When you turn on a hot-water tap, it can take a long time before hot water actually arrives at the faucet. That’s not surprising, considering all the cold water that has to flow out first. For example, a typical bungalow in the San Francisco Bay area — where we work — might have a 60-foot-long 3/4-inch-diameter hot-water supply pipe with an additional 10-foot-long 1/2-inch branch connection to the sink. That much pipe can contain more than 1-3/4 gallons of cold water. Assuming a flow rate of 2.2 gpm, it would take at least 47 seconds before hot water flowed out of the faucet. If the homeowners have a green conscience and have installed a 1.5-gpm reduced-flow aerator on the faucet to save water, they’ll have to wait even longer — 70 seconds — for the hot water to arrive. They’re wasting both water and time.

According to Gary Klein, an expert on water distribution systems, a typical family of four wastes about 10,000 gallons of water per year waiting for hot water. “Average hot-water usage is about 20 gallons per person per day, with a very large variation,” Klein says. “About a third of that, or 7 gallons, is water that runs down the drain while...

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