About Humidifier Room Size:
Humidifier "room size" refers to the recommended room size (usually stated in square feet) that many humidifier manufacturers publish for each model humidifier they produce, in order to help you select the best sized humidifier for your room or area. A recommended room size is not a guarantee that the humidifier will be able to raise the relative humidity (RH) level of any particular room size to any specific RH level. Room size recommendations should be used only as rough guidelines when shopping for a humidifier, as these recommendations are subjective, are based on ideal environmental conditions, and can vary from manufacturer to manufacturer. Each manufacturer bases their recommended room sizes on the maximum amount of moisture the humidifier is capable of producing per day, assuming the humidifier is operating in perfect environmental conditions, with the humidifier constantly running on its maximum speed setting.
No two room environments are exactly the same, and the RH level even in a single room can often vary greatly throughout the course of a single day. A humidifier is only responsible for the moisture it adds to a room. Once that added moisture exits the humidifier and enters the room air, each room has many factors working to remove that added moisture and dry out the air again. This is a constant struggle between room conditions and humidifier output that changes from room to room, day to day, even hour to hour, depending on a countless number of factors, including the outdoor temperature and RH level, the indoor temperature, how often your heat is running, the type of heat being used (forced air heating, baseboard heating, wood stoves, etc.), the location of forced air ducts and return air ducts, the number and location of windows and doors, how well each window is insulated, the height of the ceilings, how often the door to the room is open or closed, etc.
As a result of these various factors and room environments, which cannot possibly be encompassed by a single room size recommendation, the actual RH level that you will be able to achieve with any given humidifier will depend on your unique room environment. We choose to publish manufacturer's recommended room sizes because that is how most humidifiers are sold today. However, after years of our own testing of every humidifier we sell in real world conditions, we have found that you will get the best performance by using the highest output humidifier possible that will fit in your room and budget. If achieving the highest possible RH level for your room is of the utmost important to you (even more important than other factors such as noise level or filter replacements), then it is our recommendation that you select as large a humidifier as possible, to ensure that you get as much additional moisture into your room as possible.