My hot water tap is supplied by a combi boiler which heats the water on-demand, supplied by the same cold water source as my cold water tap. As a result, I'm confident that the water is reasonably fresh and clean. I don't use it for brewing tea or coffee, but I'm happy to boil vegetables and rice in it. It saves a couple of minutes bringing the water to the boil.

My parent's hot tap is supplied from an insulated immersion heater tank. The same water can sit in there for days; it may heat and cool several times in that period. It is supplied by a header cistern in the attic. Last time I looked at the header cistern, there was a crop of dead flies floating on the surface, and some unidentifiable gunge settled at the bottom. *This* hot water is suitable for bathing and cleaning; it's not suitable for cooking.

If you don't know the details of your plumbing; and you're not sure it's safe; don't cook with water from your hot tap.