Steam vs. Evaporative Humidifiers: Which Costs Less? – GreenBuildingAdvisor

Building Science

Steam and evaporative humidifiers both turn liquid water into vapor—but they get the energy to do it in very different ways.

Recently on the Fine Homebuilding Podcast, we had an interesting conversation about the cost of humidification, specifically the differences between steam and evaporative humidifiers. Both turn liquid water into vapor—but they get the energy to do it in very different ways.

Is one method less expensive than the other? The answer is a little complicated. One thing we can be sure of is that humidifying a house isn’t free. Changing liquid water into vapor takes energy. The question is: Where does that energy come from?

The physics

Humidification requires water to change from a liquid to a vapor, and that change of phase requires energy. At atmospheric pressure and water’s boiling point, it takes about 970 Btu to change 1 lb. of liquid water into vapor. (Interestingly, we only need to remove about 145 Btu to change 1 lb. of liquid water at 32°F into ice.)

For the examples in this blog, we’ll use gallons of water instead of pounds. One gallon of water weighs about 8.34 lb., so changing a gallon of water at its boiling point into vapor requires roughly 8,090 Btu.

Before going further, we need two definitions: sensible heat and latent heat. Sensible heat changes the temperature of a substance without changing its phase. Latent heat is the energy absorbed or released when a substance changes phase—for example, when liquid water becomes water vapor.

An easy way to see the difference is to start with cold tap water. Before that water can be boiled into vapor, two energy transfers are required: Sensible heat raises the temperature of the water to its boiling point, and latent heat changes the water from liquid to vapor.

Let’s say we start with 1 gal. of 50°F tap water and add enough energy to turn it into steam. First, we need to…