Water heater not at correct temperature at random times. Occurs inconsistently.

We own a 10 year old Rheem 40 gallon natural gas water heater that is behaving inconsistently. On certain mornings the hot water runs out near the end of the first shower. The thermostat dial temperature has never been changed and we started noticing this behavior about six months ago. Some mornings we have hot water and others we don't. On those occassions where it is luke warm, the water heater recovers normally and we have hot water at the faucets about 20 minutes after. It appears that the tank is not holding its temperature overnight. I have eliminated the following causes:

1. Dip Tube - About 5 years ago, my first one went bad and I was able to replace it with the newer non-degradable type. I confirmed that this wasn't the problem by taking the temperature from the heater's drain valve and measuring it against the nearest water faucet in the house. The water faucet was considerably hotter than the drain valve. If the dip tube was busted, the faucet water would be luke warm and the drain water would be hot.

2. Thermocoupler - The pilot is always lit and the heater is able to fire up when the thermostat dial is set higher.

Is this a fault thermostat?

Thanks,

Jon
Los Angeles, CA

Yep it sounds like it...

On a 10 year old water heater, just put in a whole new one...

The temperature probe inside the tank can get coated with hard water deposits that slow its reaction time. Sometimes it gets too hot before it turns off the burner and gets too cold before it turns on the burner. Way too many hard water deposits in the bottom of the tank can sometimes get higher than the probe as well also delaying the cycle of the thermostat. Turn off the water, drain the tank, disconnect the gas line, and disconnect the gas, pilot, and thermocouple lines from the bottom of the gas valve, unscrew the gas valve, clean the threads in the tank, install the new gas valve, reconnect the gas lines, reconnect the gas service, light pilot, etc. Easy peasy.

It could be a few things, although a bad thermostat would NOT usually "fix" itself periodically. You have to determine whether the tank is full of the proper temperature water BEFORE you start using hot water to see if you are starting with a full tank or not. Once you determine that, then you can go about figuring out why the volume is erratic.

Thanks

Thanks for the feedback. I'm glad that you were able to identify that the probe might be suspicious. This morning, the water temp was perfect. The thermostat is probably on the fritz. I'll proceed as suggested.