Gravity systems are among the most simple of hydronic heating systems, as well as the most common in small houses. They are also very reliable, being able to be in use for more than forty years without significant repairs and they use only the natural laws of physics without needing extra sources of energy or expensive equipment. The disadvantages of such systems are: total horizontal length should not be more than 30 metres because of low pressure of circulation; slow start-up because of high warmth capacity of water and, again, low circulation pressure; danger of freezing of the expansion tank if it is located in an unheated area.
The principal scheme of gravity systems consists of a boiler, supply and return pipes, heating devices, and an expansion tank. The hot water from the boiler goes through the supply pipes and vertical pipes into the heating devices, giving out part of its warmth, and then goes back to the boiler through the return pipe, where it gets reheated to the necessary temperature and the cycle repeats. All horizontal pipes in the system should have a slight downward slope in the direction of the water: the hot water which went up through the vertical pipes because of its expansion, and because of the pushing effect of the returning cold water, goes horizontally by itself and the cooler return water goes back to the boiler by itself. The incline of the pipes makes it easier for bubbles of air to go upwards to the expansion tank: gas is lighter than water and therefore aspires to go upwards and the sloping parts of the pipes help the air not to remain but to go to the expansion tank and then to the atmosphere. The expansion tank creates a constant pressure in the system by receiving water which has expanded due to heating and returning cooler water to the pipes. Water in the system rises up because of expansion during heating, and under the force of gravity circulation starts because of the difference in densities between heated water (going up in the vertical pipe) and cooled water (going down in the vertical pipe). The pressure of gravity is used to give motion to the water and to overcome the resistance of the pipes. These resistances are caused by the friction of water against the walls of the pipes and also of other resistances in the system. These other resistances are: branches and turns in the pipes, fittings, and the heating devices themselves. The more resistances there are in the system, the more gravitational pressure is needed. To lower the friction, large diameter pipes are used.
Circulational head Pcirc = h(ρret — ρsupp) depends on (see Figure 1):
- the vertical distance between the centre of the boiler and the centre of the lowest heating device (h). The greater this distance h, the better the circulation of the warmth carrier will be;
- the densities of cold water (ρret) and hot water (ρsupp).
How does the circulation head get created? Imagine that inside the boiler and radiators the temperature of the warmth carrier changes in jumps (not gradually) on the central axis of these devices — and in fact, this is not far from what really happens. Thus, in the upper part of the boiler and radiators there is hot water, while in the lower part there is colder water. Hot water has a lower density and therefore lower weight than colder water. Imagine that we cut off the top parts of the heating contour (see Figure 2) and leave only the lower parts. So, what will we see? We will see an example of the principle of communicating vessels, which is well-known to us from school physics. The top part of one vessel is located higher than the top part of the other; the water under the force of gravity wants to go from the upper vessel to the lower one. The heating contour is a closed system, in which the water does not go away as it does in communicating vessels, but wants to be on one level. That is, the high column of cold heavy water after the radiator constantly pushes the lower column of water before the boiler and pushes the hot water up from the boiler, and thus a natural circulation is created. The higher the centre of the radiators is located above the centre of the boiler, the greater is the circulation head. This difference in height is the main requirement for circulation head. The slope of the supply pipes to the radiators and return pipes from the radiators to the boiler simply assists the process, helping the water overcome the resistances of the system.
In private houses, it is best to put the boiler below the heating devices, for example in the basement. In apartments, when the boiler is put on almost the same level as the radiators, to increase the circulation of head, it is better to put the boiler in a «hole», that is, directly on the slabs. Of course, precautions must be taken against fire: a thin layer of cement, along with sheets of asbestos and steel, can be placed under the boiler.
The second thing on which circulation depends is the difference between the densities of hot and cooler water. Here I note that gravity systems belong to the group known as «self-regulating systems». Changes in temperature cause changes in quantity of water. Changes in the density of hot water will lead to an increase or decrease in the natural circulation pressure, and therefore of the quantity of water being circulated. In other words, when it is cold outside, it will get cold inside also; turning the boiler on to full extent will increase the heating of water and make its density much less. When the water comes to the heating devices, it gives out warmth to the cool air in the room, and its density increases very much. But if we look at the part of the formula in Figure 1 which is in parentheses, we see that the greater the difference between the densities of cool and hot water, the greater the circulation head. Therefore, the more the water is heated in the boiler, and the more it is cooled in the radiators, the faster it starts to «run» (circulate) in the system, and this continues until the room becomes warm. Then, the cooling of water in the radiators will be slower than before, and its density will not be much different than that of the water coming from the boiler, and circulation head starts to decrease gradually. The water no longer runs very quickly in the pipes, warming the room, but goes more sedately. But when the temperature in the room starts to go down again, either because of colder weather outside or because of open windows, the circulation head starts to increase again, and the water starts to run more quickly, trying to compensate. Thus, self-regulation of the system is occurring: simultaneous changes of temperature and quantity of water provide the heat emission from the heating devices necessary to support an even temperature in the rooms.
Gravity heating systems can be: two-piped with upper setting, two-piped with lower setting, and one-piped with upper setting.