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.
A mycelium block can spread to any dirt block within one space above, sideways or three down. The mycelium need light level 9+ above it and the dirt needs light level 4+ above it, and must not be covered by any light-impending block i.e water, lava, ice or any opaque block. Mycelium also dies under the same conditions as grass: when covered by one of the light impending blocks above.