We talked a little about this a while back.
I know there are people who use forums for advice but sadly this 'open forum advice' often becomes misleading.
There has been discussion elsewhere on other forums about the use of correct water for model steam engines.
Without questioning those effluent steamologists with seemingly far superior chemical knowledge here is the most important information and advice published by the 3 major manufacturers.
....Fill the boiler..... ideally using distilled water...
Use distilled water plus 5% tap water.
Roundhouse (copied from their website).
Well, water is just water isn't it?.
Actually no, it's not quite that simple.
Our miniature steam boilers are made mostly of copper, with some bronze and silver solder and there are also brass steam fittings for the steam to negotiate before it exits to atmosphere. Although all these items are made from non-ferrous metals and therefore do not rust, they are subject to chemical attack in other ways. Here is a quick run down of the common sources of water with their pros and conn s.
Tap water - in soft water areas, this is fine but in hard water areas it can quickly lead to a build up of 'lime scale' on the inner surfaces of the boiler, fittings and pipe work. The easiest way of establishing what your water is like is to look inside your kettle. If it's thick with white 'lime scale' deposits, avoid using it.
Rain water - free of charge and quite good if you can get sufficient quantity. It must be filtered to get all the dirt, grit and other crap out of it and the filters used by home brewers and wine makers does an excellent job.
De-ionized water - this is often sold for use in steam irons and the general opinion amongst the small scale live steam community is that it should not be used. Because of the way it is 'purified', it can cause long term problems by slowly removing zinc from the brass fittings - commonly called de-zincification.
Purified water - tricky one this, as it is not always clear how purification has taken place. Shops that sell it will variously tell you it is de-ionized, or distilled. If you can't be sure that it is distilled, don't use it.
De-humidifier water - another good source. A de-humidifier is a bit like a fridge in reverse, and the water that collects in the tank is quite safe to use.
Refrigerator/deep freezer ice - good. It is basically moisture in the air that has frozen and once thawed out is good to use if you can get sufficient quantity.
Distilled water - the best water you can use. Unfortunately, it is difficult to get hold of. If buying distilled water, be very sure that it actually is. Some retail outlets will sell 'purified' or 'de-ionized' water (for steam irons or car batteries) and tell you it is distilled but it is actually quite different.
You will probably find that a combination of these will supply all your needs but if you store or stock pile collected water, be sure to filter it well before use and change the containers from time to time. If not, you will find algae and other deposits forming in the water which will cause steaming problems. Dirt, algae and other debris can cause the water to foam as it boils and this will cause priming to the cylinders and syphoning at the safety valve.
I personally use twice filtered rainwater in containers regularly replaced.