It is possible that throughout the winter months, your CPAP unit will use more water for humidification than usual to help compensate for the drier, warmer air from heating your home.
To make your water chamber last and your unit continue to function at its best, you must use distilled water.
Distilled water is the collection of evaporated water which is free of minerals and other solids. When heated, regular tap water (which contains these particles) will leave a film on the heating plate; this is known as calcification. This calcification can prevent the plate from heating to the point of creating the necessary humidity, which can cause your unit to overwork, thus creating bigger problems within the unit itself.
Distilled water can be purchased inexpensively at any grocery store.