The HumidAire 3i Cleaning Plug allows you to seal the H3i water chamber during cleaning. 1. Add warm soapy water to the H3i chamber 2. Seal the chamber with the yellow cleaning plug 3. Swish the water around inside the chamber 4. When you're finished cleaning, remove the plug, drain the water, and rinse the inside of the chamber with clean water This yellow plug is identical to the plug that is included with the HumidAire 3i.