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The Little Known Energy Used by Humidifiers

Aug 29, 2008
As winter approaches, the thoughts of servicing or checking your furnace grows in priority. Part of this may include the humidifier. The havoc created by dry winters can vary from annoying dry skin to damaging wood floors and furniture. Humidifiers can take care of these winter dry problems. However, they do use energy. While the energy is not a major contributor to your utility bill, not understanding their energy use adds hidden costs.

First the physics of a humidifying air must be explained. Air contains water vapor; the amount of this water vapor is called humidity. When referring to the actual amount of water vapor in a particular sample of air it is called absolute humidity, humidity ratio, or moisture content. Did you ever wonder why your favorite weather personality reports a value called dew point temperature? This value directly correlates to the moisture content of the air. The higher the dew point, the higher the amount of water vapor. If the actual temperature (dry bulb temperature) falls to the dew point temperature, water starts to condense out of the air. Since morning is usually the coolest time of the day, we often see this affect as dew.

An important physical aspect of air is that it can only hold so much water vapor. It is important to us to know how close the amount of water vapor is to the maximum amount air can hold. This is called relative humidity and is usually expressed in percent. Air at 100% relative humidity cannot hold anymore water vapor. If the air is at 50% relative humidity then the amount of water vapor can be doubled before reaching the maximum limit.

Why is relative humidity important to us? Our body cools itself mostly by evaporating sweat. If sweat evaporates quickly then we feel cool. If sweat evaporates slowly then we feel hot. The ability of sweat to evaporate directly relates to the relative humidity of the air. When the relative humidity is high, say 80%, there is already so much moisture in the air that sweat does not evaporate very fast. The result is a feeling of being too hot.

There is another important issue about relative humidity. The maximum amount of water vapor decreases dramatically with temperature. Air at 85 degrees Fahrenheit can hold about 10 times more water vapor than air at 25 degrees Fahrenheit. This is why it is dry in the winter. The cold outdoor air just cannot hold much water. As you heat your home the temperature is increased but not the moisture content. So a winter outdoor relative humidity of 90% becomes 30% when the air is heated to a comfortable indoor temperature.

So let's add moisture to the air. This is the purpose of a humidifier. There are two ways water can be added to the air. One is to evaporate or boil water and force the steam (water vapor) into the air. The other is to force liquid water into the air in small enough drops it turns to vapor quickly.

It should seem obvious energy is needed in the first method. Water is heated by some means, usually electric resistance coil, into a vapor. The advantage of this is that the vapor is pure; the contaminants in the liquid water are left at the humidifier. The water vapor has no impact on the temperature of the air.

Now is time to explain the big myth about humidifier energy. The second method involves spraying small drops of liquid water into the air. Atomizing, ultrasonic, cool mist, air washing, evaporative humidifiers all belong to this category. This can be done with far less energy than by heating the water. Because the humidifier does not use very much energy, manufacturers will often advertise that these devices save energy. While the units do not use much energy, energy is still needed. You simply cannot convert liquid water into water vapor without energy.

So where does the energy come from? It is taken from the air. As liquid water is injected into the air it evaporates into a vapor. The energy is taken from the surrounding air. This slightly lowers the temperature of the air. Assuming we want to humidify without a temperature drop, then that air must be reheated. So the furnace provides the energy if it is not provide at the humidifier. While the humidifier may be efficient, is your furnace?

Bottom line is ALL humidifiers use energy. Your selection of humidifier types should be based on other criteria. By the way, Energy Star does not qualify humidifiers. Differences in energy use between different types of humidifiers are moot. If you desire to save energy, limit the time the humidifier operates. The problems associated with dry air tend to occur when relative humidity drops below 30%. You only need to humidify up to 30%, anymore is wasting energy. But, will a higher relative humidity feel warmer and result in a lower setting on the thermostat? No because the human body can not differentiate between values of relative humidity below 50%. The ability of the body to evaporate sweat does not change much when relative humidity is low. In the summer, the body can feel changes in relative humidity when it is over 50%. Proof of this may be at your grocery store. Some grocery stores have lowered the relative humidity but not the temperature in the refrigerated and frozen foods area. People complain when the temperature is lowered but not the relative humidity.
About the Author
Gary Transmeier has worked over 25 years providing paying clients methods to save energy in their buildings. Now he offers ways to save energy to everyone. At The Home Energy Place you will find details on over a 100 ways to save energy in your home.
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