A heat trap is a valve or loop of pipe that allows water to flow into a water heater tank but prevents unwanted hot-water from flowing out of the tank. Heat traps have balls inside that either float or sink into a seat, which stops convection. These specially designed valves come in pairs. The valves are designed differently for use in either the hot or cold water line.

 

Pipes on the top of a water heater with two heat traps installed, one in the hot water line and one in the cold water line

 

If your storage water heater doesn't have heat traps, you can save energy by adding them to your water heating system. They can save you around US$15–$30 on your water heating bill by preventing convective heat losses through the inlet and outlet pipes.

 

A pair of heat traps costs only around $30. However, unless you can properly solder a pipe joint, heat traps require professional installation by a qualified plumbing and heating contractor. Therefore, heat traps are most cost effective if they're installed at the same time as the water heater. Today, many new storage water heaters have factory-installed heat traps or have them available as an option.