Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Bonjour,
Be careful, demineralized water will tend to want to remineralize, which it will do by attacking the metal that receives it.
Do not hesitate to use tap water from time to time, provided of course if it is not (too) calcareous and completely empty the boiler when it is not in use. Personally, I put it under atmospheric pressure and put aside its filling cap to avoid the rising of lubricating oil.