Hot water puzzle

Recently, the hot water in one portion of our house has lost its temperature--it's really more lukewarm than anything. There doesn't seem to be a decline in pressure or flow, just temperature.

The temperature does not seem to have decreased in other parts of the house, which are closer to the water heater.

We have a 40-gallon gas water heater that serves the entire house. We have copper plumbing. We have a pier-and-beam house with relatively easy access under the house. I haven't found any leaks. The water heater is about 10 years old and the house is about 50 years old.

I have noticed this problem in the past, but it has never lasted for more than one day--it always seems to have cured itself. But this is going on a week now.

Any thoughts?

Is it in all faucets or just when you take a shower?

Where do you live? Is there a correlation between outside temperature and your max hot water temp in that one room?

Air temperature is not a problem

I'm in Texas--this has nothing to do with air temperature.

Also, this is in all faucets (sink and shower/tub in two bathrooms).

Scratching head

Got any temperature controlled valves in that branch? Maybe the guts are stuck, mixing the hot and cold. A bad tempering valve? A hot water recirculation system? A bad check valve could mess things up.