Distilled water for radiators?

This spring I disassembled an engine from a 1965 MGB that has been in the family for the last 30 years. It has always been filled with a 50/50 mixture of Prestone and tap water. There was virtually no rust anywhere in the cooling system. Now I am hearing that you should only use distilled or de-ionized water. Based on my experience, it would be a needless complication, although rather minor in cost and nuisance. By the way, as far as I know, the MG still has the original radiator.

Also, if you go to the Prestone web site, they show a radiator being filled by a garden hose.

Is there any factual information that would cause one to believe that the use of tap water is harmful?

I would imagine that it depends on your tap water, and the amount of dissolved “stuff” in it. Water quality varies a lot across the country. If you have very hard water, I suppose its possible that mineral deposits could develop and potentially clog the small conduits in a radiator, but it depends on if the chemistry of coolant, and whatever else… the metal its in contact with, would allow for that to happen. Any water chemists out there want to elaborate?

It doesn’t matter what kind of water is mixed with the coolant. Be this tap water, distilled water, de-ionized water, bottled water, dog water, etc… As long as the cooling system is maintained on a regular basis, the antifreeze at the proper mixture will prevent any corrosion from ocurring.


I believe it depends on the quality (hardness) of your tap water. Personally, I only used distilled water in my cars, it is very cheap and available everyplace.

In most areas there is no problem using tap water. There are some places where the water is not so good, usually on the acid side and if it overcomes the buffers in the coolant, there can be some problems so using distilled water will not hurt. That said, I have never bothered and I would guess few people do and very few experience any problems by using tap water. It is important to change the coolant as it appears you have been doing. If it has worked for the last 40+ years, I suggest keep doing what you have been doing.

What they said. If the tap water is hard, it might be marginally better to use distilled water to fill. I should still be O.K., even, to flush with tap water. You will see little problem with water quality in a closed system like this compared to a chilling tower, steam boiler or water heater where the water is constantly being changed. There, corrosion and scaling are huge (expensive) issues.

Even acidic water should not be a problem as it will not have significant buffering capacity compared to fresh coolant. I would not use dog water, use nothing yellow in there unless it comes out of a coolant jug.

De-ionized water has no minerals in it, and there for is a sponge looking to absorb minerals, like the radiator, and other metals in the cooling system, Not what I think you would want. A matter of fact, plumbing for Di-ionized water needs to be stainless steel or plastic.

It’s not a problem when mixed with the correct antifreeze in your cooling system. The intent of using distilled water is to avoid minerals from the water being deposited in the cooling system.

distilled and de-ionized water is not the same. Although after doing some research I did read were Di-ionized water is preferable.

I would just buy pre-mixed antifreeze/coolant. That would easily resolve the issue. Tap water is fine though, as long as you change it on schedule. Don’t give the antifreeze/coolant chemicals a chance to stop working. Drain and refill every two years.

That is correct, I use distilled water in cooling systems. Even de-ionized water should be harmless when mixed with antifreeze containing corrosion inhibitor packages. I certainly would not use straight de-ionized water in a cooling system.

You say, “…it would be a needless complication, although rather minor in cost and nuisance.” So, OK. Use the purest water you can buy; unless, you mean, “…it’s major in cost and nuisance.” And, “I want to ensure that whatever minerals are in my tap water do get deposited on the inner surfaces of my radiator and the rest of the cooling system.”

Tap water is fine.

Some people insist on using deionized or distilled water, and then they dump it into a filthy cooling system. Real smart.

Some tap water probably isn’t bad but if you have well water in an area that is full of lime, you shouldn’t use tap water. I have used bottled drinking water by the gallon and I would recommend it as well as distilled or filtered water. Reverse osmosis… Oh well. There is factual information but I can’t find it right now. Where did I put that factual information?

If you have a filthy cooling system, you have bigger problems than using tap water.

I’m not sure why we would bother to argue about distilled water that costs about $0.50/gallon at the corner store. If you are going to buy $40 worth of antifreeze every couple of years, why are we discussing an extra $1.00 every two years.

I think the most important thing is to make sure the cooling system is leak free. So air is not getting into the system re oxygenating the coolant making it corrosive.

I agree that it’s not a cost issue, probably would only take a couple of gallons. However, consider this: an automobile radiator is a closed system. If it is working properly, you are not adding water routinely, as you would in a coffee pot, home heating boiler, or an industrial boiler. In those applications, you are evaporating water, not recirculating it. Even in those applications, you can evaporate a huge volume of water before it produces significant scaling or sediment in the mud drum, in the case of marine and industrial boilers. One of my neighbors builds steam locomotives (about 3 feet long) as a hobby. These are total loss designs, which means that none of the condensate is recycled, thus all steam is the result of evaporation of new water. He now uses tap water for these applications and generally runs 300-500 gallons through one in the course of a summer. (Sets up in a park and gives rides to kids.) He does an acid scaling of the boilers every few years. Also, a steam iron evaporates a fair amount of water and most people just use tap. Mine is about 10 years old and shows very little scale. We have a steam humidifier in our house that runs most of the winter. Every spring I take it apart and clean the scale out because the owners manual says to. It’s less than a teaspoon full.

My conclusion is that in a closed system where there is no evaporative loss, there simply isn’t enough mineral content in two or three gallons of water to make any difference. One day when you have nothing better to do, just boil 2 gallons of water until it is completely evaporated and see how much scale you actually have left.

However, the cost is negligible and if it makes you feel better, do it.

It depends what you consider negligible. Last year I replaced the radiator in one of my cars (400+K miles, 25 years old) and reduced the normal operating temperature of the engine by about 5C. Clearly there was enough accumulation of minerals and/or corrosion products to affect the heat transfer over the life of the radiator. That’s enough of a difference for me to use nothing but distilled water when I replace the coolant, I expect the new radiator to last considerably longer.

It also depends on the mineral content of your tap water, some areas are much worse than others. That said, if you are only going to keep you vehicles 5 or 10 years it probably doesn’t really matter.

At 400+k out of a radiator I think you were doing great. I think I would keep up whatever habit you had.

But, that being said I think most new radiators’ week point is that they have a lot of plastic in them and that goes out before the water has any real effect on the copper/aluminum.