Hot Water Recirculation Systems: How They Work - Fine Homebuilding Article
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Hot Water Recirculation Systems: How They Work

Whether controlled by a demand switch or by simple gravity, hot water recirculation systems put hot water where you want it fast.

In "Sensible Plumbing", FHB #216, master plumber Dave Yates addresses basic whole-house plumbing design. A more advanced plumbing technique is the integration of a recirculation system.
A recirculation system is designed to provide hot water to each of the home's hot-water taps instantly. Beyond providing convenience and increased comfort, a recirculation system saves thousands of gallons of fresh water annually that would otherwise go down the drain as a homeowner waited for hot water to travel from the water heat heater to the faucet or showerhead. (For more on the significance if this water savings, see "Bringing the Water Shortage Home," FHB #212.) There are two basic systems: those that rely on an electrically operated pump, and those that are gravity fed and rely on thermosiphoning. 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, has studied recirculation systems and has found that a demand-controlled circulation system-one that operates on a pump activated by a switch or motion sensor-uses the least amount of energy and saves the most water. It also costs the least to operate. However, whole this system provides hot water quickly, it doesn't do so as quickly as other setups.

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

2. The circulation pump houses a temperature sensor and a check valve that prevents water from entering the return line. The pump moves ambient-temperature water 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 when it’s applicable. 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 needs to 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, closest to the fixtures.

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.
From Fine Homebuilding216 , pp. 18-19
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