How do I clean the filter for a Humldifier?

Answer 1

soak it in CLR, then rinse it out with water,

Answer 2

if it the type that fit onto the ducts, somewhere near the heater, and has a filter that looks something like a small air filter. These should not be cleaned but should be replaced. They have some chemicals in them to help take care of the mineral deposits that are naturally in the water.

One these types is made by Honeywell, but there are many others.

Otherwise the other answer is a very good way to clean.