards honest usage estimation.

Signs it's time to drain regardless of the schedule

The formula gives you a planned interval, but water can degrade faster than expected. Drain when you see any of these:

• Chemistry is hard to hold. pH and alkalinity keep requiring large corrections and still drift quickly. Sanitizer readings are unstable despite consistent dosing.
• Persistent foaming. A little foam immediately after adding chemicals is normal. Foam that sits and doesn't clear, or that comes back after defoamer, signals high organic TDS.
• Unpleasant odor. A strong chemical or musty smell even when sanitizer levels are correct and CC is below 0.5 ppm.
• Hazy or dull water. Water that won't clear after shocking and extended filtration, despite all readings being in range.
• Skin or eye irritation during soaking despite balanced water — another sign that dissolved byproducts have exceeded what balancing can address.
• CYA above 50 ppm (chlorine spas) — once CYA is high in a spa's small volume, partial drains can't bring it down fast enough. A full change is the practical fix.

How to drain and refill your hot tub

1. Run a line flush before draining. Add a spa line flush product to the water and run the jets for 20–30 minutes. These products dislodge biofilm and scale that accumulates in plumbing lines, jets, and the heater — stuff that your filter and routine chemistry don't reach. If you skip this step, biofilm gets left behind and seeds your fresh water.
2. Turn off the heater and jets, then drain. Most spas have a drainage valve on the equipment panel. Attach a garden hose and route water away from landscaping (chlorinated or brominated water can harm plants at high concentrations). A submersible pump speeds up the process significantly — draining via gravity valve can take 1–2 hours; a pump takes 10–20 minutes.
3. Clean the shell and waterline. While the spa is empty, wipe down the shell with a spa-specific cleaner. Pay particular attention to the waterline — that's where body oils and scale accumulate most visibly. Rinse thoroughly before refilling.
4. Clean or replace the filter. Water change day is the ideal time to do a deep filter clean or replace a worn cartridge. A clogged or degraded filter compromises water quality faster than most other factors.
5. Refill through the filter housing. Insert a garden hose into the filter housing (not directly into the spa) to fill. This purges air from the plumbing lines before the pump starts, preventing an "air lock" that can cause the pump to run dry.
6. Balance fresh water before heating. Test and balance alkalinity, then pH, then calcium hardness before turning the heater on. It's much easier to balance water at room temperature than at 104°F, and balancing cold water is more efficient since heat accelerates chemical reactions and pH drift.
7. Add your sanitizer and establish your system. For a chlorine spa, dose to your target FC (3–5 ppm). For a new or reset bromine spa, add sodium bromide to establish the bromide bank, then activate with MPS or a small chlorine dose. See Chlorine vs. Bromine for Hot Tubs for setup details.

Partial drains: useful but not a substitute

If a full drain isn't practical, replacing 25–30% of the water (draining and refilling roughly 100–150 gallons of a 400-gallon spa) will dilute TDS, CYA, and other accumulated compounds proportionally. A 25% partial drain reduces TDS by 25%.

This can extend your interval or address a specific problem — high CYA in a chlorine spa, for example — but it doesn't reset the clock. Dissolved solids that remain in the 75% of water you kept are still there and will continue to accumulate. Partial drains are a useful maintenance tool, not a substitute for periodic full changes.

Track your spa water age

PoolChem Tracker logs your water change date alongside your chemistry readings — so you always know how old your water is and when it's time to drain.

Related reading

• Hot Tub & Spa Chemistry Basics — target ranges and how spa chemistry differs from pools
• Chlorine vs. Bromine for Hot Tubs — how to set up or restart your sanitizer system after a water change
• How to shock a hot tub — when shocking helps and when it doesn't
• Reverse osmosis vs. draining a pool — the pool equivalent of managing dissolved solids buildup