I hadn't really thought about it - but I bet that all of the diffusers I have going throughout the house are adding a substantial amount of moisture - PLUS all that essential oil goodness.
the place i lived in the attic for several years had hot water heat and the radiators had little valves to burp them once in a while if there was air in the sytem. it really wasn't supposed to be there.
In my childhood home, we had this huge ugly gravity furnace in the basement. With long tubular arms, reaching towards the ceiling. It did produce nice warm air in the winter, but it sure was ugly.