Never use hot water from your geyser to cook or even make coffee !!!
The Geyser is made out of steel and is finally sealed with a glass vessel to protect the steel from the water. This also prevents the tank from rusting on the outside.
Inside the geyser is an anode which protects the geyser from the water. However, the water starts to attack the anode inside the tank. This results in the geyser rusting inside the tank and the eventually the water starts to go brown, forming a thick rusty sludge. Hot water from the geyser could even give you a metal taste.
Drinking water havested from the hot water tap could have toxins or dangerous bacteria, including arsenic, radioactive heavy metals, chromium and mercury. Rather use water from the cold water tap to make your coffee or cook.
Pictures below are the inside of a geyser !!!