3. Put a recirculating pump under the sink

As you stand around waiting for hot water to arrive at your bathroom sink—or, worse, wander off to do something else while the tap is running—watch what's flowing down the drain: not just water, but all the energy that went into heating it. But let's say you could press a button and have hot water in an instant. That's the work of a clever device called an on-demand recirculating pump. Installed under the sink, the pump captures the not-yet-hot water before it exits the tap and shoots it back to the water heater. The process repeats until the water gets hot enough, at which point the pump shuts off, you turn on the tap, and the steamy stuff flows. Obviously, this setup saves water. Less intuitively, it also saves energy. That's because the water going back to the heater is usually slightly warm, so reheating it takes less energy. And the pump moves water faster than a typical faucet can, which means less heat loss in the pipes—and less waiting time for you.

*Metlund D'Mand S50 Recirculating Pump: $268 (includes all fittings for copper pipe)

4. Insulate hot-water pipes

Without insulation, your house's hot-water pipes act as a gigantic radiator, transferring heat to the air so efficiently that any water in the pipes—even if it left the boiler at a toasty 105 degrees—is barely lukewarm 15 minutes later. So if you wash your hands to prep dinner, then need to rinse a pot, you'll have to wait for hot water all over again. The solution: Insulate pipes wherever you can reach them by encasing them in rubber or polyethylene foam tubes. The tubes come with an adhesive-coated slit down the middle, so you just ease them over the pipe and press the ends closed. Seal the seams with duct tape. Tests show that insulation can double the cool-down time in 1/2-inch pipe and triple it in 3/4-inch pipe.

Tubes to cover 45 feet of 3/4-inch pipe: $55
Roll of duct tape: $5
*Total: $60

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