Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney.
Mycelium is a block added in Minecraft the same time as the introduction of the Mushroom Biomes in Minecraft 1.9pre1. Mycelium can only be found and only can be found in Mushroom Biomes. Mycelium resembles dirt in that it can be broken with a shovel and it without a 'Silk Touch' enchanted tool it only drops dirt when broken. Mycelium also lets off a small particle effect the same as Bedrock.
The mycelia and the hyphae are both important parts of fungi or more commonly known as the mushroom. The two terms refer to the building blocks of fungi. The hyphae (plural form, singular form is hypha) is the name of the long, individual pieces that comprise a mycelium, overall a fungi.