Let's go with the notion that temperature is actually a container. Let's call it a bucket. The higher the temperature, the bigger the bucket. The bigger the bucket, the more water it will hold,and vice versa. The amount of water in the bucket effects our sense of comfort. Too much water is uncomfortable, as is too little. Because temperature dictates the quantity of water in the bucket, too little usually occurs in winter, and too much occurs in summer.
Adding more water into the bucket is called humidification. Emptying the bucket is called dehumidification. Because a central heating and cooling system is circulating and treating the air in your house, it becomes a convenient place to alter the water content of your air as well as the temperature. Dehumidification is a natural byproduct of a well designed cooling sytem. The cooler air has a smaller bucket and cannot carry as much water as before it was cooled, so it condenses inside your cooling system with little extra effort. Raising the water level in the bucket is not a natural side effect of the heating system, and requires a special effort.
A humidifier is the machine that performs this function. During heating season, the relative humidity indoors will get to 13% or lower. The desirable level is 30-35%. Humidifiers come in a variety of designs, from floor and table models, to furnace mounted units. Furnace mounted units can be by-pass, steam, mist, wick, and powered. Some mount in the ductwork while others are attached to the plenums on the furnace. Whatever the model, mineral deposits from the water will reduce the effectiveness of the humidifier. Be sure to turn the water and humidistat off before servicing.
Special chemicals can be used to clean mineral deposits, but one of the safest is vinegar. A 50% mixture of vinegar and water will usually dissolve or help clean away deposits. If the humidifier has a drum with a belt, replacement of the belt at the beginning of the heating season can be the easiest fix if the water has a high mineral content. If the humidifier has screens or is the wick type, some corrosion can occur as well as build up.
Heat pumps and electric furnaces usually have steam humidifiers to increase output. They are line voltage powered and must be unplugged or disconnected before servicing to avoid injury. With the exception of some mist type humidifiers, all have a reservoir. Stagnant water sitting in the reservoir during the off season can be a breeding ground for bacteria. Turn the water off and clean the humidifier if possible when heating season has ended.
Other than mineral deposits, the most common problem is sediment in the water supply. The float inlet valve has a tiny hole that easily clogs. The float is usually made of plastic, and if cracked will fill with water and sink. The valve will be stuck open and the reservoir will overflow. Use caution when scraping deposits off the float. If the deposits cannot be removed, replace the float or it will sink and cause an overflow.