A giant terrestrial steam-punk robot full of water that you power by using fire to turn said water into steam wouldn't really fit with the aerial-esque theme Air has. Maybe a hot air balloon?
I can't picture this being Air/Fire since you need water to create steam, while not being able to say the same about air. Air's domain is air and gaseous substance, but I don't agree that it has control over the excess product of water and fire merging. If you think it does, why not say most creatures belong in Air since they need air to exist, life due to the fact the majority are alive or for that matter be in the water domain since we're about 75% water and I assume animals aren't all that different.