Domakihub When to Replace Pool Water: Signs, Timing & Steps
Know when your pool needs a full drain — and how to do it safely without damaging your shell.
Reviewed by Domakihub pool pros · 2026-06-23 · 5 min read
Pool water does not last forever — dissolved minerals, cyanuric acid, and combined chloramines accumulate over time until no amount of chemicals can restore balance. The video above walks through exactly when draining is the right call and when it is not worth the risk. This article pairs those principles with a practical step-by-step process tailored to Greek homes, holiday rentals, and hotel pools that deal with hard water and intense summer sun.
Why Pool Water Degrades Over Time
Every time you add chemicals, every swimmer that enters the water, and every litre lost to evaporation leaves behind a residue. Calcium, cyanuric acid (the stabiliser that protects chlorine from UV), total dissolved solids (TDS), and phosphates all build up steadily. In Greece, where mains water is notably hard and summer temperatures push evaporation rates high, this accumulation happens faster than in cooler, softer-water climates.
Once TDS climbs above roughly 3,000 ppm or cyanuric acid exceeds 100 ppm, the water resists treatment. Chlorine becomes ineffective, algae blooms return despite shock doses, and the water looks perpetually dull or hazy. At that point, no amount of chemicals fixes the underlying problem — the water itself must go.
When a Full Drain Is Not the Answer
Draining a pool carries real risk. An empty fibreglass or vinyl-liner shell can float out of the ground if the water table is high — a genuine concern after winter rains in many parts of Greece. Concrete shells can crack under hydrostatic pressure or bake and craze in summer heat when exposed and empty. For these reasons, a full drain should be a last resort, not a routine maintenance step.
Partial draining (replacing 20–30 % of the volume) solves many problems — mildly elevated cyanuric acid, moderate TDS, or temporarily high calcium — without the structural risks. A reverse osmosis (RO) filtration unit, now available from specialist pool companies in Greece, can recondition water on-site without any draining at all, and is the preferred option for large pools or during dry summers when water is scarce.
5 Steps to Replace Your Pool Water Safely
- 1
Test the water first and document the numbers
Before scheduling a drain, run a full water analysis: TDS, cyanuric acid (CYA), calcium hardness, pH, and total alkalinity. Many pool shops in Greece offer free testing. Only proceed with a full drain if TDS is above 3,000 ppm and CYA is above 100 ppm, or if the water has been contaminated by a severe algae outbreak, heavy metals, or a flooding event. Record the results — you will need them to balance the fresh fill correctly.
- 2
Check the water table and pick the right time of year
Never drain a pool during or shortly after heavy rainfall. A high water table can push an empty shell upward off its base, causing thousands of euros in structural damage. In most of Greece, the safest window is late spring (April–May) or early autumn (September–October): the water table is lower, temperatures are mild enough that an empty concrete shell will not crack from heat exposure, and you have time to refill and balance before peak season or winter.
- 3
Lower the water to a safe level, then drain completely
Use a submersible pump or your pool's waste port to drain water down in stages. As the level drops, inspect the walls and floor for scale, staining, or algae colonies — note any areas that need brushing or acid washing before the refill. Once the pool is empty, work quickly. Aim to have the shell exposed for no more than 24–48 hours in summer to minimise sun stress on the surface.
- 4
Clean, inspect, and repair before refilling
With the pool empty, brush down the walls and floor, apply a diluted acid wash if calcium scale is heavy, and rinse thoroughly. Inspect all fittings, skimmer baskets, return jets, and the main drain cover for cracks or wear. This is also the right moment to replace worn O-rings, re-grout tiles, or carry out any resurfacing. For hotel and rental pools, a professional inspection at this stage keeps you compliant with Greek health and safety regulations for commercial pool operators.
- 5
Refill, then balance within 24 hours of reaching full level
Fill slowly using a hose sock or inlet sock to prevent staining from iron or manganese in the mains water — common in many Greek municipalities. Once the pool reaches the normal operating level, run the filter and begin balancing: start with total alkalinity (80–120 ppm), then pH (7.2–7.6), then calcium hardness (200–400 ppm), and finally add stabilised chlorine to bring free chlorine to 2–4 ppm. Test again after 24 hours before allowing swimmers back in.
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FAQ
- How often should I completely replace my pool water in Greece?
- Most residential pools in Greece need a full drain and refill every 5 to 7 years, assuming regular chemical maintenance and partial water changes along the way. Hard water and strong UV can push this closer to every 3 to 5 years. Pools serving holiday rentals or hotels with high bather loads may need attention sooner. Test TDS and CYA annually — those numbers tell you more than any fixed schedule.
- Can I drain my pool in summer to clean it?
- It is best to avoid a full drain in the peak of the Greek summer (July–August). High ambient temperatures can stress or crack an empty concrete or plaster shell within hours, and UV exposure damages exposed fibreglass. If you must drain in summer, work during the coolest part of the day, refill within 24 hours, and keep the empty shell shaded with a tarpaulin if possible.
- Is there an alternative to fully draining the pool to fix high cyanuric acid?
- Yes. A partial drain (replacing 30–50 % of the water volume with fresh water) can dilute cyanuric acid if levels are only moderately elevated — say 80–120 ppm. For severe cases above 150 ppm, or if TDS is also very high, a full replacement is usually more cost-effective. Reverse osmosis filtration is a water-saving alternative that strips CYA and TDS without emptying the pool, and is increasingly available through pool service companies in Athens, Thessaloniki, and popular tourist areas.