I have a gas powered 40 gallon AO Smith water heater that is about 8 years old. It is not maintaining hot water temperature, as it used to. The pilot light is not the problem. I have raised the "thermostat" a couple times. Is this a symptom of a failing water heater? Is it just the heating element and can that be replaced? If I have to replace the water heater, what price should I expect to pay, including installation?