Using reverse osmosis (RO) water in your car's radiator is generally safe and a significant improvement over tap water, but it should not be used alone. The ideal coolant is a 50/50 mix of RO water and a quality antifreeze concentrate. Antifreeze contains essential corrosion inhibitors and lubricants that pure water lacks, which are critical for protecting your engine's aluminum, cast iron, and solder components from rust and scale buildup.
While RO water is highly purified, removing over 95% of dissolved minerals like calcium and magnesium that cause scaling, it is not a complete coolant solution. Using it by itself can lead to increased electrolysis and corrosion because the lack of minerals makes the water more "aggressive" in seeking ions from the metals inside the cooling system. This is why distilled or deionized water is specified by manufacturers.
The following table compares the mineral content of different water types, showing why purified water is preferred.
| Water Type | Total Dissolved Solids (TDS) | Key Minerals Present | Suitability for Radiator |
|---|
| Hard Tap Water | 200-500 ppm | High Calcium, Magnesium | Poor - Causes scaling |
| Soft Tap Water | 50-150 ppm | Moderate Sodium | Fair, but not ideal |
| Reverse Osmosis (RO) Water | 5-25 ppm | Very Low Minerals | Good, when mixed with antifreeze |
| Distilled Water | 0-5 ppm | Negligible Minerals | Excellent, when mixed with antifreeze |
The correct procedure is to always mix the water with antifreeze first, then add the mixture to the radiator. This ensures the inhibitors are properly diluted and active throughout the system. For most climates, a 50/50 blend provides protection down to -34°F and raises the boiling point, enhancing engine efficiency. Check your vehicle's owner's manual for the manufacturer's specific recommendation, as some modern engines require specialized coolants.
