Is it safe to reheat old water in an electric kettle? I usually throw away the remaining water in the electric kettle before refilling it if the water is more than about a day old. In theory wouldn't boiling kill any impurities the old water might have? How long would the water stay drinkable in the kettle (assuming you boil it again before drinking)?
Maybe I'm wrong, but there is no safety risk at all when boiling old water. There is however something like taste. The reason the old water is discarded is because, after boiling, it lacks oxygen and will taste stale. This is also the reason why coffee is made with 95ºC hot water, or why Moroccan tea is poured from above (to oxygenate)...

So, as long as you boil it before drinking it should be safe.