There may be sediment buildup in the tank bottom, which can cause this. Shut off the electricity to the unit, drain off some water from a faucet that should be at the bottom outside of the tank and see what comes out, if a lot of dirty water and sediment comes out that may be it. Let it flush until clear water comes out. You will probably have to hook a garden hose on the faucet to get the water to a drain or outdoors. Be sure to shut off the power to the heater first! Then let the heater fill back up with water and turn on the power and wait for it heat up.
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There may be sediment buildup in the tank bottom, which can cause this. Shut off the electricity to the unit, drain off some water from a faucet that should be at the bottom outside of the tank and see what comes out, if a lot of dirty water and sediment comes out that may be it. Let it flush until clear water comes out. You will probably have to hook a garden hose on the faucet to get the water to a drain or outdoors.
Be sure to shut off the power to the heater first!
Then let the heater fill back up with water and turn on the power and wait for it heat up.