Suddenly, after returning from vacation , we get hot water fast to upper floors. 3 story house, 75 gallon gas hot water heater, no recirculating system, no leaks, 2 occupants. For years it took about 2 minutes to get hot water from the top floor shower, or most any faucet. We turned the heater down while on vacation, which we have done many times. Return home and reset water heater to original setting. For some mysterious reason we now get hot water in about 15 seconds. What has caused this change?

Have you noticed, they weather has been much kind, this season?
Best Wishes

Yes. I did think about the mild weather. However, even in the summer time it would take nearly two minutes to get hot water previously. Something is radically different now. I even held a stethescope on the hot water pipe out of the water heater to listen for a leak, and there was nothing. Even if there were a leak causing the water to run continuously, that would only affect the branch piping where the leak was. This phenomenon occurs at all the hot water faucets.