Hydrogen sulfide is the gas behind the rotten-egg odor, and your nose is remarkably good at catching it. People can detect it at concentrations well below one part per million, which is why a faint trace at the tap seems so strong.

Older homes were plumbed with air chambers to absorb this shock, short capped stubs of vertical pipe near fixtures that hold a cushion of trapped air. Air compresses; water does not. When that cushion is present, the shock wave pushes into the air pocket instead of the pipe.

Hard water is water carrying dissolved minerals, mainly calcium and magnesium. Those minerals are invisible in the glass, but when the water is heated or left to evaporate, they drop out of solution and cling to whatever surface is handy.

Most homes are comfortable and safe at 40-60 psi. Plumbing code draws a hard line at 80 psi: once static pressure exceeds that, a pressure-reducing valve is required because above it the risk of damage rises quickly.

Hard water is loaded with dissolved minerals, and those minerals come out of solution as scale wherever water is heated. Both heater types deal with that, but in different places.

Before anything else, check whether the discoloration appears in the hot water only or in both hot and cold. This single observation points you in the right direction.

Diagnosing short hot water starts with two questions. First: did it change, or has it always run short? A heater that used to be fine and now runs out points to something developing inside the tank.

The popping, rumbling, or banging that happens when the heater runs is the sound of sediment. Hard water — common across the Phoenix area — carries dissolved minerals like calcium and magnesium that settle out over time and collect as a layer on the bottom of the tank, right over the burner on a gas unit.