"We got hot-water heat pumps."
Jim and Rachel King, Rosharon, Texas

How They Work: Like other heat pumps, this scaled-down version extracts heat from the surrounding air and transfers it to another medium—in this case, to water stored in a conventional standby tank. The AirTap, as it's called, measures less than two cubic feet, weighs 48 pounds, and sits on top of your existing hot-water tank. AirGenerate, the Houston company that makes the device, says it uses only one-quarter of the electricity drawn by a conventional electric hot-water tank.

What These Homeowners Did: Jim and Rachel, who live with their two kids in a 3,750-square-foot house, had been heating water with propane—at a monthly cost of $130. "You'd hear the flame go whoosh all day long," says Jim. "It was like hearing money go down the drain." When they learned about the AirTap earlier this year, they decided to buy one for each of their two 50-gallon hot-water tanks. A contractor installed them in about three hours.

What They Learned: "You still hear a bit of noise sometimes, like a bathroom fan running," says Jim, who is relieved to know his kids are no longer sleeping near a propane flame—one water tank is upstairs. And how's the water? "We haven't had a cold shower since we turned off the propane."

Keep in Mind: The manufacturer says an AirTap should last 10 years but won't perform as well or last as long if the existing tank is in poor condition.

Payback period: 7 months
Their Cost: $450 each, or $900 total, uninstalled
Yearly Savings on Propane for Hot Water: $1,560

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