The purified water you would get from a laboratory RO unit should absolutely not be used. Pure water is very corrosive, and the purer it is, the more corrosive. And lab-grade water is some of the purest available. In my experiment linked above, I had visible dezinc pitting in brass test samples after only 120 days with lab water. Unfortunately the experiment is in a members-only section and is not visible to the general public. I'll talk to the mods there and see if it can be copied or moved somewhere where others can see it. Otherwise I'll just re-post the whole thing here.
But to summarise, very pure waters such as deionised water (which is what a lab water system produces) are corrosive. Less pure is distilled water, however there is some evidence that distilled water is still too pure and possibly corrosive in steam boilers. People often say that rainwater or dehumidifier water IS distilled water, but this is only in theory. The time they each spend in the atmosphere is very different, and affects the purity. Rainwater has been drifting around as clouds for possibly days before finally falling, and in that time it picks up a lot of dust, soot, and dissolved acidic gases from industry etc. Dehumidifier water spends much less time in the atmosphere, yet it is still an open system. A steam distillation setup is a closed system and the water is therefore more pure. The few manufacturers who know their stuff about water suggest adding some tapwater to pure waters such as deionised or distilled. This will add some minerals to it and stop the corrosivity. Marklin and Regner both advise this, and it is excellent advice. If you use a very soft water such as deionised with 5% tapwater, and then empty the boiler after use, you will have no problems with scale, and no problems with corrosion.
My experiment showed that dehumidifier water was the best of all - the brass surface was very clean and devoid of any scale buildup, and there was no pitting. That's why I was wondering how dunnyrail knew this.
Regards,
Kevin