Wet Central Heating Systems
Originally, systems were designed to circulate hot water by convection currents; because these systems had a very low circulating pressure, however, larger pipe sizes were required. More modem systems use a pump to speed up this circulation. Because of this faster velocity, it is possible to use smaller pipe sizes which use less water and thus heat up more quickly. Installing a pump also enables freedom in system design; for example, the water can be pumped to circulate below the level of the boiler as it does not rely upon convection currents. The water which is heated up in the boiler is often also used to circulate, either by pump or by convection currents, to an indirect hot water storage vessel, giving a domestic hot water supply.
One pipe system This system reduces installation costs, less pipe work being required. It does, however, have several disadvantages when compared to the more expensive two pipe systems. Firstly, the first heat emitter in the system passes its cooler water back into the main flow pipe; these results in the heat emitters at the end of the heating circuit being cooler than those at the beginning; therefore, careful balancing of the system is essential. Secondly, because of its design, the pump only forces water around the main flow pipe and not through the individual radiators, these being heated only by convection currents. For this reason, the heat emitter used must offer only a very little resistance to the natural upward flow of hot water.
Two pipe system With this system, the water is not only pumped around the circuit but also through the radiators, giving them a much faster heating up period. Balancing out the heat to each radiator proves to be reasonably simple. k is not uncommon for the first radiator in the system to have its lock-shield valve just fractionally opened when balancing; this is due to the minimal fictional resistance to the flow through this heat emitter.
Two pipe reversed return system (three pipe system) This is a special design of the two pipe system in which the length of each heating circuit to each heat emitter in the system is about the same. When the cooler water leaves the first heat emitter in the system, it does not simply join the return pipe and travel back to the boiler, as in the two pipe system; instead, it travels to the furthest point in the system and upon receiving the return water from the last heat emitter, runs back to the boiler return connection. This ensures that frictional resistance to the water flow is the same to each radiator. Therefore, although the three pipe system can prove a little more complicated and expensive to install, balancing of the system proves to be a simple task.
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