Through evaporation. Humidifiers expose their stored or flowing water to an air stream that evaporates the moisture. Units mounted to the warm side of a forced-air heating system evaporate more water into the air than other units, because warm air can hold more moisture than cooler air.
It depends. Generally, the colder the weather outside, the more humidity you need inside. That's because colder air tends to be drier air and, as the outside temperature drops, the moisture is drawn out of the house. To gauge the humidity level in your home, use an accurate hygrometer. Most people find that humidity levels between 40 and 50% are comfortable during the heating season.
You need to disassemble your dehumidifier to get the tray out (see your owner's manual for instructions). Then you can clean it with ordinary household cleaners and cleaning tools.
That depends mostly on the level of humidity you want to maintain. Furnace-mounted units produce humidity only while the furnace blower is on. You may be able to set the blower to run continuously (usually at the thermostat). But if you need greater humidity in only one area of the house, we recommend that you supplement the humidity in that area using a free-standing humidifier unit.