Short answer: change the water in your hot tub every 3–4 months. Sooner if you use it heavily; with very light use, you can treat four months as the outer limit. No amount of chemistry can compensate indefinitely for what gradually builds up in the water — and for winter, a simple rule applies: don’t drain a hot tub that’s in active use; only drain it if you won’t be using it for an extended period.
Why change the water at all if I have chemicals?
A hot tub works with a small volume of water (typically 1,000–1,500 litres) at 36–40 °C. From a water-chemistry standpoint, that’s an extreme environment: each bather has an order of magnitude less water available than in a pool, and warm water speeds up every chemical and biological process.
Every soak introduces sweat, cosmetics, sunscreen, body oils and laundry-detergent residue from swimwear. Disinfectant oxidizes these substances, but the breakdown products don’t vanish from the water — they accumulate. Chemistry keeps the water safe, not eternal.
TDS: the invisible ceiling on water lifespan
TDS (total dissolved solids) is the sum of everything that has dissolved in the water: minerals from tap water, residue from every dose of chemicals, and salts formed by the oxidation of contaminants. Every gram of product you add to the water stays in it as dissolved solids.
As TDS rises, disinfection efficiency drops, the water loses its sparkle, foams more easily, and holds a stable pH less well. Past a certain point, no amount of dosing helps — the only solution is fresh water.
Combined chlorine: when disinfection “runs out”
When chlorine reacts with nitrogen-containing substances (sweat, urine, cosmetics), it forms chloramines — so-called combined chlorine. This barely disinfects at all, yet it irritates eyes and skin and is the source of that classic “chlorine” smell. Paradoxically, the more a hot tub “smells of chlorine”, the less effective chlorine it usually contains.
Shock oxidation temporarily breaks down combined chlorine, but in old water with high TDS, chloramines form faster and faster. A practical rule of thumb from the industry: if you have to keep adding more disinfectant just to hold the same reading, the water is spent. The US retailer Epic Hot Tubs makes the same point in its 2025 draining guide, citing the 3–4 month interval based on Pool & Hot Tub Alliance recommendations.
So how often should you change the water? A simple formula
The 3–4 month interval is an average. A more precise estimate for your household comes from a well-established industry rule:
hot tub volume in litres ÷ 3 ÷ number of bathers per day = days until the next change
- 1,200-litre hot tub, 2 people a day: 1,200 ÷ 3 ÷ 2 = 200 days — in practice, round down to the 4-month maximum.
- 1,200-litre hot tub, 4 people a day: 1,200 ÷ 3 ÷ 4 = 100 days, so a change roughly every 3 months.
- Smaller 900-litre hot tub, 3 people a day: 900 ÷ 3 ÷ 3 = 100 days.
Treat the result as an upper limit, not a fixed commitment. After a party with six guests, or a period when the hot tub sat unused without disinfectant, fresh water takes priority over the calendar.
Signs it’s time for an immediate change
- Foam that won’t go away even after a defoamer and a shock dose — a sign the water is saturated with surfactants and organics.
- A musty or sharply “chlorine-like” smell that persists after treatment.
- Cloudiness that doesn’t respond to chemicals — you’ve tried shock, a clarifier and a filter change, and the water still hasn’t cleared within 24–48 hours.
- Unstable pH and alkalinity — readings “drift” within days of every adjustment.
- Eye or skin irritation after bathing even though test readings are within normal range.
If two or more of these apply at once, there’s no point investing further in chemicals. At that stage, changing the water is both cheaper and faster than fighting old water with chemistry.
Changing the water step by step
Draining and refilling the water itself is simple; it’s the steps around it that make the difference between “drain and refill” and a genuine water reset. Plan for 3–4 hours of work plus heating time.
- Clean the plumbing before draining. With the tub still full, dose it with a pipe cleaner and run the jets at full power for 15–30 minutes. This loosens biofilm from the plumbing — brown foam often appears. Biofilm is exactly why fresh water sometimes “turns” within a week.
- Switch off the power and drain the tub. Use the drain valve or a submersible pump (significantly faster). Suck out any remaining water from recesses and jets with a wet/dry vacuum — standing water would contaminate the fresh fill.
- Clean the shell. Use a non-abrasive cleaner made for acrylic, rinse thoroughly and vacuum out the residue. Pay particular attention to the waterline, where grease and oils build up.
- Deal with the filter. Soak the cartridge filter in cleaning solution overnight — ideally alternate between two cartridges. Replace a filter older than 12–18 months outright: a clogged cartridge can ruin even fresh water and is a common cause of the FLO flow-error message.
- Fill through the filter compartment. Insert the hose into the filter well — the water fills the system from the plumbing side and reduces the risk of air pockets in the pump. Fill to the manufacturer’s marked level.
- Turn on the heater and start the chemistry. First test and adjust alkalinity (80–120 mg/l) and pH (7.2–7.6), then hardness if needed, and finally dose the initial disinfectant shock. You’ll find the complete first-fill dosing routine in our guide hot tub chemistry step by step.
- Wait for operating levels. You can get in once the temperature reaches its target and free chlorine or bromine drops to operating level — usually within 24 hours.
Winterizing: drain it, or keep it running?
This is where the most expensive mistakes happen. Intuition says “winter means drain it”, but for a hot tub it’s exactly the opposite: a hot tub that stays in operation is the best protected against frost. Its water is warm, it circulates, and most hot tubs have automatic freeze protection that starts the pump when the temperature drops.
Why not to drain a hot tub you use in winter
A poorly drained hot tub is more at risk in frost than one that’s full and heated. Water always remains in the plumbing, pumps and heat exchanger that gravity alone can’t remove — and it’s exactly this water that splits pipework and cracks pump housings when it freezes. Repairing pipework that has cracked inside the shell’s foam insulation is one of the most expensive service jobs there is, often costlier than a full year of running the tub.
Winter bathing is also the best thing a hot tub has to offer. Running costs do rise in freezing weather, but with a good thermal cover they stay within reasonable limits — we’ve worked out the actual figures in our article hot tub running costs.
When to drain a hot tub for winter instead
Draining only makes sense if you genuinely won’t be using the hot tub over winter — typically at a cottage visited once a month, during a long absence, or wherever unsupervised power outages are a risk (freeze protection doesn’t work without power either). If so, winterize it thoroughly:
- clean the plumbing and drain the water as you would for a normal change,
- loosen the unions at the pumps and drain their bodies and the heater element as well,
- blow out the remaining water from the jets and plumbing (a wet/dry vacuum in blow mode) and suck it out of any recesses,
- remove and dry the filter, and wipe the shell completely dry,
- cover the hot tub with its thermal cover and a tarp to keep out water and rodents.
A half-hearted, quick drain without blowing out the plumbing is the worst possible option — it combines the risk of frost damage with the certainty of biofilm in spring.
Decision table: what to do with your hot tub in winter
| Situation | Recommendation | Note |
|---|---|---|
| I use it year-round | Keep it running | Thermal cover, lower the temperature to 34–36 °C between soaks, check the water level |
| I bathe only occasionally (1–2× a month) | Keep it running, lower the temperature | Economy mode / 30–32 °C; heating up before a soak takes a few hours |
| Cottage without regular supervision | Drain and fully winterize | Including blowing out the plumbing and draining the pumps |
| Absence longer than 4–6 weeks | Drain, or arrange supervision | An alternative is app-based monitoring plus someone checking in person |
| Frequent power outages in the area | Draining is generally advisable | Freeze protection doesn’t work without power |
| Indoor hot tub (heated space) | Leave it filled | No frost risk; the usual 3–4 month change interval applies |
Overview: hot tub water maintenance intervals
| Task | Interval | Why |
|---|---|---|
| Testing pH and disinfectant | 2–3× a week | A small volume of warm water goes out of balance quickly |
| Rinsing the cartridge filter | Once a week to once every 14 days | A clogged filter reduces both flow and chemical effectiveness |
| Deep-cleaning the filter in solution | Once a month | Removes greases that rinsing alone can’t shift |
| Shock oxidation | Once a week / after heavy use | Breaks down combined chlorine and organics |
| Complete water change | Every 3–4 months | Resets TDS, biofilm and combined chlorine |
| Replacing the cartridge filter | Once every 12–18 months | The fibres wear out and stop filtering effectively |
FAQ: the most common questions about changing the water
Can I extend the interval with better chemicals?
Partly. Showering thoroughly before bathing, regular shock oxidation and a clean filter can push the interval towards the four-month upper limit. But no product can reverse TDS build-up — every dose of chemicals actually increases it. Any interval beyond 4 months is always a compromise on water quality.
Is it enough to partially drain and refill the water?
As an emergency fix between full changes, yes — draining off a third of the volume dilutes TDS and improves the water’s behaviour in the short term. It doesn’t replace a complete change, though, since it does nothing about biofilm in the plumbing or deposits on the shell.
Where am I allowed to drain hot tub water?
Ideally into the sewer system. Before draining onto the garden, stop dosing disinfectant and let the water sit for 2–3 days so the chlorine can off-gas — otherwise it will damage the lawn and plants. For local rules on disposing of pool and spa water, follow your municipality’s regulations.
How long does it take for fresh water to heat up?
A standard 3 kW electric heater raises the temperature of 1,000–1,200 litres by roughly 1.5–2.5 °C per hour. So going from 10 °C cold water to 38 °C takes around 12–18 hours; heating is faster and cheaper with the thermal cover closed.
After changing the water, do I have to start the chemistry from scratch?
Yes. Fresh water from the mains or a well has its own alkalinity, pH and hardness, which need to be tested and adjusted before the first dose of disinfectant. We describe the initial treatment procedure after filling in our guide hot tub chemistry step by step.
Changing the water every 3–4 months is the cheapest “chemical treatment” you can give your hot tub: it resets TDS, clears out combined chlorine, and restores a sparkle to the water that no product can manufacture. Everything you need for a fresh start — pipe cleaners, testers, disinfectant and replacement filters — is available in the water treatment category at poolspashop.eu.
