A.O. Smith AHD 290 - Manual (Page 62)

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Anode:
In order to prevent corrosion of the enamelled hot water tank, the tank is fitted
with a magnesium anode with a ¾“ screw plug. This anode has a life expectancy
of 2-5 years. It is nevertheless important to check that the anode is intact at all
times. This should be done by inspecting the anode every 2 years and replacing
it if it is corroded and measures only 6-10 mm in diameter. To inspect the anode,
turn off the power supply to the system and remove the front cover. The hot water
tank has to be drained before the anode can be unscrewed. To do this, turn off the
cold water supply, attach a hose to the drain valve to ensure the water runs to the
nearest drain. When draining the water from the tank, turn on a hot water tap to
prevent negative pressure in the tank. When the tank is empty, the anode can be
unscrewed and inspected. When the anode has been fitted again, close the drain
cock and turn the valve water supply back on to fill the tank with water. When the
tank is full and the cover back on, the power supply can be switched on again.
3.3 Demounting / Putting the unit out of service
The following actions must be taken:
Disconnect the power supply from the unit and disconnect other connections.
Shut off the fresh water supply valve and connect a drain hose to the drain
valve in order to drain the water. While draining, a hot tap water valve should
be opened to avoid under pressure in the tank. The air duct connections are
uninstalled and remaining air channels must be closed to avoid condense water
entrance to the building.