Question: I recently saw on television a new type of home heating appliance that produces only water as a product of combustion. I thought that would mean that the fuel was hydrogen- but that is not the case it is a canister of some kind. Have you heard of this type of heater (it looks like a wood stove) and requires no chimney. Answer: Weird- I have not heard about it. Vent Free gas appliances produce mostly water vapor as a by product. Maybe you're speaking about fuel cells, which do produce only water vapor as a by-product and can operate with any fossil fuel or even water as a fuel.