Can you help us with a problem with our gas-fired water heater? It delivers plenty of hot water in the morning, when my husband takes a shower. In the evening, when I use the same shower, it produces only lukewarm water. I don’t use hot water before taking a shower, so I’m stumped by the problem.

My first thought was that there’s gaspressure trouble in the evening, so I ran that theory by Kenny Hart, a secondgeneration master plumber. “A gas-pressure problem that occurs only in the evening is possible but unlikely,” Hart says, pointing out that this would also make itself known at other gas appliances, such as a stove or heating equipment.

He says that, if the water heater’s temperature is properly adjusted and it works well in the morning, the problem probably doesn’t lie with the appliance either. The remaining suspect: the distribution system. “One of the first things I look for is a problem with a hot-water recirculating pump,” he says.

The pump cycles water through the water heater to ensure that hot water is instantly available at sinks and showers. If the recirculating pump is on a timer and that’s switched off in the evening, there’s your answer. Reset the timer and you’ve got it made. Also, a simple plumbing mistake or a failed component (such as a check valve) would allow hot and cold water to mix when the pump is off.

“Since modern plumbing systems moved beyond hot on the left and cold on the right, they’ve become more problematic,” Hart says. When you add in modern conveniences like recirculation systems and devices that protect against scalding, back flow and thermal expansion, “you need to add pages to your troubleshooting book”.