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I have a gas water heater about 15 years old, I flush it about once a year. The water seemed too hot ( measured 135 F at a faucet) , it had not been that hot before. There are about 10 temperature marks on the control[ It was on #4 , I moved it to #2 . Good temp, then 110 F. Wife thought it too cool so I set to #3 ; temp now 120 F. That was a week ago , today it is definitely cool ( not measured but about 100 F). I turned it back to #4 ; the burner came on and the water is now warm. I never had the temperature drift in a water heater before , Can this happen? The heater is a generic ( "State") from plumbing supply chain , not a big box store. There is no "bumping" to indicate stuff in the tank bottom.

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Yes a thermostat can malfunction and it's certainly possible that what you're seeing is related to a bad one. You might consider replacing it because an erratic thermostat can be dangerous.

At 15 years old, however, you might consider replacing the entire unit.

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    I agree and at 15 years old this is beyond the normal service life I have seen for a generic or even high end residential unit. – Ed Beal Mar 29 at 15:29
  • I agree with both jwh20 and Ed Beal: probably a bad thermostat and that your $, especially if considering the -labor- cost to replace the thermo, is much better invested with a new unit that has, by default, a new thermo already in it – Ack Mar 29 at 18:24

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