Full boil is right for exactly two things in a mug: black tea and instant noodles. Everything else you pour hot water over tastes better a little below 212 degrees, and the reason is chemistry, not snobbery.
What temperature changes in the cup
Hot water pulls compounds out of leaves and grounds in order. Aromatics and acids come first, sugars next, and bitter tannins last. Hotter water accelerates the whole sequence, so the bitter end arrives before you finish steeping.
Green tea and the 175 rule
Green tea leaves are barely oxidized and give up tannins fast. Boiling water scorches them into something astringent and flat. Around 175 degrees, the same leaves taste sweet and grassy. Variable-temperature kettles in a small-appliance catalog like https://peach-street.net/ exist for exactly this one problem.
Coffee lives between 195 and 205
Below 195, extraction stalls and coffee comes out sour and thin. Above 205, you strip harsh compounds and get ashy bitterness. That ten-degree window covers pour-over, French press, and drip.
Black tea really does want the boil
Fully oxidized leaves need the energy. Assam, Ceylon, and breakfast blends brewed at 190 taste weak no matter how long you wait.
The no-thermometer method
- Boil, kill the power, and leave the lid open.
- Roughly 30 seconds off the boil drops a full kettle about 10 degrees.
- Two minutes gets you near 195, good for coffee.
- Four to five minutes lands close to 175 for green tea.
- Pouring into a room-temperature mug or pitcher costs another 10 degrees instantly.
Why a glass wall helps here
You can see when the rolling boil settles into stillness. It is a crude gauge, but it beats guessing at a stainless box with no window.
Altitude quietly moves the target
Water boils near 202 degrees in Denver and lower still in the mountains. Coffee brewed at altitude is already at the cool edge of the window, so waiting after the boil there makes things worse, not better.
Reboiling is not the crime people claim
Reboiled water loses dissolved oxygen and tastes slightly flatter. That is all. It is not dangerous, it does not concentrate anything harmful in a home kettle. Fresh water is better because it is fresh, not because the old water turned toxic.
Boiled is not filtered
Boiling kills microbes. It does nothing for chlorine taste, hardness, or metals, and it slightly concentrates minerals as steam leaves. If tap water tastes bad cold, it tastes bad hot. Filter first.
Where to put your attention
Grind size, leaf quality, and ratio move the cup more than any single degree. But once those are stable, temperature is the cheapest remaining upgrade, and a timer plus an open lid costs nothing.