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 extend the area in which a mushroom or fruiting body can acquire nutrients. Like fungi, mycelium have rigid cell walls made of chitin that allow them to grow through or into environments that would typically not allow other cell structures. Mycelium also link high plants through the root system and allow them to swap nutrients in adverse climatic and growing conditions.