Another thing to consider is it a single element, or a dual element? If it's a dual element one of the heaters may be shorting out.

or one of the thermostats...the main things that go wrong with water heaters are the heating elements and thermostats...other than that there is really nothing else that can go wrong other than it leaking or it being too small for the home....40 gallons is not a very big water heater...you will use at least 10 gallons or more in a normal 10-20 minute shower (depending on how hot you like your water of course)...there is a setting normally on the bottom thermostat as to the temperature you want the water to be heated to...it also takes some time to replace the hot water you use...If you use half the capacity of the tank ( already heated water) it could take a half hour to heat the cold water that flows into the tank as you use the hot water...I have seen many homeowners try to get by less expensively by replacing their old unit with a smaller water heater in their home, only to find out that it isn't a sufficient  capacity to be practical, and some builders will put the cheapest unit they can get their hands on to instal even if it's too small to be practical. Also, if it's really cold outside and your water heater is in the garage, it may take longer to heat, and not stay hot as long...I would think that would be a slim chance of that being the problem though

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I called the plumber.  Found out it was still under warranty.  It had a faulty thermostat on the top, which is why we could shower for 15 minutes with hot water and then have none right after.  The bottom thermostat worked just fine, but they replaced both.

 

Again, as always, thanks to everyone for their help. I love being able to ask questions like this and get everyone's input.  

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“It is the job of thinking people not to be on the side of the executioners.”
― Albert Camus

I called the plumber.  Found out it was still under warranty.  It had a faulty thermostat on the top, which is why we could shower for 15 minutes with hot water and then have none right after.  The bottom thermostat worked just fine, but they replaced both.

 

Again, as always, thanks to everyone for their help. I love being able to ask questions like this and get everyone's input.

As I suspected. Glad you were able to get it repaired under warranty. Now I have to get mine fixed.

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Men that know their way around a tool box are sexy...just sayin! :yes:

I'm quite capable of walking around a toolbox thank you very much. 

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