Building on this post....
by ndroman21 (2019-08-23 10:54:08)

In reply to: The system does not waste water, it recirculates it.  posted by voidoid

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....hot water supply systems are usually built as a straight line run from the water heater to the nearest fixture to the next fixture.....so on and so forth, until they end at the furthest fixture from the water heater.

If you make the system a loop by installing a small (usually 1/4" or 3/8") line back from the furthest fixture to the water heater, you can keep a small amount of hot water constantly circulating through the line.

There are two methods for this. The gravity method works if you have a significant elevation change between the water heater and the last fixture (for exxample, WH is in basement, last fixture is on the 2nd floor). Because hot water is less dense than cold water, gravity will keep a constant flow of hot water rising in the line, and the colder water will travel back down through the re-circ loop.

If there isn't enough elevation change for gravity to do the work, a mechanical pump can be installed to do the same thing, with a thermostatic valve that activates the pump when the water at the last fixture cools.

As voidoid noted, this doesn't waste water, it wastes a small amount of energy heating water that may not be used at that time. But becasue the recirc line is ususally small, there is very little water being reheated.

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