Different manufacturers have different requirements for how often they'd like you to service your hot water unit to keep it performing at it's optimum. It's also handy to keep the servicing up to date to protect your warranty. As a rough guide, try to maintain regular maintenance by a licensed plumber for your unit:
Continuous flow - annually
Electric and gas storage - every six months, you should lift the lever on the Pressure & Temperature Relief valve for approximately 10 seconds (as per your Owners Instructions). Then a service every five years.
Solar - every five years
Heat pump - annually
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