It's the water

And a lot more...








It's The Water

and a lot more


Easily overlooked and often misunderstood, the chemistry in the water we use in a cooling system is becoming more important all the time. As engine designs become more sophisticated, so does the coolant mixture requirements that are used. Since a 50/50 mixture of coolant is half water, dissolved minerals and chemicals in the water we use have the potential to interfere with the intentions of the inhibitors in the coolant. It should be no surprise to anyone that tap water varies from place to place. What you may not know is that depending on which part of the world you are from, the tap water varies even more significantly.

  • In Japan the tap water is generally very pure and so the inhibitor packages in Asian coolant is made for pure water. Generally Asian cars require Silicate Free coolant.
  • In Europe the water generally has a very high dissolved mineral content, and so European coolants are formulated with this in mind. Generally, European cars require Phosphate and Borate free coolants.
  • In the USA we also have rather high mineral content as well as harmful sulfate content. Additionally, high chloride levels are often found in treated tap water. As a country, our water purity varies greatly nationwide. Dihydrogen Monoxide is more commonly know as water.

Commonly held thoughts regarding specifications for tap water when

used in a coolant mixture for automotive uses:

  • the total hardness is less than 100 ppm total hardness and
  • the Chloride level is less than 50 ppm and
  • the Sulfate level is less than 40 ppm and
  • there is less than 250 ppm TDS (total dissolved solids).

It has been widely suggested that if your tap water does not meet the above minimum standards, then distilled water should be used. Conventional wisdom also suggests that 'Spring Water" or 'Drinking Water' should be avoided, and 'Well Water' should never be used under any circumstance.


With the purity of water such a wildcard, it is not surprising that the trend of packaging automotive coolant is in the "Ready to Use" or pre-diluted form. Currently both Toyota and Honda offer their own brand of premixed coolant. European and American OE manufactures'  have so far resisted the pre-diluted  trend, and still offer concentrated coolant.


Independent coolant packagers (Prestone, etc.) are on both sides of the fence, offering both concentrated and pre-diluted coolants. However, the trend is changing, depending on the  particular store, it is now easier to purchase pre-mixed coolant (50% water) than the traditional concentrated coolant (0% water).


Water, in its chemically pure state is nearly a perfect electrical insulator. It is however, highly polarized which can make very strong electrolytic solutions conductive enough to carry significant electrical currents. Water has long been called the universal solvent which means it will easily dissolve other substances and is often a benchmark characteristic.


Distilled Water is produced by first boiling or evaporating water, then concentrating and condensing the water vapor back to a liquid state. This form of water purification is excellent for removing minerals, chemical contaminates, and viruses. The distillation process takes a significant amount of energy since the water must change states twice. Bio-chem Laboratories might actually double distill water when making ultra pure processes. Distilled water is generally held to be ideal for use in an automotive cooling system as it is very pure and not capable of conducting electricity. However, this extra purity may in fact point us in the wrong direction when dealing with the challenges of automotive electrolysis. Pure water will adapt to the environmental conditions around it, and may act to enhance the galvanic process (electrolysis). Since water is rather good at dissolving things ( There is a reason it's called The Universal Solvent ) pure water will reach back into the water jackets of the engine block and absorb sediments that were once dormant. With no other dissolved minerals to fight it, this pure water has unwittingly now become part of the problem. That is why I characterize distilled water as a double agent: It is great in a new clean maintained cooling system. However distilled water is not the right choice when our cooling system presents symptoms of chronic galvanic cell corrosion (electrolysis). 


Deionized Water is purified water that has passed through a ion exchanger resin which exchanges a hydrogen ion and hydroxide ion for dissolved minerals. Deionized water is not as pure as distilled water, but is easier, faster and cheaper to produce.  Di water is pure enough for a wide variety of uses including cooking and drinking, however bacteria and virus's can still be present in deionized water. For automotive uses deionized water is ideal for topping lead acid batteries, and mineral free exterior washing. Deionized water is sometimes also named and marketed as de-mineralized water. Deionized water almost has no place inside the automotive cooling system, as the mineral free properties may work against us just as in the distilled water. Using Di water for general maintenance when there are no symptoms of galvanic corrosion might be ok, but using DI water when a cooling system is infected


Reverse Osmosis Water systems purify water by using a number of steps. First the water is filtered down to remove rust and other sediments, then a smaller filter to remove even more contaminates. Usually at this point an activated charcoal filter is introduced to remove chlorine and other organics that will interfere with the RO process. Next the pressurized water is in contact with a thin film membrane that further purifies the water by allowing osmosis. Once the water has migrated beyond the membrane, it again has contact with an activated charcoal filter that removes any particles left over from the previous steps. UV light is sometimes exposed to the water in some cases if the need dictates. Smaller RO water process use no electricity and the water does not change states. No pumping is needed when the beginning water pressure is sufficient. Typical  smaller RO water units produce 2-5 gallons of water a day, making them ideal for home or office water needs. However, smaller household RO units are not very water efficient and can use (reject) 15 gallons of water for every gallon they make.  RO units are often coupled with soft water systems to remove the sodium chloride ions left behind by the soft water process.


Chlorinated Water is a problem for our automotive cooling systems. Chlorine and Chloramines are added to our public drinking water supply to disinfect the water from pathogenic disease. While disease free drinking water is the desired result, the chlorine charge creates problems for our metals. The Chlorine is a nonmetallic Anon, it reacts with the Aluminum in the cooling system which is a Cation. Since the electrostatic attraction between the positives and the negatives bring the particles together, we now have an ionic compound. 


Hard Water is water that has higher than normal concentrations of dissolved minerals, typically calcium and magnesium. While these are not considered harmful to human health, these minerals can play havoc on modern household plumbing as the minerals tend to fall out of suspension and adhere to the insides of certain types of pipe. Soft water systems have a resin that exchanges sodium ions for calcium and magnesium ions, allowing the water to be "soft". Softened water is not good in an automotive cooling system, and a known good source of water is advisable over soft water. Well water and/or ground water is generally known for having a higher than normal "hardness" factor as well as iron and sulfate content and should be avoided in the automotive cooling system.


Tap Water can also sometimes be harmful to an automotive cooling system if the levels of dissolved solids and chlorine are high enough. The Chlorine ions can be very reactive and corrosive to aluminum and other metals including stainless steel.  When Chlorine is added to water, it reacts to form a pH dependant equilibrium mixture of Chlorine, Hypochlorous Acid, and Hydrochloric Acid (CL2 + H2O -> HOCL + HCi). Additionally, naturally occurring Sulfates (SO4) found in tap water are generally not removed by public water systems and can also contribute to rapid metal failure. Reverse Osmosis and Distillation water treatments containing an activated charcoal stage are very effective at removing Chlorine, Chloramines, and Sulfates and is a great final stage treatment for any city water used for drinking, cooking, or any automotive use.




North America
Water Hardness
Water Hardness        








Properties of Water




Heat transfer fluids have thousands of applications involving the movement of heat energy, lubrication, and protection of surrounding metals, but none more important or abundant that in an automotive cooling system. Water is a hugely abundant natural resource that has virtually unlimited uses to mankind. For the purposes of this discussion, we will limit our focus to the use of water in an automotive cooling system as a heat transfer fluid. In the fields of physics and engineering all fluid measurements were historically based on the behavior of water. Therefore, this discussion will begin with water as a reference heat transfer fluid against which all other fluids and their characteristics can be compared. While being commonly available and the least expensive of all possible options, water has features and idiosyncrasies that make it ideal for some applications and totally useless in others.


In a cooling system, plain water has three fundamental disadvantages:


  • First, for low temperature applications, plain water provides no freezing protection, becoming a solid at 32F. If that were not enough, this transition to a solid is accompanied by expansion with enormous potential force capable of severe damage to contained components. If this expansion takes place in the engine, the freeze plugs may push out preventing engine damage. However the radiator and heater have no such expansion protection and will expand to the point of faiure.

  •  Second, water has a limited upper level temperature range governed by its sea level boiling point of 212F. Exhaust emission levels are better controlled when the engine temperature is in the 200F - 210F area, so a boiling temp of 212F does not give enough room for controlled heat transfer.

  •  The the third disadvantage is the inherent lack of corrosion protection. The presence of dissimilar metals within the fluid circuit can create an electrochemical cell which promotes galvanic corrosion potentials. The purity of the water can also amplify the corrosion potential. Even though pure water is considered an insulator of electricity, without a balanced balanced corrosion inhibitor plan in place, pure water will accept the voltage polarity differential between metals and allow electron movement.


Historically, all these issues have been addressed by the addition of a hydrocarbon solvent called Ethylene Glycol along with various corrosion inhibiting additives. The resulting mixture with water will be recognized as ordinary automotive antifreeze. The most common mixture ratio of Ethylene Glycol (EG) and water is a 50/50 mix in which the freezing point is lowered to –34 F and the boiling point is raised to 228F (at sea level). For a maximum freezing point protection the ratio can be increased to 67% ethylene glycol and the balance (33%) water. This will lower the freezing point to –84F. 


Often the observation of the beneficial increased EG concentration is noticed and the thought of running a 100% concentration arises.  Since pure EG freezes at about 10F there is no freezing benefit over a 50/50 mix at all. Worse yet, the use of straight EG will result in a 25% reduction in heat capacity, or heat carrying capability compared to an 50/50 EG mix. In an automobile the cooling system (the radiator) we would need to have an increase in size, by approximately 25% to provide the same cooling capacity. Additionally, pure EG can saturate the soft rubber cooling system hoses, weakening them causing premature failure.


Heat CapacityThe First Requirement: Since the primary purpose of heat transfer fluids is to carry heat, it is reasonable to talk about water’s ability to absorb and hold a quantity of heat energy. As energy is absorbed into a fluid, its temperature increases. In the case of water, the definition of the “BTU” (British Thermal Unit) is the amount of energy required to increase the temperature of one pound of water by one degree ˚F. In the metric system of measurements, this energy is measured in terms of the “Joule” which is defined by the ability to raise one kilogram of liquid by one degree ˚C. These values are called “Specific Heat Capacity.”


As we mentioned earlier, all characteristics of fluids are compared to the performance of pure water. The heat capacities of various fluids are all compared to the heat capacity of water as a percentage or a fractional factor called “Specific Heat Ratio.” Think of it as a correction factor. For example; The specific heat capacity of water = 1.0 BTU/lb-˚F.

The specific heat capacity of pure ethylene glycol = 0.57 BTU/lb-˚F The ratio of specific heats is 0.57 divided by 1.0 which is 0.57 In the metric system (SI units); The specific heat capacity of water = 4.19 kJ/kg-˚C The specific heat capacity of pure ethylene glycol = 2.38 kJ/kg-˚C Thus, The ratio of specific heats is 2.38/4.19 = 0.57 Specific heat capacity values will also vary slightly with temperature./p>


To answer a very common question, and as shown above, water has better thermal heat capacity than pure Ethylene Glycol, so a 50/50 mix of Water and EG will give us the best thermal properties combined with the best freezing/boiling/corrosion protection.


Specific Gravity: We just said that Specific Heat defines the ability of a certain weight of fluid to hold heat energy. It should be readily apparent that for any given volume capacity of a cooling system, there is going to be a fixed weight of fluid circulating within it. Therefore what goes hand in hand with Specific Heat is Specific Gravity—the density of a given fluid compared to the density of water. The higher the Specific Gravity, the more dense is the fluid and therefore the heavier a given volume of fluid is. And the heavier the fluid circulating in a given volume, the more heat energy it can hold.


Thermal Conductivity: As with metals and other solids, in addition to their abilities to hold heat energy, fluids have the ability to conduct heat energy. Obviously, this is an important factor since it governs the ability to put energy into a fluid and then get it back out again. The units for thermal conductivity are defined as a conversion factor that relates the ability to move one BTU of energy through a square foot of material one foot thick for each 1˚F of temperature differential from one side of it to the other. This value is typically defined for a specific temperature or temperature range.


Viscosity—The Resistance To Movement: Once we have a fluid that can carry the heat volume desired in the temperature ranges of expected operation, and we understand its thermal conductivity—the ability to move energy into and out of it, the next question that arises is just how easy the fluid will be to move around. While certain fluids will have the ability to absorb and hold enormous amounts of heat energy, they are so viscous that the pump horsepower required to move them around would be prohibitive. Additionally, their viscosity might change with temperature making them ideal as long as they are hot but they become solids at ambient temperature (e.g. liquid sodium used in some nuclear reactors). Additionally, viscosity affects cavitation tendencies within water pumps. In pumps where there is a low pressure area that drops fluid pressure below its vapor pressure there is a tendency to create cavitation with the ensuing erosion effects. In this case a fluid with a higher viscosity will have less of a tendency to cavitate.


Surface Tension: Let We’ve all seen how water beads up when it is poured onto a smooth surface. This tendency to create beads instead of flowing out uniformly over the surface is due to what is called surface tension. It is the measure of the ability of any given fluid to wet the surface from which it is to conduct heat energy. Surface tension is measured in units of dynes per centimeter and the lower this value, the more “wettability” of a surface a fluid has. There are products available containing special surfactants (e.g., Red-Line’s “Water-Wetter”) to cut surface tension of water for the purpose of improving the ability of water to transfer heat energy. Reducing the surface tension of a cooling system solution is generally considered to improve heat transfer, however when using surfactants in a galvanic cell contaminated cooling system, reducing the surface tension will actually promote electron movement, making an existing electrolysis problem even worse.


Corrosion: It should come as no surprise that one of the chief disadvantages of water as a heat transfer fluid is its proclivity to induce corrosion of metals. Most heat transfer systems are comprised of metals, and dissimilar metals at that, serving to exacerbate the corrosion potential. Most practical heat transfer systems are fabricated from various metals which are selected based on their weight, their conductivity, and their ease of manufacture. Given the inherent corrosiveness of water as an operating fluid, we are faced, therefore, with the need to inhibit the corrosion rate of the materials. The most common metals found in heat transfer systems are cast iron, steel, aluminum, copper, brass and the constituents of solder (tin, zinc and lead). Cooling system additives that allow exposed metals to "plate-up" offer additional protection against corrosion and should be considered when active problems are being remediated.






Ethylene Glycol: A 50/50 Ethylene Glycol-Water mix is the best and most economical heat transfer fluid for our automotive cooling system. For optimal results the coolant mix must have the appropriate levels of inhibitors and buffers matching both the requirements of the engine, and also for the water that is being used. A coolant supplement is often used when the water purity is imperfect, or if the engine has dissolved solids that seep back into suspension changing the coolant chemistry, or if there are underlying issues that need to be addressed. The cooling system must be kept free of dirt, rust and pH lowering elements (acids) if we want the cooling system remain trouble free. A glycol concentration higher than 50% can be of some benefit in extremely cold climates, as well a concentration lower than 50% can aid slightly in heat transfer during extremely hot climates with a properly operating pressurized cooling system.. It should be noted however that changing the concentration more than 15% in either direction (from 50/50) can cause unintended results.


While so called Universal Coolants may meet the EG requirements, the inhibitor additives, plating elements and pH buffers could never match the original intent of the OE coolant mix. It is for this reason I recommend using the OE coolant whenever possible.


Water: The best water available should be used, however distilled or deionized water may not be the best water for a cooling system suffering from electrolysis. Water that is void of the buffering effects of calcium and magnesium may tend to adopt the low pH characteristics of an acidic cooling system, allowing aluminum to be an unwitting donor of electrons. It is this reason that I do not use or recommend soft water, distilled water or deionized water in a cooling system suffering from electrolysis.


Almost all vehicle manufactures recommend that distilled water be used in the coolant mix. When performing regular routine maintenance on a vehicle free of galvanic corrosion (electrolysis) distilled water is just fine. The problem with distilled water is that it is too "willing", meaning that an electrolysis infected cooling system is actually being fed by aluminum and distilled water.


My conclusions are my opinions, and are based on case studies, 30 years automotive experience, continuing education, research in the form of under hood experimentation, and countless phone calls from frantic car owners and mechanics nationwide. I understand that my opinions about distilled water may be controversial and not widely held, and so I invite you to do your own research, and formulate your own opinions. I would love to hear them.

Further In Depth Reading...

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