A passive hot water recirculating system, also called a "gravity loop," is a method of reducing water waste by keeping hot water available at the tap.
There are a couple of things required in order for this to work, and it may or may not be appropriate to all locations.
- First of all, the water heater needs to be in the basement, with the faucets being connected located on the floors above. Water heaters on the same level, or in attic spaces above the faucets, will not work with a gravity loop.
- An additional hot water line extension from the faucet back to the water heater makes a complete loop.
- Then, operating on the simple principle that cooler water will naturally be heavier and will fall, and warmer water will likewise rise, the gravity loop helps keep warmer water available so that you don't have to run the faucet as long in order to get hot water.
In other instances, when gravity alone will not work, recirculating the water is done Grundfos, one manufacturer of such pumps, the annual operating cost will be less than $20 per year.. According to
Taco, another manufacturer, offers the D'MAND system, which uses the existing cold water supply line as a return line to the water heater, making it better for retrofit installations. The D'MAND is operated only when needed with either a push button or an occupancy sensor, so that hot water is circulated only when needed, and the use of electricity is minimized.
Other systems avoid being run unnecessarily by using a timer on the circulating pump. However, these systems typically also have backflow preventers or valves in place to stop gravity loop circulation when it might not be useful. In a case such as that, the valves would prevent the system from circulating passively, and turning the timer off will defeat the purpose of the pump.
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Read Kevin Burrell'sQ&A "Do hot water recirculation systems work? What kind would you recommend?"