Depending on the quality/hardness of your water or how often you use the unit with oils, you may need to clean the tank at least once a week and change the water in the tank after each use. Clean the tank using roughly 20ml of normal household bleach. Put the bleach into the tank itself with a full tank of cold water, give a good shake and leave to stand for half an hour. If residue is particularly bad or has been left for over a week you can use a dish brush or bottle brush to clean the remaining residue. Rinse out the tank thoroughly with warm water and leave to dry.
We recommend to clean the base of your Hygro+ once a week with a solution of white vinegar and water - roughly 2 parts vinegar to 1 part water until it fills the base. Let the base stand for half an hour with the vinegar solution in it. Rinse out with some warm water being particularly careful not to immerse the electrical part of the unit in water. Allow to dry completely.
The chimney and the oil tray can also be soaked and cleaned in a weak solution of bleach and water to lift any oil residue.