Siphon brewers get hot, but not all of them get hot enough. The water should be between 195-205°F (90-96°C). If you're using a gas stove, you might be fine. If you're using a halogen or butane burner, check the flame.

A quick way to tell: if the water is boiling violently, it's too hot. You want a gentle simmer. If you have a thermometer, use it. If not, let the water come to a boil, then wait a few seconds before adding the coffee.