How long are you willing to wait for hot water when you turn on the tap? That's a question I began asking in 1993, while researching the design of hot-water systems for the California Energy Commission. Most people told me that they want "time to tap" to last only a few seconds. But in the real world, it often takes a minute or more before hot water begins flowing out the faucet.

That's because traditional methods of sizing distribution piping are based on maintaining adequate system pressure. Plumbers often use oversized pipes to overcome the pressure drop caused by excessive pipe lengths and sharp changes in direction. It's not unusual to see a 3/4-inch-diameter supply line installed where a 1/2-inch-diameter pipe will...

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