How often do you need to clean out a teapot? I usually heat water up for some tea and then re-heat that same water up again later. Is that safe?
Are you talking about a teapot or a kettle? If you reaheat the water in a teapot, you’d be heating…tea.
Ah yes. A kettle. Heh, sorry 
I’d imagine you would clean it out the same way you’d clean out a coffee maker. I don’t drink coffee, so this is from memory, but I think you run a batch of vinegar though it, and then a couple batches of water, that might do the trick.
(I don’t have a bottle with me to look, and for the love of god PLEASE read the label before doing this but CLR might be applicable in this case.)
This may be completely fictitious but I seem to recall hearing or seeing something some advice that reboiling water for tea reduces the number/effectiveness of the anti-oxidents…
Whether this is false, an impact on the tea, or a property of the water itself being boiled more than one I have no idea, but might provide you some more search criteria
That would only be true if there were antioxidants in your water. The stuff that comes out of my filtered tap water is pretty much H[sub]2[/sub]0 and some trace minerals. Is it possible you heard about reboiling the tea itself?
Every time you boil water, you lose some of the water through evaporation. Any impurities in the water, however, do not evaporate. Hence repeated boiling increases the oncentration of impurities in the water.
How significant a problem this is depends on the nature and concentration of impurities to start with but, in principle, reboiling water intended for consumption is not a good idea.
(Of course, if you suspect that the water contains living impurities then you should boil continuously it for an extended period - ten minutes or so. This will increase the concentration of impurities, but the negative effect of this shouldbe outweighed by the benefit of killing the living organisms. But modern electric kettles will not boil water continously; they switch off once the water comes to the boil. And repeated reboiling will not kill organisms effectively.)
But the main objection to reboiling water is probably that it indicates you boiled more water than you needed first time round, which is ineffecient.
Dharkon, you mis-heard. When water boils, its oxygen content is reduced, and the theory is that oxygen is needed to extract full flavor out of tea leaves (lots of Google results, but no scientifically rigorous ones about this theory, so no cites); therefore, de-oxygenated water makes for flat-tasting tea.
[ul][li]Our very own Science Advisory Board’s word on making tea.[/li][li]The gubmint explains how oxygen gets boiled away.[/li][*]The readers of New Scientist weigh in with their opinions.[/ul]
…the best taste-neutral solution (no pun intended) is to add a couple of tablespoons full of bicarbonate of soda (baking powder) to a full kettle, boil it up and leave it overnight to cool and soak. Rinse out - clean as a whistle (why are they so clean? Whoops, hijack!)
Remember to tell whoever is going to make the tea in the morning what you have done though - in case they follow the “reboil the water in the kettle” procedure you see questioned here. As an aside I always use fresh water for any beverage brewing - think I would notice the taste difference…
Oh, the baking poweder things works great for teapots too, especially if you tip them up to soak the spout too. Just boil in the kettle and transfer to the pot and soak. About once a month or so for the purists…