Thanks for the honorable mention, Fred!
Code everywhere requires a pipe from the tpr valve to be no more than 6" off the floor. As I recall.
I resurrected the gas hot water heater in the church house weeks ago. Bad thermocouple, typical and cheap and easy fix. The exhaust pipe had a 3" aluminum adjustable elbow to start, went up through the shed roof through an insulated double wall setup, and terminated about 6" from the rake board at the other roof. I extended and braced the pipe last summer to bring it at least 2' from the nearest roof line. Replaced that elbow with galvanized. Vacuumed out the mouse droppings from the inside bottom.
When I got the old inefficient heater fired up for the first time in 7 months, since the pastor left with her family, it started in with the crackling and popping sounds- must be a lot of mineral buildup on the inside. When a water heater starts doing that, it's either put up with it or replace it.
The church had the well water tested there, apparently potable, but I won't drink it, it has a distinct sulfer smell.
I would be absolutely skeptical about a mouse getting in a closed system, however, I found a mouse in the gas tank of my Jimmy when I took the tank down!!!!! At first I thought fiberglass fibers were clogging the fuel pump screen! It was mouse fur! My only conclusion was that it must have been a junkyard gas tank put in!