The OP's question was about boiler water for steam, not about HX water for shots. Focus people!

I think that many could not know that in the Semi the boiler is not automatically filled.

For what concerning lead, I think, and thus underline, that cannonfodder is joking as his smily says. Lead is a toxic metal and it would be better to chose another metal as sacrificial anode.

@algue: But lead is so DELICIOUS!!! ....and yes, you're correct, Jim's review on the Semiautomatica is fantastic....it's his review that ultimately sold me! The described "impracticalities" were no big deal to me--and if something can look THAT good, and still function at a respectable, even great level for its intended purpose, then I MUST HAVE IT (I buy cars with a similar rationale).

@3-oxocyclopentanecarboxylic acid (which C6H8O3 do you prefer, anyway?): Thanks for the suggestions and references! Great ideas!

@everyone else: Yes, true, distilled sucks for making drinks as much as it does for drinking by itself....but lucky for me, like Jim points out, I can fill the boiler separately and then pick whatever de-LISH-ious, minerally water I'd like to fill the reservoir for making espresso, and just descale the group & HX portion however often I need to....which, if I used my tap, would be about every other day (I actually have to clean my glasses with vinegar if tap water is left to evaporate in them even once. I can practically cut my water with a knife....just like my air. Yes, I live in L.A.).

Thanks so much for everyone's input!! I really appreciate your thoughts!!

P.S. @algue: I LOVE how you described it: the "sacrificial anode". I know, I know....that's just what it is...but as a non-chemist, it's wonderfully colorful terminology for a chunk of metal. Now I really want one. Maybe cannonfodder can suggest one for me if the lead doesn't work out so well

Ah, chemistry humor.

I've seen recently a boiler from large commercial machine with several leaks due to corrosion of brazed connections.
The cafe it came from was using direct tap connection without any filter/treatment system and water was from desalination/RO plant supplying our town.
It is low in minerals and there is no scale build-up when boiled in a kettle although minerals contents is very similar to the bottled water from natural source I use.
Distilled water, despite bad taste, is quite corrosive.

For what it is worth, I thought I would just add: NEVER USE DISTILLED WATER IN BOILER. Even water that is too soft is unhealthy. If you do not find any scaling in your boilers then it is not a good sign!!

Water needs to be hard enough to form medium scaling inside boilers, to quench thirst, and to have a taste signature.

witoldb wrote:Distilled water, despite bad taste, is quite corrosive.

Distilled water has No taste at all. The taste comes from the pipes it flowed through as well as the containers it was stored in. So the bad taste itself is a result of its corrosivity.

Not just metals or metal salts, even more unknown plastic molecules leach into distilled water. Stay away from distilled water as well as overly soft water if you intend to drink it.

Distilled (or DI, or any other very pure) water is not corrosive, nor is it unsafe to drink. The presence of electrolytes in water makes it MORE corrosive, not less. Don't believe me? put an iron or steel bolt (not stainless) in a dish of distilled water, and another in a dish of salt water, and see which rusts first. Or another experiment you've already done: every time you use your steam wand, the water that re-condenses inside the wand is distilled water. Ever had your steam arm corrode and fall apart?

As to safety, you'll be hard pressed to find a legitimate conclusive medical opinion one way or the other. Crackpots exist on both sides of the issue, attributing both unrealistic benefits and unrealistic dangers to the drinking of distilled water.

It is true that under the right circumstances consuming EXCLUSIVELY distilled water can leave you depleted of electrolytes. If for example you are an athlete and routinely sweat your ass off, and drink only distilled to replenish, sooner or later you will have a problem. This is also the reason sports drinks like gatorade are a thing. Most ordinary people get more than enough electrolytes through their diet that this is not a concern. I like a little bit of minerals in my water because it tastes better, but that's a separate issue.

Further anecdotal evidence: a friend of mine (and his wife) has been drinking DI water as his drinking water almost exclusively 10 years or so with no ill effect. He also has his espresso machine plumbed in to his DI system, and the machine has not required any service at all for the over 5ish years he's had it.

From the American Galvinizers Association:
Pure water, also known as de-ionized or distilled water, is usually very corrosive to zinc coatings due to the presence of dissolved oxygen and carbon dioxide. Corrosion rates of steel increase with aeration of pure water; dissolved oxygen in pure water is five to ten times more aggressive than carbonic acid
---

So it's not the pure water, but what is in the water including gasses that cause the corrosion. When water is heated in the boiler the first thing that happens is most of those gasses are driven out of the water. Purging the dry air or even steam releases that dissolved gas - my hypothesis anyway.

I think the "dangers" of pure water are mostly fiction, but a bit of gas in the water could significantly increase some corrosion if water isn't heated and sits there long enough.

right, dissolved gases are the culprit, and having some trace minerals doesn't make that problem go away. You could potentially have more gasses in very pure water, but unless you go out of your way to aerate it it doesn't mean you will. And as you say heating will degas anyway.

My wife, the industrial water treatment chemist, says the culprit is distilled water, itself. Water is a solvent, period. The purer the water, the stronger its solvent properties.