One-piped gravity systems are made only with a high-setting supply pipeline, where there are no vertical return pipes. Compared to two-piped systems, one-piped systems are easier to install, a smaller quantity of piping is required, and they look better.
One-piped heating systems are divided into two types (see Figure 10).
In the scheme without bypass, the vertical supply pipeline is absent, and the radiators are connected to each other in sequence vertically. The hot supply water goes in sequence through all the radiators from top to bottom, starting from the top radiator and becomes cooler as it goes to the low radiators. Thus it is hot in the upper rooms of the house and cold in the lower ones. To balance the heating contour, radiators with more sections are installed in the lower floors. In a non-bypass system, one should not install globe (regulator) valves because while decreasing the flow or closing the valve by one or another radiator, the supply of water will be decreased or stopped in all the radiators which are connected below in the same vertical pipeline.
Also in such a scheme, one cannot regulate the air temperature in the rooms. If the house is two-storied, then it is impossible to have heat on only one floor. Non-bypassed systems were quite popular in the middle of the twentieth century, when the main aim was economy of piping. More recent years have shown that this scheme should not be used, and nowadays it is not used.
Using a locked-section scheme (bypass) some water from the vertical pipeline goes to the upper radiators and the rest of the water is directed through the vertical pipeline to the radiators which are below. The water in such systems gets cooled a little less and therefore the difference between temperature on high and low floors is less. In essence, it is an improvement over the non-bypass scheme, because a locked section (bypass) is made between the supply and return pipes of the radiator. The diameter of the pipe of this locked section should be one size less than the diameter of the radiator pipes. Thus the water which comes from above is divided into two streams: one goes to the upper radiator and the other, through the bypass, goes to the lower radiators. If the diameter of the bypass as that of the radiator pipes, then the water in the radiator will stop circulating because the hydraulic resistance in the radiator is more than in the bypass. Water always flows in places of lower resistance: why would it flow through a radiator if it can calmly flow through the locked section, the diameter of which is the same as that of the pipe from which it has just come.
However if you install pipes in the bypass of the same diameter as those of the radiators for the purpose of balancing the heating system, the quantity of water coming into the radiator must be regulated by the valves which are installed on both the supply and bypass pipes. Thus by closing (opening) the valves on the supply pipe of the radiators or bypass pipe, it is possible to regulate the arrival of the water into the radiator or vertical pipeline. For example, it is possible to cut off the radiator completely and direct all the water to the bypass and thence to the lower radiators on this vertical pipeline, or the opposite, to close the bypass and direct all the warmth flow into the radiator.
In modern heating systems, these two valves on the bypass and supply line are replaced by one, which is called a three-way valve. Depending on the placement of the closing baffle, a three-way valve simultaneously opens the path of the water to the radiator and closes the path to the bypass, or the opposite — closes the bypass and opens the path to the radiator. Such valves can be equipped with an electric drive which is plugged into a special device, a controller. The controller measures the air temperature in the room or the temperature of the water and sends a command to the three-way valve, which increases or decreases the supply of hot water into the radiator, and the rest of the water goes into the bypass.
As in the two-pipe system, in a one-pipe system it is possible to make «dead end» or «continuous flow» motion of the return of the water. Using «continuous flow», all the loops of the heating contour have the same length and it is possible to balance such a system. With «dead-end», to balance the temperature of the water is quite difficult, because there is an unbalance not only in the length of loops, but also in the height of the vertical pipelines, whereas in the two-pipe system the temperature imbalance was only in the loops.