Distilled water is a very BAD idea.

Nature abhors a vacuum. Water has an affinity to gather minerals. If distilled water is used, it's like a vacuum. It will start gathering minerals from the available sources it's in contact with, i.e. the pipes, heat exchangers, fittings, etc... Basically, it will corrode things faster.

Clark,

Do you have anything to support the contention that distilled water somehow sucks in "minerals"?

There are no minerals in metal plumbing components.

Corrosion (oxidation) takes place only in presence of oxygen...I don't see a connection between use of distilled water and accelerated corrosion in a purged closed loop system.

I've heard a similar argument (debunked) about using distilled water in automotive cooling system. Hardness molecules wreak much more havoc than any distilled water corrosion issues.

I respect much of what you've posted in the past, but this flunks a smell test for me.


If I'm missing something major here, by all means let me know...

Maybe I'm misunderstanding what I was told by an engineer/geologist friend of mine. It seemed to make sense to me that distilled water, being virtually void of minerals, would act somewhat like a vacuum in nature. Don't know, I guess. Sorry for the bad info. It sure made sense when he explained it to me, but I can't remember all of his points exactly. The basic gist was what I posted earlier... Thanks for the correction

I don't know if it is bad, and I'm certainly no metallurgist or chemist.

I do know that in high pressure, high temperature boilers a huge effort is made to remove every last possible mineral and bit of oxygen in the water that is boiled into steam. Minerals cause scale, reducing heat transfer and ultimately leading to tube failures / burnouts. Oxygen causes corrosion.

Not sure how / if that bears on this.

While distilled water has a theoretical pH of 7 (neutral), it actually has a lower pH (as low as 5 which is acidic). The reason for the lower pH is that by distilling water you take out all the buffering capacity the water naturally has so if the water comes into contact with air it will grab the CO2 and transform it into acid. This is somewhat like what your friend mentioned Tuffluck.

Because distilled water is more acidic, it can cause corrosion. I can't say how serious it would be in a geo system, but I can say that if we didn't treat the condensate of industrial boiler systems (which is in essence distilled water), the pipes that the condensate would flow down would not last a year.

The true problem lies with the distilled water coming into contact with air, if it did not come into contact with air it will work great.

As far as the definition of corrosion:
corrode - cause to deteriorate due to the action of water, air, or an acid; "The acid corroded the metal"; "The steady dripping of water rusted the metal stopper in the sink"

Interesting.

I suspect pH 5 would be bad news for the metal in a geo heat exchanger. I wonder if the choices of antifreeze (various alcohols, or glycol) affect this?

Not sure I agree with you Ona.

CO2 concentration in air is about 350 PPM. That wouldn't seem high enough to increase the H+ ion concentration by a factor of 100 (pH is a log scale). The cause of corrosion in outdoor environments (like the condensation you mention) is primarily due to oxygen. And a pH of 5 in distilled water would likely be from SO2, not CO2. But since power plants have fairly sophisticated SOx control systems we rarely hear of acid rain anymore.

The pH of drinking water runs from about 6 - 8.5. Even the low side pH (6) does not usually cause problems with pipe.

Actually Clark, you and engineer are both right and both wrong. In theory metal would move to the distilled water. But the rate of movement depends upon the type metal. Lead can not be used in solder for drinking water due to its fairly rapid movement into the water. However, there is no issue with copper moving into nearly neutral water. And titanium would have no issue with it. Copper will however move into water with low pH (a la the green "whites" in overly acidic drinking water).

In the real world the systems are not 100% closed. Therefore in hotwater circulation systems they generally add corrosion inhibitors. But in truth, they don;t use distilled water in them either.

So, Clark you are right and Engineer is mostly right too.

And yes engineer, the alcohol or glycol used does have a significant impact upon corrosion and heat transfer.  Methal alcohol (wood alcohol) is very corrosive. Ethyl alcohol (drinking alcohol) is less corrosion but still is corrosive.  Same for ethylene and propylene glycols.   The viscosity and heat transfer characteristics of the fluid change significantly as you increase the alcohol or glycol concentrations.

Whew... fingers are tired ... I type too slow ......

Good stuff, Alex.

Evidently I was a bit out over my skis in my response to Clark.

Considering all that, what would be your recommendation for the "best" antifreeze in a typical closed loop (non-dx, of course) system consisting of a bunch of poly pipe and the usual flow center and coax heat exchangers?

That is a loaded question my friend ... sort of like asking which do you like best: Ribeye, New York or Del Monico steaks (I like ribeyes *smile*).

the answer really depends upon the level of protection required. If just a little (say down to 25 F) I would likely consider propylene glycol. You can actually drink the stuff since it is used as a food additive. I would probably go with ethyl alchol plus corrosion ihibitor next.

Propylene is harder to pump and provides less temperature suppression than ethylene glycol but is safer and has better heat transfer capacity.
Ethyl alcohol works well also but may be harder on seals and pump parts.

So the answer really is.. "it depends"