The hot water heating system in the home is often a mystery as well as a source of annoyance to the home mechanic. Either it fails to deliver the needed amount of hot water, or the system constantly rumbles and makes other strange noises. Often, the water is either too hot or not hot enough.
Before taking up the various causes for these deficiencies and what can be done about them, it is necessary to understand how the water is heated.
Water is a very poor conductor of heat. This can be easily demonstrated by taking a bucket full of ice and applying heat to the top. The ice on top will melt, while that underneath will remain frozen. The only way that the water at the bottom of the pail can be warmed is by circulating it to the top of the pail where the heat is applied. When water is heated it expands and becomes lighter. Consequently, warm water will rise to the top, while the cold will remain at the bottom.
If you were to join a U-shaped piece of pipe to a tank full of cold water and apply heat to the pipe, the heated water would rise to the top of the tank and the cold water would sink to the bottom, flow through the pipe and be heated. At first, circulation of water would be slow, but as steam began to form, the circulation would become more rapid.
The water back
To demonstrate how this principle operates in a home hot water system, let us select what is probably the simplest kind of mechanism—the water back. This consists of a cast iron box containing either a baffle plate or a U-shaped section of pipe. The box is fitted into the kitchen range and is connected to the hot water storage tank.
Cold water flows from the bottom of the tank into the water back, is heated, and flows back into the tank. As warm water is lighter than cold, it rises to the top of the tank. In rising, it displaces the cold water, forcing it to the bottom of the tank and through the pipe into the water back; this process continues indefinitely.
While the principle of operation is simple enough, poor performance is often caused by improper connection of the water tank with the house plumbing or with the heating element.
The incoming cold water should be piped into the tank from the top and carried to within a few inches of the bottom. If the cold water supply is allowed to flow directly into the top of the tank, it will mix with hot water and produce a constant supply of lukewarm water. The hot water line from the tank to the outlets should, of course, be connected at the top of the tank. Many hot water failures are due to the manner in which the tank is connected with the heating element.
For example, a common case is that in which the water in the upper portion of the tank becomes too hot, while the water at the lower portion remains cold. This occurs when the cold pipe between tank and heater is placed too high on the side of the tank.
The water in the tank below the level of the cold pipe is not affected by the circulation inside the tank, and, as water is a poor conductor of heat, the water below remains at a low temperature. This condition can be corrected by lowering the cold pipe on the tank. When this is done, nearly all the water in the tank will flow through the heating element and provide an adequate supply of evenly heated water.
Another reason for an inadequate hot water supply is the kind of pipe connecting the heating element to the supply tank. Sharp angles in a pipe will give marked resistance to the free circulation of water.
To have free and rapid circulation of water between heater and tank, be sure that the cold pipe is as nearly horizontal as possible, and the hot pipe slants up from the heater to the tank. For any of these pipe connections, use 45-degree angles at the bends instead of the sharp, 90-degree elbows.
While most hot water supply tanks found in the home are the vertical kind, the horizontal tank is frequently used, particularly in large houses. In this tank, the cold water supply must be brought to the bottom where it can flow through the heating element. If the cold and hot pipes are too close together, the circulation of water will be limited to a small portion of the total capacity of the tank.