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.