Salt and Chlorine Use in Baltresto Hot Tubs: Water Care Guide
The foundation of our hot tubs is a fiberglass shell that can be equipped with various equipment: wood-fired stoves made of stainless steel, electric heaters, pumps, filters, and hydromassage systems. Aeromassage systems and LED lighting are not affected by the chemical composition of the water. However, every other element (including the shell itself) has its own resistance limit to aggressive chemical environments.
1. Basic water parameters (Foundation)
Before adding salt or disinfectants, ensure the chemical balance of the water is correct. This is the foundation – without it even safe chemical concentrations can cause rapid corrosion.
Ideal water parameters:
- pH: 7.2 – 7.6
- Alkalinity: 60-120 mg/l (ppm)
- Calcium hardness: 100-300 mg/l (ppm)
- Iron: max 0.1 mg/l (ppm)
- Copper: max 0.2 mg/l (ppm)
- Manganese: max 0.05 mg/l (ppm)
- Phosphorus: max 0.01 mg/l (ppm)
- Nitrates: max 50 mg/l (ppm)
2. Equipment: Limit values
2.1 Filter, hydromassage, heat pumps and non-titanium electric heaters
- Salt: max 250 mg/l (ppm)
- Chlorine: max 3.0 mg/l (ppm)
(Chlorine above 3.0 ppm is critical for this equipment: the aggressive environment quickly destroys rubber seals in pumps, plastic impellers and membranes, leading to leaks and equipment failure. Strict dosing is essential.)
2.2 Titanium electric heater
- Salt: max 35,000 mg/l (ppm) (Titanium is resistant even to sea water levels).
- Chlorine: max 3.0 mg/l (ppm)
3. Salt use: norms and limitations (at water temperature up to 20°C)
(Note: salt becomes more aggressive when the water is heated. See the summary table below for limits up to 38°C.)
3.1 Wood-fired stoves (AISI 316 stainless steel)
In cool water stainless steel resists chlorides better:
- Up to 10,000 mg/l: Safe operation, the metal oxide film remains intact, warranty preserved.
- Above 10,000 mg/l: Risk of pitting corrosion and issues in welded areas increases.
3.2 Fiberglass shells
Gelcoat handles salt water very well:
- Up to 20,000 mg/l: Suitable for long-term use, warranty applies.
- 20,000 – 35,000 mg/l: Gelcoat withstands the load but wears faster – no warranty.
4. Chlorine use (at water temperature up to 20°C)
(Note: chlorine’s oxidizing effect increases with heat. See summary table for limits up to 38°C.)
4.1 Wood-fired stoves (AISI 316 stainless steel)
- Maximum free chlorine: 3.0 mg/l (ppm).
Chlorine is a strong oxidizer. In cold water the stove tolerates it better, but when heated (or with constant exceedance) it destroys the protective film on stainless steel. This can cause rust on walls and welds.
4.2 Fiberglass shells
- Dissolved chlorine up to 3.0 ppm is completely safe for gelcoat.
Concentrated chlorine is the main risk. Long-term high doses or chlorine gas buildup under the cover can cause color fading and osmosis (bubbles in the coating).
Summary table
| Equipment | Max salt | Max chlorine (≤20°C) | Max chlorine (38°C) |
|---|---|---|---|
| Filter, hydromassage, heat pumps, non-titanium heaters | 250 mg/l | 3.0 mg/l | 1.5 mg/l |
| Titanium heater | 35,000 mg/l | 3.0 mg/l | 1.5 mg/l |
| Fiberglass shell | 20,000 mg/l | 3.0 mg/l | 1.5 mg/l |
| Wood-fired stove (AISI 316) | 10,000 mg/l (5,000 mg/l at 38°C) | 3.0 mg/l | 1.5 mg/l |
5. Important rules for adding chemicals
- Never throw chlorine tablets directly into the tub. Direct contact causes chemical burns (white spots) or osmosis bubbles. Use floating dispensers.
- Remove the thermal cover during shock chlorination for 1-2 hours. Chlorine gas buildup damages the cover and accelerates corrosion of metal parts.
- Dissolve salt in advance. Do not pour it as a pile on the bottom. Undissolved salt creates high-concentration zones that damage gelcoat. Mix thoroughly with the pump.
