1️⃣ Aggressive Water (High or Low pH)

Water that is too acidic or too alkaline becomes chemically aggressive.

Ideal Spa pH:

7.2 – 7.6

🔴 Low pH (Below 7.0)

Acidic water:

• Attacks aluminium

• Causes pitting corrosion

• Weakens the heater chamber

• Destroys internal seals

• Shortens element life

Aluminium reacts with acidic water because hydrogen ions strip away its protective oxide layer, allowing corrosion to occur.

🔴 High pH (Above 7.8)

High pH:

• Encourages scale formation

• Reduces chlorine effectiveness

• Allows mineral deposits to form inside heater tubes

• Restricts water flow

• Causes overheating

Scale build-up traps heat and stresses the PTC element.

How Chlorine Overdose Damages Aluminium

Many spa owners use 20g chlorine tablets designed for swimming pools.

This is often too much for a small inflatable spa.

How Much Chlorine for an 800 Litre Hot Tub?

A standard 20g tablet is designed for much larger volumes.

For an 800 litre spa, you typically need:

👉 ¼ to ½ of a 20g tablet at a time

Using a full tablet can cause:

• Chlorine levels above 5 ppm

• Chemical aggression

• Rapid corrosion

• Seal degradation

• Heater chamber pitting

Always test chlorine levels and aim for:

3–5 ppm free chlorine

The Hidden Danger of Multi-Functional Chlorine Tablets

Some chlorine tablets contain:

• Stabiliser (CYA)

• Flocculants

• Algaecide (often copper-based)

Why Is Green Water a Warning Sign?

If your spa water turns green, many assume algae.

However, in small hot tubs it’s often:

👉 Copper spike from algaecide

Copper in water reacts with oxidisers and can:

• Turn water green

• Stain liners

• React with aluminium heater chambers

The Chemistry: How Copper & Chlorine Corrode Aluminium

Aluminium normally forms a protective oxide layer.

When exposed to:

• High chlorine

• Copper ions

• Imbalanced pH

The protective layer breaks down.

This creates:

• Galvanic corrosion

• Pitting damage

• Electrical leakage risk

• Premature heater failure

Copper ions accelerate electrochemical reactions, especially in warm water environments.

Your heater core becomes the sacrificial metal.

Limescale – The Silent Heater Killer

Hard water contains calcium and magnesium.

When heated:

• Minerals precipitate

• Scale forms inside the heater chamber

• Heat transfer becomes uneven

• The PTC element overheats

• The unit may trip or show errors

Scale is one of the most common causes of heater failure in hard water areas.

How to Protect Your PTC Heater Core

✔ Test Every Few Days

Check:

• pH (7.2–7.6)

• Free chlorine (3–5 ppm)

• Total alkalinity

• Calcium hardness (where applicable)

Do not guess. Test regularly.

✔ Avoid Overdosing Tablets

Never drop full-size swimming pool tablets directly into an 800L spa.

Use:

• Smaller doses

• Floating dispenser

• Regular testing

✔ Avoid Copper-Based Algaecides

Inflatable hot tubs rarely need algaecide.

Proper chlorine management prevents algae without introducing copper.

✔ Descale Regularly

If you live in a hard water area, preventative descaling is essential.

Scale build-up:

• Restricts flow

• Causes overheating

• Shortens heater life

Protect Your Heater with LayZRepair No-Scale

Our LayZRepair No-Scale treatment is designed to:

• Reduce limescale formation

• Protect heater chambers

• Improve heat transfer

• Extend PTC heater lifespan

• Maintain efficient flow

Ideal for:

• Hard water areas

• Frequent spa users

• Heater protection maintenance

Regular use can significantly reduce internal scaling and prevent costly heater damage.

Signs Your Heater May Already Be Damaged

• Repeated tripping

• E01 / E02 / E03 errors

• Burning smell

• Slow heating

• Discoloured water near outlets

If you suspect damage, stop using the heater and inspect or book a repair before further corrosion occurs.

Final Advice

Most heater failures are preventable.

Balanced water is cheaper than replacement parts.

Testing every few days takes minutes.
Replacing a heater costs far more.

If you’re unsure whether your heater core is damaged or your water chemistry is correct, we’re happy to help diagnose the issue.