Wouldn’t it be nice not to have to wait so long for the water in the shower to get hot? It’s possible, and this is how it works.
In a standard hot water distribution system, the hot water in the pipe cools off to room temperature after the flow is turned off. Then, when hot water is desired at a later time, it must be allowed to run until all this room-temperature water is drained out of the pipe, and hot water from the tank reaches the faucet. The length of time that this takes is proportional to the distance of the faucet from the tank. For a long run of pipe, this can be greater than one minute (see Figure 1).
Note that, inside your water heater, hot water that you take out comes from the top of the tank, and the replacement cold water runs down an internal tube into the bottom of the tank. Hot water, like hot air, always rises above that which is colder because it is less dense. By taking the water off the top of the tank you are always getting the hottest water.
In case you are interested, temperature is simply the vibration energy of the molecules of a substance bouncing against each other. That amounts to pressure that pushes them farther apart, which reduces the number of them in any given volume, i.e. the hotter fluid is less dense.
Fortunately, there is a way to make use of hot water rising above cold to always have it available near your bathroom or kitchen faucets, even if they are far from your water heater. It’s called a “Recirculating Line” (See Figure 2). This is a pipe that runs from the far end of the hot water distribution pipe to the bottom of the water heater. Water continuously recirculates slowly around this loop due to natural convection, i.e. hot water rises, and cold water falls.
Because recirculation is relatively slow, there is some heat loss along the distribution pipe.
Despite this moderate temperature drop along the pipe, the water available at the faucets along the distribution system is much hotter than room temperature and is generally satisfactory for immediate use. With the recirculating line in place, the time required to get hot water from any faucet depends on its distance from the main distribution pipe. Where the length of the branch pipe is short, the time to get hot water can be reduced from over a minute down to mere seconds!. Because the circulation is slow, it is important to insulate the outgoing hot water pipes. The temperature of the water that is quickly available from a recirculating line is hotter if insulation is in place to minimize the heat loss along its journey. Plumbers are generally familiar with putting in a recirculating line. However, though pipes running inside a wall are already well insulated, plumbers will usually not insulate the outgoing hot water pipes that are out in the open (as in the basement of a ranch house) unless you insist on it.
Unlike the simple arrangement shown in Figures 1 & 2, many homes have water lines that branch out to reach remote sinks, showers and tubs, and can go to higher floors as well. No problem! A recirculating line can also be extended to reach the remote ends of more than one hot water line branch.