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I haven't seen it, but there is water on the floor and also on top of a hose that is rolled up next to the water heater.

It is a A.O.Smith electric ECS 40 200. Both upper and lower thermostats are set to 120°, but water temp from the tap is 149° and jumps back and forth to 151° and 149°. The unit is 12 yrs old.

Should I start with replacing the thermostats?

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I suspect that one of you elements have gone to ground and is running away. As the elements age the sheath will eventually split. The elements always have 110-volts available. This voltage will go to ground and drive current indefinitely. Recommend you renew the lower element. Remove the lime deposits while you are at it. Should be good to go.

  • Is there any electrical measurement that can be done to verify that the element is failing so that it is constantly on? – Jim Stewart Feb 22 '18 at 21:05
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Water is 'hotter' because hot water rises to the top and thermometers are usually half-height, so once the water around thermometers is 140, water on the top of the boiler may be 150, it's called 'stratification'.
You may only have to replace the 'over-pressure' valve that may have some sort of concretion. Another cause may be your expansion thank, it may have to be charged or replaced (to recharge, hot water lines have to be de-pressurized). If expansion thank fails, over-pressure is released by the relief valve wetting the floor pressure thet normally is 'damped' by the thank.

  • The over pressure, relief valve is new. I saw it dripping and replaced it. – philetus Feb 21 '18 at 20:36
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Is there any chance there is a check valve in the supply line ? I had one and found the pressure relief valve spit out several drops of water each time the heater cycled. The check valve prevented the expansion of the heated water from pushing back into the supply. There was nothing wrong with anything.

  • There is a check valve where the hot water comes out of the water heater. – philetus Feb 23 '18 at 14:12

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