What is Central Heating?
Central heating is the heating carried out by establishing separate boiler rooms in the basements of the buildings to be heated, and by sending the heat produced in the boilers to the heaters placed in the desired environments by means of a carrier fluid.
Hot Water Heating Systems
A hot water system generally consists of a hot water boiler, water carrier pipes, heating elements, circulation pump, expansion vessel, automatic control devices and spacers. Hot water with a temperature below 110 °C is used as the heating fluid. The majority of hot water systems are open to the atmosphere and the water temperature does not exceed 90 °C. The hot water produced in the hot water boiler is conveyed to heating elements such as radiators and hot air apparatus placed in the volumes to be heated by pipes. Here, the hot water that cools down and leaves its heat to the room volume returns to the boiler.
Main Systems Applied in Hot Water Facilities
a- Bottom distribution and bottom collection heating system,
b- Heating system with top distribution and bottom collection,
c- Top distribution and top collection heating system
a-Bottom Distribution and Bottom Collected Heating System
In these systems, the main supply pipe coming out of the hot water boiler, which is usually placed in the basement, comes to the circulation pumps suction collector. The pump outlet collector acts as a distribution collector. Distribution is made from the distribution collector to the desired points at the level of the basement ceiling with the horizontal main supply pipes. From these points, the water reaches the upper floors through vertical pipes called feeding columns. Hot water is connected to each radiator from the supply column with branches. Return columns join with horizontal main pipes collected in the basement. Thus, the water collected from all radiators reaches the return collector. There is an expansion vessel at the top of the building. This vessel is connected to the boiler inlet and outlet by means of the flow and return safety pipes, with no valves in between. In addition, all outlet columns are connected to the expansion vessel with an air pipe. Horizontal pipes and branches should be sloped. Thus, the air to be formed in the system is automatically discharged towards the top point.
In Figure 2.1 and Figure 2.2, the bottom distribution and bottom collection heating system with open expansion tank and closed expansion tank is seen.
b-Top Distribution and Bottom Collection Heating System
This system is applied in buildings with no terrace and full basement. It is possible to heat all floors at the same level, that is, homogeneously. For this reason, it is known as the best working system.
In the top-distribution-bottom-collection heating system seen in Figure 2.3, the water reaches the attic with the main feeding column coming out of the boiler. From here, it reaches the vertical columns in the roof with distribution pipes with a 1% or 2% inclination. The radiators are fed with hot water by vertical feeding columns and branches. The return is the same as the previous system.
Figure 2.4 shows the top-distribution-bottom-collection heating system with closed expansion tank. Air tube, 1/2” air discharge valve and automatic air vent should be installed at the top point of the flow pipe leading to the upper floor.
c-Top Distribution and Top Collection Heating System
If there is no place to pass the pipes in the basement, a top distribution and top collection heating system called umbrella system can be used. Since the system works against natural circulation with the help of a pump, 12.5 mmss should be added for 1 m of vertical pipe in the pressure loss calculation of the 90/70 °C system. (25 mmss should be added to the total of the flow and return pipes.) It is a system that is the worst undesirable in terms of heating. It is applied in compulsory situations and is a system that is rarely applied today.
Figure 2.5 shows the top distribution and top collection heating system.
Air Evacuation in Hot Water Heating Systems
In hot water heating systems, air prevents water circulation in the system and causes corrosion. It is possible. Pipes and radiators that make air do not work well. In fact, circulation in one part of the system may come to a complete stop. The amount of air in the water depends on the temperature and pressure. The air that melts in the water when it is cold is released as a gas when it is heated. The higher the speed of this air, which is entrained with the water, the harder it is to separate from the water. It is useful to stop the pump while taking air from the heating system. The main sources of air formation in hot water systems are; are fresh water and open expansion tanks fed to the system. The molten air entering from these sources is released by gasification in the upper floors where the pressure is low during heating in the boiler.
In practice, horizontal pipes are given a slight upward slope (1% or 2%) in the flow direction to direct the movement of the air generated to the highest point in the piping system.
Vent pipes are located between the roof. Possibility to take vent pipes to the roof It is possible to connect it to the safety flow pipe or another column by collecting it under the ceiling in places where it is not available. In places where there is no possibility to exit the vent pipe, the end of the flow column can be raised 500 mm with a 1/2” pipe, and the air tube or air vent can be placed on a pipe.
Freezing Prevention in Hot Water Heating Systems
In the design of hot water systems, necessary precautions should be taken to prevent the water temperature from falling below the freezing point. If there are pipes passing through unheated volumes, the temperature of the water in these pipes will drop below the freezing point and cause the pipes to crack.
In the heating of large buildings, there is a greater likelihood of sub-freezing elements in the system this way. As long as the circulation continues, that is, the pump runs, there is no freezing. Even if the boiler is not working, any freezing does not occur before the water temperature in the whole system drops below the freezing point.
For this reason, the circulation pump should be operated continuously in cold climates, at night and on weekends, in non-operating workplaces. If the system is to be stopped for a long time, then all the water in the heating system must be completely drained. It has been seen in practice that the small amount of water remaining in the lower parts of the radiator sections freezes and causes cracks at those points.
In hot water heating systems in cold regions, there is a possibility that the still water in the roof installation may freeze, even if the system is operating. In order to prevent this situation, the expansion tank between the roof, all air pipes, safety pipes should be insulated very well.