Warranty for this specific model:

Inner Tank:

•  5 years if supply is from roof tank
•  5years warranty If supplied by a pump, said pump must be a Flow pump & not exceeding 2.5BAR of pressure. A Pressure Vessel (10% of water heater volume) must be installed
  
• If no vessel is installed, no warranty is offered on a pump supply
• If supplied by government mains, a pressure regulating valve (set to 2.5BAR) and a Pressure Vessel must be installed for the warranty to be valid