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 p