Alde Recommends replacing the Glycol Fluid every 2 years. Glycol is an item that must be replaced before it stops providing protection (lubrication and rustproofing) 

Replacing the glycol should only be done at a certified service center or dealership. Truma uses Alde Antifreeze, a high-performance heat transfer fluid specifically designed for an aluminum heating system.

PLEASE NOTE THAT WE HAVE CHANGED GLYCOL IN 2021. PLEASE SEE THE FOLLOWING LINK FOR MORE INFORMATION, https://nucamprv.freshdesk.com/support/solutions/articles/44002156353-2021-glycol-change