I'm not at home at the moment to give you all the model info on my water heater - but thought I'd get the discussion rolling on a possible general cause given the symptoms.
First, some background:
it's a 12 y.o. 40-gallon gas water heater. I've done my best to keep it serviced over the years by regularly opening the drain and getting as much sediment out as possible. We do have hard water and no softener. Last summer I found the thermal expansion tank above the water heater had failed (bladder ruptured), so I replaced that. At the same time, I completely drained the tank at that time and replaced the anode rod, which had almost completely worn away. But it gave me a chance to inspect the tank from the top with a flashlight and flush it out good. I was actually surprised by how good it looked inside - but of course, I couldn't get a close look at the bottom. I also replaced the drain valve (had a cheap plastic/nylon one) and the pressure relief valve at the top with a matching rated one. At 10+ years, I figured better safe than sorry.
What I also found at the time is that the water pressure in my house was WAY high. With some direction from some of these forums, I tested the pressure at an outside hose spigot which was reading 85psi. Things started clicking then - probably the reason for the expansion tank failure. Probably the reason I found one of the bathroom hot water shutoff valves "popped" off earlier this year (which I thought my wife had somehow accidentally shut off before.) I didn't know if the city's pressure to the house changed? Or if the inlet pressure regulator valve had failed? But I was able to adjust the regulator down and it had been holding steady as of sometime in November. I've not checked it recently because I had no reason to. But maybe I do again....
The new symptom is little to no hot water - or just luke-warm water at best - when you turn on the upstairs shower in the morning to it's hottest setting. it's happened twice in the past week - but not two days in a row. It's been about 5 days apart. Got up in the morning to take a shower. Shower was the first thing I turned on - and at it's hottest setting, it's only luke-warm. Even after letting it run a full minute! I took my luke-warm shower (maybe 5 minutes), got dressed, and went down in the basement to check the water heater, fearing the worst. No leaks. Nothing from the pressure relief valve. Drain valve is closed and no leaks. Tank was full. Expansion tank still functioning fine. The gas was on and I heard the water being heated (crackling noise it makes). So I manually tested/opened the pressure relief valve and heard air escaping...quite a bit... before water finally began to flow. I've tested that on several occasions before and never heard that much air have to escape. So that concerned me.
I went up to the kitchen and turned on the hot water, and we had hot water.???
I thought maybe it was just the trapped air - and a one-time issue. But it happened again this morning according to my wife. She left earlier than I did - but said she had to take a luke-warm shower - as did one of our kids after her. When i got in the shower an hour later, it was perfectly fine. In fact, water so hot, I had to cool it down.
So, CLEARLY, something isn't right here.
Replacing the water heater was actually already a starred item in our major expenditures this year. I just figured anything we got beyond 10 years on it would be a blessing - but we shouldn't wait for catastrophic failure.
I can get back on here tonight with my water heater's specifics if need be - but I hope this would be enough to at least start honing in on the problem(s). Thanks in advance for your time!