whole house humidifier

I have a 2 story house, approx 3500 sq/ft, with forced air heating in NY. There are 2 furnaces, one in the basement for the first floor and one in the attic for the second floor. I would like to add a humidifier.

Contractor 1 wants to put in an evaporative humidifier on each unit. For what it’s worth, an AprilAire 600A downstairs and an AprilAire 400 upstairs. The attic furnace would actually have the humidifier located in a closet so that the water line won’t freeze. He says that a humidifier on the first floor only will not properly humidify the upstairs because the moist air is heavier and will not rise. Also, when the bedroom doors are shut, there is no way for the moist air to enter the room. That all seems to make sense.

Contractor 2 wants to put a steam humidifier on the downstairs unit only. He wants to put in a General Steam unit with an auxillary blower so that it runs even when the heat is not actively blowing. He feels that the upstairs unit could fail and leak and it is not a good idea to install one there. He says that the moist air will rise and mix with the upstairs air. Again, it seems to make sense, especially the part about potential water damage.

I’ve googled and googled and can’t come up with anything conclusive. I did find that steam units can be expensive to operate. If anyone has one, can you give me an idea of how much? I will discuss that issue with my contractor further.

What do I do?

I am not sure what you should do, but I have two comments.

First, moist air is lighter than dry air, though only by a little bit. Since a water molecule has one oxygen and two hydrogens, and the hydrogens have very little mass, it is only a little more than half the weight of an oxygen molecule (with two oxygens), and somewhat over half the weight of a nitrogen molecule (with two nitrogens that are 7/8 the weight of oxygens). However, this difference is not big enough to matter much, because even very humid air is only a percent or two water.

Second, I have studied and bought Aprilaire equipment, and spoken with the people there. I think they make very nice products. Though, as I remember, their humidifier (which is not a product I’ve bought) runs hot tapwater over evaporator plates once and then sends it down the drain, which seems pretty expensive and wasteful to me. I think only a small fraction of this water evaporates. I haven’t calculated costs, though.