I and my local club use rain water, I believe this is a common practice. There may be some small contamination as it falls and I always filter to remove debris. Noel.

Buy DI water and put a sample of the metal(s) of concern (a cleaned up bit of silver soldered copper and a bit of brass for example) in the bulk container. This will will scarifice enough ions in a week or two to protect your boiler.
ou could also use an ion exchange resin water filter jug e.g. Brita to make your own DI.
Personally I think the issue is over inflated. The problem is possibly over inflated from issues in high cleanliness processes like semiconductor manufacture.

An example of my direct experience relates to aircraft gas turbine engines. It is standard practice to wash the compressor and tubines of engines with water (and other materials including detergent or ground walnut shells) to remove dirt and corrosive deposits. We operateed to and from islands at low levels so salt was an issue both for buildup causing loss of compressor efficency and chlorine causing corrosion of the turbines. So we reguarly washed the engines with distilled water that was locally sourced in 25l containers from an industrial supply company. Despite this a “hot section” inspection revealed corrosion on a turbine. one blade had a “blister” about 2mm across. This was analysed and found to be a sulphur compound. This was unexpected. Investigation revealed that the distilled water was contaminated with sulphuric acid. The suppliers main customer was a battery company who also bought H2SO4. This came in identical returnable containers. The supplier didn’t bother seperating the water and acid  containers and their other customer didn’t acre mecause the mixed them anyway.
So we switched to a different water suplier. I found out who when I was going into town and was asked if I could pick up some water from Boots. There were too many 5l containers of BP grae distilled water to fit in my car. It was expensive too. I suggested we bot an Elgastat DI water “filter” and made our own DI. The leaching issue was brought up as the engine manual only said distilled. The manufacturer confirmed that DI was OK to use. We never had a corrsion issue after switching to self-droduced DI. Note that Nickel is a metal that is used in turbine blades and is leached by DI.

Robert.

On 30 August 2025 at 10:38 Richard Simpson Said:

On 30 August 2025 at 09:45 renardiere7 Said:

Surely de-ionised water is the complete opposite of an electrolyte?

So where does the advice not to use it come from?  And why?  Or have we simply fallen into a trap of believing old stories based on myth?  I really don’t know.

An electrolyte by definition contains ions. It is a material that conducts electricity by movment of ions rather than by movement of electrons. So renardiere is correct DI  is most cetrtainly not an electrolyte. In practice the purity of DI is tested by measuring it’s electrical conductivity. The lower the conductivity the purer the water.
Putting DI in contact with a source of ions turns it into an electrolyte to some degree.

Robert.

Whole world of difference between  21st century marine practice on 1,000 psi superheated boilers supplying turbines and 19th century steam locos chuffing along.

But in the 1920s, according to my grandfather who sailed on them,  Grimsby trawlers ran seawater in their boilers. They just shovelled the salt out at the end of every 3-week sojourn at sea. More primitive even than the railways!

If you read Wardales book about the Red Devil he waxes lyrical about keeping total dissolved solids (TDS) high. I think this is to control priming, but clearly keep it below the level at which scale develops

Some of the de-ionising methods are aimed at domestic steam irons, kettles etc so primarily interested in removing the calcium to avoid it furring things up. They do this not by removing the salts but swapping them for something that doesn’t settle out when heated. So not actually de-ionising nor distilling equivalent. These types can then swap back with the copper in your boiler, especially the zinc when we still used brass fittings.

We used to have a rainwater collection and settling tank in our old site and the leaves that came off the roof were left in because nobody could be bothered to clean it and as tannin is used for boiler treatment it was considered beneficial.

Since everyone with a workshop ought to be running a dehumidifier if not living in the Sahara it makes sense to use the output. Just note they catch a little oil vapour, dust, and the condenser gradually corrodes releasing a very small amount of contamination. Use a coffee filter if unable to use a settling tank.

On 30 August 2025 at 12:34 SillyOldDuffer Said:

Another difference between Railway and Marine practice!

As opportunities to clean boilers at sea are limited and the consequences of breakdowns are severe, it pays to take extreme care with boiler water.   Steam loco boilers are less demanding.  They were maintained daily and regularly washed out, plus locos can easily be towed back to the depot if they break down.   So railways tended to fill their boilers with ordinary water, only treating if it was exceptionally bad.    Agricultural traction engines filled up from any handy ditch.

Model boilers are different again.

There isn’t a black and white answer because it depends on the Requirement.   What does the owner want and how much is he prepared to spend?    We want the best for our models but dislike spending money!   It’s about compromise.

Choices:

• Tap water is mostly ‘good enough’ for those living in a soft-water area, otherwise water-softened hard tap-water is similarly ‘good enough’. Safe to drink.   But water-softening chemicals only remove the Calcium and Magnesium salts responsible for scale.  There will be some Chlorine, Oxygen and Carbon Dioxide in it, plus small quantities of Iron, Copper and Lead from the pipework.
• Rain-water is generally pure and cheap.   Purity is affected by how it’s collected: running over dirty roofs, pollution, and maybe dirt rendered airborne by wild-fires and storms.   Should be filtered and I wouldn’t risk drinking it!  Contains dissolved gases, Oxygen, Carbon Dioxide, and Nitrous Oxides, maybe Sulphuric Acid from coal burning.
• De-ionising removes metal salts and some gases. Unsafe to drink because it doesn’t remove bacteria or viruses, otherwise very clean.  Unlikely to damage a model boiler.  Perfumed deionised water for steam irons is probably harmless, but no-one has tested it.   Main disadvantage is cost.
• Distilling removes almost all contamination, but is very pricey.

Cheap sources like freezer frost, dehumidifier and tumble drier water are pretty filthy in my experience.  Whilst the salts that cause scale are removed, the water contains bits of fish-finger, dust, and lint.  I wouldn’t drink any of them and no one knows if they will damage a model boiler or not.

The chemistry inside a boiler is on the extreme side.  High-pressure, high-temperature water, steam, plus whatever gases were in the water and air when the boiler was filled.  Certainly capable of breaking down anything organic, dislodging old flux, corroding metal, and dezincifying any handy Brass.  Conditions vary wildly between hot and cold cycles.

I suspect most damage is done when boilers are out of service.  Damp Oxygenated air left for weeks on end inside is slow-but-sure corrosive.    My book on full-size recommends either drying out the boiler thoroughly and sealing after filling with dry Nitrogen, or, filling completely with thoroughly boiled pure water to remove dissolved gases, and then sealing.   In both cases the boiler is stored full of cold inert material, air kept out!

In practice, Richard can safely use de-ionised water if he wishes, and definitely need not waste precious spondulicks on Distilled Water.  In a hard-water area rain-water is best value for money – provided there’s a space for a water-butt, and a filter.

Dave

On 30 August 2025 at 12:34 SillyOldDuffer Said:

Another difference between Railway and Marine practice!

As opportunities to clean boilers at sea are limited and the consequences of breakdowns are severe, it pays to take extreme care with boiler water.   Steam loco boilers are less demanding.  They were maintained daily and regularly washed out, plus locos can easily be towed back to the depot if they break down.   So railways tended to fill their boilers with ordinary water, only treating if it was exceptionally bad.    Agricultural traction engines filled up from any handy ditch.

Model boilers are different again.

There isn’t a black and white answer because it depends on the Requirement.   What does the owner want and how much is he prepared to spend?    We want the best for our models but dislike spending money!   It’s about compromise.

Choices:

• Tap water is mostly ‘good enough’ for those living in a soft-water area, otherwise water-softened hard tap-water is similarly ‘good enough’. Safe to drink.   But water-softening chemicals only remove the Calcium and Magnesium salts responsible for scale.  There will be some Chlorine, Oxygen and Carbon Dioxide in it, plus small quantities of Iron, Copper and Lead from the pipework.
• Rain-water is generally pure and cheap.   Purity is affected by how it’s collected: running over dirty roofs, pollution, and maybe dirt rendered airborne by wild-fires and storms.   Should be filtered and I wouldn’t risk drinking it!  Contains dissolved gases, Oxygen, Carbon Dioxide, and Nitrous Oxides, maybe Sulphuric Acid from coal burning.
• De-ionising removes metal salts and some gases. Unsafe to drink because it doesn’t remove bacteria or viruses, otherwise very clean.  Unlikely to damage a model boiler.  Perfumed deionised water for steam irons is probably harmless, but no-one has tested it.   Main disadvantage is cost.
• Distilling removes almost all contamination, but is very pricey.

Cheap sources like freezer frost, dehumidifier and tumble drier water are pretty filthy in my experience.  Whilst the salts that cause scale are removed, the water contains bits of fish-finger, dust, and lint.  I wouldn’t drink any of them and no one knows if they will damage a model boiler or not.

The chemistry inside a boiler is on the extreme side.  High-pressure, high-temperature water, steam, plus whatever gases were in the water and air when the boiler was filled.  Certainly capable of breaking down anything organic, dislodging old flux, corroding metal, and dezincifying any handy Brass.  Conditions vary wildly between hot and cold cycles.

I suspect most damage is done when boilers are out of service.  Damp Oxygenated air left for weeks on end inside is slow-but-sure corrosive.    My book on full-size recommends either drying out the boiler thoroughly and sealing after filling with dry Nitrogen, or, filling completely with thoroughly boiled pure water to remove dissolved gases, and then sealing.   In both cases the boiler is stored full of cold inert material, air kept out!

In practice, Richard can safely use de-ionised water if he wishes, and definitely need not waste precious spondulicks on Distilled Water.  In a hard-water area rain-water is best value for money – provided there’s a space for a water-butt, and a filter.

Dave

I can’t comment on de-ionised water as I have a bigger engine that consumes 50+ litres per day and would be too expensive and you have already mentioned that you have fitted water filters.

You also noted that you only used the boiler a couple of times, so if it is only going to be use occasionally, flush the boiler out after use with scale remover that is used to clean coffee machines and irons after use before storing.