[fintstone]

There are a couple of types. In a new build or where you desire/are able to run a return line from the far end of the house to the input or cold water side of the hot water heater (can be tubing...doesn't have to be as large as supply line). The pump has a timer set for periods when you are home. It will pump enough of the cooled water from the far end of the line back to the hot watt heater...similar to you manually running the faucet to get it warm except you don't waste water or the energy used to preheat the water down the drain. If it is on the most distant place in the system...all faucets in the line that are closer to the heater also benefit. Usually the pump is mounted at the hot water heater.

The second type is mounted at the most distant point...usually under the sink. You don't run a line to it from the hot water heater, but attach it between the hot and cold lines under the sink. It pumps a small amount of the cooled hotwater into the cold water line to keep the hot side instantly hot. It also runs on a timer to save energy. It just need to come on a little while before you use it. It also benefits other faucets farther back in line depending on how far off the main supply they are located. If the house is big enough...,you might need another on another wing.

Both only pump very small amounts so they are pretty efficient. Both do need an electrical outlet to power the small pump.