Using Boiled Tap Water vs. Distilled Water in Engine Coolant: What’s the Best Practice?

When it comes to mixing water with engine coolant, the choice of water can significantly impact the performance and longevity of your vehicle's cooling system. You might be tempted to boil tap water and cool it down as a cost-effective alternative. However, is boiled water truly suitable, or should you use distilled water instead?

Understanding the Difference: Boiled vs. Distilled Water

Boiling tap water and then cooling it does not produce distilled water. Distillation is the process of boiling water to produce steam and condensing that steam back into liquid. During this process, impurities and minerals are separated from the water. Boiling tap water, however, leaves all the initial impurities and minerals in the water.

Boiled water may still contain some minerals and contaminants present in the tap water, making it less ideal for use in engine coolant. These impurities can cause scale buildup, reduce the effectiveness of the coolant, and potentially lead to system failure over time.

Why Use Distilled Water in Cooling System?

For mixing with engine coolant, it is best practice to use distilled or de-ionized water. This will help prevent mineral buildup in the cooling system and ensure optimal performance.

Distilled water is a preferred choice because it has a TDS (Total Dissolved Solids) level of 0, which means it is free from all minerals and impurities. This makes it a safer and more reliable option for your cooling system. Even bottled spring water, while better than tap water, may still contain some impurities, especially if not distilled.

Mineral Content and Its Impact

High mineral content in tap water can lead to scale deposits in the cooling system over time. These scale deposits can clog passages and affect the efficiency of the cooling system. If distilled water is not available, using bottled spring water can be a better choice than tap water, but it is still not as ideal as distilled water.

Everyone's tap water is different, and the mineral content can vary significantly. Using distilled water ensures consistency and reliability in your coolant. For this reason, it is highly recommended to use distilled or de-ionized water when possible. The 50/50 premix coolants are a convenient and reliable option since they are already mixed correctly.

Electrolysis and Mineral Buildup

Another reason not to use tap water for engine coolant is the potential increase in electrolysis. Distilled water is less likely to increase electrolysis because it has no mineral content that can react with the coolant additives. Water softeners, which replace calcium ions with sodium ions, can be even more problematic as they do not remove other contaminants from the water.

Mineral buildup and electrolysis can lead to corrosion and degradation of your vehicle's cooling system components. Regular maintenance, including draining and flushing the cooling system according to the manufacturer's recommendations, is essential to prevent these issues.

Testing Tap Water Quality

Knowing the quality of your tap water is crucial. You can purchase a inexpensive TDS tester on Amazon or AliExpress. In Canada, my experience testing tap water during the winter showed that it had a TDS level of around 200. Even distilled tap water tested at approximately 25 parts per million, which is still higher than pure water.

Pure water wants to combine with just about anything, including some carbon dioxide from the air, which makes it slightly acidic. Antifreeze manufacturers typically anticipate this and compensate for it in their additives.

Geographical and Environmental Differences

The TDS level of tap water can vary significantly depending on geographical and environmental factors. In Tapachula, Mexico, which receives about 100 inches of rain annually, the tap water TDS can reach up to 130. Coastal areas with higher salt content can have TDS levels as high as 500 to 2000 parts per million. Reverse osmosis bottled water typically has a TDS level of around 80, and my own reverse osmosis unit can bring it down to under 30.

In Mexico, some bottled water that is not labeled as reverse osmosis has the same TDS level as tap water, according to my experience. The worst commercial water I encountered was a bottle of water from Autozone in Puebla, labeled not for human consumption, with a TDS level of around 300 parts per million.

Given these variations, it is clear that distilled water is the best choice for mixing with engine coolant to ensure optimal system performance and prevent potential issues.