In “Sensible Plumbing,” FHB #216, master plumber Dave Yates addresses whole-house plumbing design. A more advanced plumbing technique is the integration of a hot water  recirculation system.

A recirculation system is designed to make hot water available at the tap nearly instantly. Beyond eliminating an inconvenient wait for hot water, a recirculation system saves thousands of gallons of water annually, water that would otherwise go down the drain as a homeowner waited for hot water to travel from the water heater to the faucet or showerhead. (For more on the significance of this water savings, see “Bringing the Water Shortage Home,” FHB #212.)

There are two types of systems: those that rely on an electrically operated pump, and those that are gravity fed and rely on thermosiphoning. Pump systems can be set up in one of several ways: with a switch, with a timer, and with a motion detector that alerts the system to start working. Improvements in pump design have made these systems more efficient than they were in the past (see the link below).

Here’s how a recirculation system works.

Demand-controlled system

Gary Klein, managing director at Affiliated International Management and a former energy specialist at the California Energy Commission, says the most efficient type of recirculation system uses a pump that’s activated by a switch or motion sensor. These systems use the least amount of energy, save the most water, and have the lowest operating costs. The tradeoff is that these systems are a little slower than other types.

1. A switch or motion sensor located near each fixture activates an energy-efficient circulation pump.

2. The pump houses a temperature sensor and a check valve that prevents water from entering the return line. The pump moves water that’s sitting in the line back to the water heater. The sensor lets the pump know when hot water has arrived at the farthest tap and shuts off the pump. Alternatively, a sensor can be placed on the return line immediately after the last tap in the system and hardwired to the pump. This will yield even lower pump run-times.

Gravity-fed system

Master plumber Dave Yates likes the simplicity of a gravity-fed system. A gravity-fed system relies on thermosiphoning, in which hot water rises to the top of the system and denser, cold water falls to the bottom. For such a system to work, the water heater must be located below the hot-water taps it will serve. While pipe insulation and short plumbing runs help to reduce standby heat loss and energy consumption, this system uses more energy than others because it is operating 24 hours a day. However, because there are no pumps to install or maintain, this system is arguably the most user-friendly.

1. Hot water rises to the top of the system.

2. Water that cools in the system is heavier and denser than the hot water being supplied and falls through a return line to the lowest point in the system, the water heater.

3. A check valve keeps water in the water heater from flowing back into the return line.

4. Cool water is heated and circulated through the hot-water supply lines, starting the thermosiphoning process over again.

More about hot water recirculation:

Making Hot-Water Recirculation Pay — New pump technology is making hot-water recirculation more efficient than ever.

Fixing a Hot-Water Problem — This Q&A Spotlight from Green Building Advisor explores a hot water problem in a system with a recirculation pump.

Hot Water Without Wasting Cold — Matt Risinger describes how a Metlund D-Mand Instate Hot Water Pump can cure a weakness common to recirculation systems.