Water quality is one of the most important — yet often overlooked — factors that determine the performance, efficiency, and lifespan of a hot water cylinder. Poor water chemistry can cause corrosion, scale build-up, and premature component failure, all of which reduce efficiency and may void your warranty.
Both Rheem and Rinnai — two of New Zealand’s most trusted hot water brands — publish clear guidelines on acceptable water quality. Following these recommendations will help you maximise performance, extend the life of your system, and protect your investment.
Why Does Water Quality Matter for Hot Water Cylinders?
Hot water cylinders are designed to operate with standard town water supplies, but water chemistry can vary significantly from one region to another. When the water contains excessive minerals, dissolved solids, or corrosive elements, it can accelerate wear and tear on the storage cylinder, heating element, and valves.
Over time, these issues can lead to:
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Premature cylinder failure
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Higher energy bills due to scale-related inefficiency
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Voided warranties due to non-compliance with water quality standards
Before installing a new water heater, it’s essential to test your water quality and ensure it meets the manufacturer’s recommended levels.Whether you have a new system or are looking to install a hot water cylinder, ensuring that your water supply meets the recommended standards is essential for long-term performance.
Recommended Water Chemistry Guidelines
Below are the maximum recommended water chemistry levels from Rheem’s product documentation and Rinnai’s EF250 water quality statement:
| Parameter | Recommended Limit |
|---|---|
| Total Dissolved Solids (TDS) | ≤ 600 mg/L |
| Total Hardness | ≤ 200 mg/L |
| Chloride | ≤ 250 mg/L |
| pH Level | 6.5 – 8.5 |
| Iron | ≤ 1 mg/L |
| Calcium | ≤ 20 mg/L |
| Sodium | ≤ 150 mg/L |
| Magnesium | ≤ 10 mg/L |
Exceeding any of these levels can lead to corrosion, limescale buildup, or component damage — all of which may void your warranty. If you’re unsure about your local water quality, it’s a good idea to have it tested by a certified laboratory or contact your local council for water reports.
What Is Total Dissolved Solids (TDS) and Why Does It Matter?
Total Dissolved Solids (TDS) refers to the total amount of dissolved minerals, salts, and metals present in water. High TDS levels can cause scale deposits on the heating element and tank walls, reducing heating efficiency and accelerating wear.
Some water test results list conductivity instead of TDS. As a rule of thumb:
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TDS (mg/L) ≈ 70% of conductivity (µS/cm)
If TDS levels approach or exceed 600 mg/L, consider installing a filtration or water softening system before the water enters the cylinder. This will help maintain efficient operation and extend the lifespan of your water heater.
Tips for Protecting Your Hot Water Cylinder
- Install a water filter or softener: These can help reduce TDS and hardness if your water supply is borderline.
- Flush your cylinder periodically: Regular flushing removes sediment and reduces scale buildup.
- Check your water quality annually: This ensures that changes in supply chemistry don’t go unnoticed.
- Schedule regular maintenance: A professional inspection can detect early signs of corrosion or scaling before they become serious.
Protect Your Investment Today
If you’re unsure about your water quality or need advice on installing a new system, our experts can help. Contact us today for a free consultation and ensure your hot water cylinder lasts for decades.
