Before buying a new humidifier, it’s useful to think about what feature you require. First, you need to know what options exist, then you can make a list of what you want and find a product that has them.
Nice big water tanks that are easy to refill. The bigger they are, the less often you’ll need to refill them, which is good. Some models are really fiddly to refill, with lots of things to unscrew and then you have to be careful putting seals back in place or they leak everywhere. We’ve only reviewed the best on this site, and although some are much more user friendly than others, none of them are too bad. Assuming you buy one of the more respectable brands you shouldn’t have too much trouble.
Low noise levels. This is something that's improved dramatically over the last few years. Many are now so quiet that most people wouldn’t notice them. For some people though, any noise can be a problem, particularly if you’re trying to sleep. Different technologies produce different types and levels of sound. Complete silence is not possible. If noise is a priority for you, read carefully about the different types and choose the one you think would be best.
A built in hygrometer (humidity meter) is particularly useful. You need to be able to monitor the humidity level in order to know if it needs to be increased or if it's high enough already. You can buy this separately, but it's easier to have it built in. With some high-end units you can choose your desired humidity level and the machine will turn on and off automatically when needed. Full humidity control, rather than mindlessly pumping moisture into the air when it doesn't need it. This saves electricity and stops humidity getting too high.
A low water indicator light is useful, so you know when to refill.
When the machine runs out of water, ideally, it should turn itself off. Not all have this auto shut off feature, which is a shame. Leaving the machine running when it’s not doing anything is wasteful, and pointless. You may not even realize the water tank is empty!
Multiple output settings. Most have at least a high and a low setting, though the difference between the two can be minimal. On most of the models we've reviewed, the differences were much more dramatic. Some have a massive range from the lowest setting to the highest. This effects mainly how much moisture is released, but also the amount of noise that's produced which is also important.
Some sort of mold and mildew protection can save you a lot of cleaning time. Anywhere water is stored is a breeding ground for all sorts of germs and bacteria. Regular cleaning is enough to prevent problems, but those who want to clean less often should look for a machine with some sort of anti-mold coatings. Some, like the Vicks V790 even use UV to create a germ free environment.
Each of these features adds to the price of the unit you buy, but most will save you time or electricity which compensates for that. You do get more value for your money if you pay extra for the best features.