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.
Mycelium is a very rare grass block variant that appears naturally only in Mushroom Biomes. Although it is rare, some can make a mushroom farm a very easy affair. Mycelium, unlike Grass, cannot grow plants (excluding mushrooms). Once a block of mycelium is obtained, it can be used to convert grass blocks into more mycelium over a period of time. This happens regardless of light level. Mobs can spawn on mycelium, however; that particular effect applies only in the Mushroom biome.