Water recirculation loops are very common in homes. They typically have a small pump to recirculate water from the water heater to remote points of use in the home. Waiting for hot water to get to a distant location can consume a lot of water and use of a pump can theoretically be cheaper than the cost of the water that would otherwise be wasted.
I think this is probably true—depending on your water heating costs and the costs of the water itself, but there is a way to do it with almost no extra cost–assuming that both approaches require extra piping.
Thermosiphon is the principle where heated water is more buoyant than cold water so it tends to rise in the pipe and then fall as it cools off. If we place this principle in the pipe from the hot side of the water heater to the bottom cooler part of the tank, this thermosiphon will create a loop of continuously circulating hot water without the need for a pump.
The following picture shows how simple it is.
In my system a ¾” supply line was run from the water heater to all the fixtures. At the last fixture the line was reduced to ½” and then run back to the water heater. The lines were well insulated. The effect of the thermosiphon can be greatly improved by leaving the insulation off of the last 10 to 12 feet of pipe before it gets to the water heater, but is not absolutely necessary.
Of course this will only work if the tank can be located lower than the fixtures. Any house with a basement can make this system work for the fixtures located on the upper levels of the home. For fixtures on the same level as the heater you will just have to wait. A check valve was installed to prevent the water from going the wrong way when fixtures at the same level as the heater (in the basement) are being used.
In my own home, this represents the guest bath and the laundry, and since they are located within a few feet of the water heater it is of no consequence.
Our kitchen is really the only fixture that takes considerable time for the hot water to arrive—and the thermosiphon works great and at no cost. It is also VERY quiet—as in absolutely NO noise that is associated with pump systems.
Charles Buell, Real Estate Inspections in Seattle
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