Is it dangerous to prepare tea directly in a water boiler? If I prepare green tea in a water boiler there is some discoloring of the container. Are there any bacteria/ molds which are dangerous to one's health which could survive between boiling intervals supposed you heat the water until it boils? How often should you clean the container thoroughly using strong detergents?
Although the initial question seems crazy enough from the start, the preamble to this answer has to make it clear that you should not use a water boiler with tea – or anything else but water – in it. This answer has to assume that the question is about: only green tea or plain water used directly in the boiler.
Doubting that there are really people so much after the IgNoble that real studies have been conducted to test this. Even if called unsolicited advice: just don't do that. Reasons and alternatives follow:
The boiler should not contain any plastic (which might leach plasticzers and other chemicals into the water) or heat spirals that are not made out of stainless steel (since some of the old type spirals may leach nickel and other metals into the water). [What to use then? Glass and stainless steel, or a traditional pot/kettle.]
Steeping too hot or for too long results in the release of excessive amounts of tannins, leading to a bitter, astringent brew, regardless of initial quality.
And, extraordinarily, Mr Gorman advocates something which - had they been invented - Orwell would surely have balked at; using a microwave to make tea. Mr Gorman, whose organisation represents tea packers, brokers and importers, said: "Usually when people's tea goes cold they reboil the kettle and make another cup. But doing this you are guaranteed to give yourself a dull cup of tea. You need freshly drawn water for a good cup because reboiling it takes out all the oxygen and nitrogen out of it.”
He added: "A better solution is to put it in the microwave for 15 to 20 seconds. When you microwave tea all you're doing is from a scientific point of view is just moving the molecules around and getting it back up to a decent temperature. It is not impacting the flavour at all."
Cleaning the water boiler with strong detergents may leave some of these detergents behind. That is presumably the most dangerous health problem with the approach outlined in the question (provided that the kettle itself is really fit for food grade application).
If the discolouration bothers you or starts to impact the boiling efficiency: soak it in soda. Tea stain will then come off so easily that you might use a very mild and eco-friendly simple soap to keep it shiny clean. Rinse thoroughly.
You won't attempt brain surgery with shards from your kettle? So it becomes an argument of common sense that you will not need absolute sterility. Boiling fresh water of decent quality will kill enough microorganisms (that will be in there in very small numbers to begin with). Even baby bottles are now free from this germ-o-phobe advice.
Finally, the inventors of tea have the following to say:
Yixing teapots are meant for use with black and oolong teas, as well as aged pǔ’ěr tea. They can also be used for green or white tea, but the water must be allowed to cool to around 85 °C (185 °F) before pouring the water into the pot. Yixing teapots absorb a tiny amount of tea into the pot during brewing. After prolonged use, the pot will develop a coating that retains the flavour and colour of the tea. It is for this reason that soap should not be used to clean Yixing teapots. Instead, it should be rinsed with fresh water and allowed to air-dry.
Other info: Never use anything but water to clean an yixing pot. Soap will be absorbed and create a permanent soapy flavor in the tea you steep. Always allow your yixing pot to dry completely between uses, and before replacing the lid and storing. A teapot stored wet with the lid on will mold. If your teapot molds, reboil for 10-15 minutes and re-season. Take a picture of your pot when you start using it so that you can compare in a year to see how it has grown.
While this 'boiling the tea directly in the kettle' is not a good idea for a number of reasons, the negative health impacts from the tea residue alone should be negligible.