"Is the water collected from a dehumidifier distilled or deionised? "
A good question, Smidget, you got me thinking again.
We're expanding the original question of whether 'deionised' and 'distilled' are significantly different in terms of radiator fluid requirements. Strictly, the technique of distillation is the act of heating a mixture so that the components are separated based upon their volatility. The component with the lowest boiling point vaporises first and the pure vapour can then be isolated and cooled so that it condenses into a separate container.
With electric de-humidifiers (and fridges which Gunger mentioned elsewhere) there is no prior purification step; ie. the heating stage which fills the condenser with water vapour and only water vapour. The air is chilled, water condenses and falls into the collection box. If there is tobacco smoke or bacteria in the air it is likely to contaminate the condensate just as Preunit suggested with rainwater. Whether you'd regard that as a problem would depend on whether you plan to put it in your radiator or drink it, but it will be limescale-free.
In this context I should have described rainwater as "deionised" or just "purified" rather than "distilled" in an earlier post, so Preunit was right to say rain isn't strictly "distilled". The degree of de-ionisation/ purification also becomes important. Brutale r gets into that with conductivity measurements on the "I'm brewing WATER!!!" thread.