I've read (and experienced) that brewing tea with a tea cosy on the pot results in "stewed" tea that is unpleasant and bitter. I've had this happen to both black and green tea, but white doesn't seem to suffer as much. Of course, black and green teas have so short steeping times that a tea cosy isn't needed, but some white teas are supposed to steep quite long. This Yin Zhen
says seven minutes, for example. Even it doesn't get bitter with the tea cosy, is the taste still damaged? Is it better to steep without the tea cosy and let long-steeped white tea become somewhat colder?
I assume this isn't an issue for rooibos and herbal infusions that don't go bitter (please correct me if I'm wrong), but what about pu erh? I've read that the aged kind can't be oversteeped, but I'm not sure if that holds for temperature as well as steeping time. And what about raw pu erh?
Actually, after writing this, I wonder if even a pot without tea cosy causes minor stewing compared to steeping in a cup without lid due to the higher insulation/temperature of the pot. What do you think?