It is very important you calculate the area where you want to run the humidifier for efficiency reason. Different unit come with different coverage number and these ranges from 250 sq. ft. up to 4,500 sq. ft respective to each unit. It is not wise to run a unit with a lesser coverage unit in a higher coverage space. Choose according to your home space if you want the unit work to performance.
Type of unit to choose
Another aspect to consider will you need a humidifier for a whole house or for single room use. According to specification, whole house unit are designed to power an entire house. Some models require installation to a furnace or HAVC system before you can use them while others exist in console whole house model requiring no installation. On the second hand, single room unit also known as portable unit work well in a room accordingly. They come in small foot print that allow for easy carriage around the room or to another room. Single room humidifiers need no installation and so you can easily purchase these types. However, among the two, your preference counts.
Mode of Humidity Control
Humidifiers usually come in digital or manual mode of humidity control. If you want your home humidity level to be regulated by the unit, consider a humidifier with humidistat. But if you want to DIY, a manual one is what you should look out for.
Some level of features makes running of humidifier easier and fun. Having certain features such as auto-shut off option, low tank level indicator or change filter alert in your unit will give you high level of control and less time checking on the unit.
You may also want to consider the type of mist a particular unit do released as there is both warm and the cool mist unit. Warm mist unit releases warm air or steam beneficial for cold period. This type is not advisable for parents with babies as they can release hotter mist sometimes. While the cool mist unit in returns disperses cool moisture air suitable for hot weather. These types are perfect for babies nursery bed, rooms etc.
Some manufacturer attached warranty to their unit and every unit has it duration of warranty. Warranty is worth considering as it will give you level for repair or replacement in case the unit is malfunctioning within the stipulated time.