Some people may be surprised to learn that vent-free appliances produce water. This occurs naturally by burning a combination of gas and air. The water is in the form of a vapor rather than a liquid. You can’t see it with your eyes.

During the cold months of the year, the relative humidity—the percentage of water vapor in the air—is low. You’ve probably experienced a scratchy throat caused by the dryness inside a home during the winter. Health studies have shown that increasing the indoor relative humidity results in a reduction of respiratory illnesses. AHAM, the trade association for the humidifier industry, recommends a 60% indoor relative humidity. To improve the level of human comfort and reduce symptoms like a scratchy throat, many people add humidification with either a portable or whole-house system. Of course, the water vapor added to the air comes not only from humidifiers, but also from a variety of sources such as dishwashers, ranges, showers, etc.

Keep in mind that a home is not airtight. Even one built to the tightest construction standards allowed by codes still has at least 0.35 air exchanges per hour. This means the higher relative humidity indoor air is constantly being replaced by lower humidity outdoor air. You’ve probably noticed cold air leaks around the windows and doors in your own home during winter. That’s indicative of air exchange taking place. While vent-free products increase the relative humidity, the air exchange decreases the relative humidity.

How does a vent-free appliance work in a really tight home? The answer is “very well.” It’s simple: a well constructed home requires less heating, and as a result, the appliance will operate for a shorter period of time and produce less water vapor. Of course, in average homes—particularly those located in the most northern climates—you may notice some condensation on cold surfaces such as the inside of the windows. However, this condition is indicative of the weather and the construction, and most likely it would exist whether a vent-free appliance is present or not.

In summary, a vent-free appliance produces some of the water vapor that occurs in the home. It serves to help humidify the indoor air, and that’s beneficial for human comfort and health. Also, tight houses are superior for preventing condensation problems, making them good candidates for properly sized vent-free appliances.