Two-piped Gravity System with Upper Setting

Two-piped Gravity System with Upper Setting


The water from the boiler goes up through the supply pipe and then goes through the pipes to the heating devices (see Figure 3). Horizontal pipes must have a slope of 0.002–0.003. From the heating devices through return and vertical pipes the water goes to the return pipe of the boiler. Every device in this system is served by two pipes — supply and return — and is therefore called a two-piped system. Water is added to the system from the water supply, as needed. But if you do not have one, then you can add water manually via the expansion tank. In adding the water from the local water supply, it is better to do it through the return pipe: cold water from the water supply mixes with the warmer water from the return pipes and increases its density, thereby increasing the circulation head during the time water is being added.

The Scheme of two-piped Gravity Heating System with Upper Setting
Figure 3: The Scheme of two-piped Gravity Heating System with Upper Setting

To improve the circulation of the warmth carrier, the main vertical pipeline (from boiler to expansion tank) should be insulated, so that the water will remain as hot as possible, delivering water to the horizontal pipes. The expansion tank can be made in two ways:  simple, without circulation of water; and more complicated, with circulation of water.

The simple type is a vessel with two pipes welded into it, or screwed with rubber gaskets. One pipe is the vertical supply line and the second pipe gives the signal of overflow from the tank. The place where the vertical pipe is connected to the expansion tank is not important; the pipe can be inserted into the tank through the bottom or through the side. It is important that it should be inserted as low as possible, in order to use the volume of the expansion tank completely. The signal pipe comes into the tank on the side wall 100 mm from the top: when the water is added to the system the tank fills only up to this level and then the water starts to go into the signal pipe, showing that the system is filling. As the system is being used, the heated water will expand and go down through this signal pipe. Eventually, when the water is heated to its maximum, the system will spit the excess water into the signal pipe, thus ensuring self-regulation of the water level in the tank. With further increases and decreases of the volume, the level of water in the tank will be changed, but there will be no overflow into the signal pipe. This type of expansion tank has two disadvantages: first, periodically (approximately twice a year), you need to check visually how much water is in the tank; and second, the tank must be insulated very well — the water in it will get cold, and with very low temperatures, it may freeze. However, these drawbacks are not serious in such a simple system. You get used to it quickly:  you need to insulate the tank just one time and you get to know when water needs to be added (once a year, or twice, etc.). Usually the level is checked and water added before the heating season starts, and you can forget about it until the beginning of the next season.

In village houses which get their heat from a boiler but which do not have a water supply or sewage, the simple design of the expansion tank can be even simpler by not including a signal pipe. A very good tank can be made from an old milk jar which has a lid and sufficient volume, once the seal has been removed. The lid, either closed or almost closed, lets the air through but keeps garbage out. When you need to add water, just lift the lid. The system is filled with water either from pails or a hose while monitoring the water level visually. The expansion tank should be filled between one-third to one-half, leaving room for water expansion. If you add too much water, the heating system will push the excess through the top of the tank since it is open. Of course in this case, water will leak through the ceiling so the owner of the house will not likely put in too much water — this is another kind of self-regulation.

Scheme of Gravity Heating with more complicated Expansion Tank
Figure 4: Scheme of Gravity Heating with more complicated Expansion Tank

Using the more complicated expansion tank (see Figure 4), four pipes are welded or screwed into the tank instead of two. Two are supply and return, and provide the circulation of water in the tank, decreasing the probability of freezing greatly. The other two are the overflow and control pipes: they oversee the level of water in the tank. While adding water to the heating system, the valve at the lower end of the control pipe is opened. As soon as the water comes out of it, then you must stop filling the system: the pipe is showing that the system and the tank are filled with water. The valve on the control pipe must then be closed and should not be opened until the next time water is added to the system. The overflow pipe works in the same way as it does with the simple tank, i.e. when there is a sudden increase in the volume of hot water, this pipe gets the extra water and spits it into the sewer system. There should be no valves on the overflow pipe. It should be noted that, in spite of these tanks providing more automatization, they are not popular in private houses because too many pipes are involved.

Gravity systems can have one or two contours. In one- contour systems, the boiler is placed at the beginning of the contour and the pipes are located on the left or right side, going as a belt around the whole house or apartment, and the length of the loop horizontally should be less than 30 metres (and preferably less than 20 metres). The longer the loop, the greater the hydraulic resistance there will be in the system (forces of friction inside the pipes). If the length of the loop is more than 30 metres, the system will not have enough circulation head to overcome this resistance. Even for 25 metres, the head of circulation would have trouble. In two- contour systems, the boiler is placed in the centre, and the pipelines (contours of the loops) are placed on both sides of the boiler, and the total length of the pipes of each contour horizontally must be less than 30 (20) metres. To balance the system hydraulically, the lengths of the loops of the two- contour system and the total number of sections in the radiators must be more or less the same (see Figure 5).

Examples of two-piped gravity systems with upper setting
Figure 5: Examples of two-piped gravity systems with upper setting


Depending on the direction of motion of water in the main pipes, the heating system can be «dead-end» or «continuing flow».

In «dead-end» heating systems, the motion of hot water in the main supply pipeline is opposite to the motion of the cooled water in the main return pipeline. In this scheme, the lengths of circulational loops are different from each other; the further the heating device is from the boiler, the longer the circulational loop; and the opposite — the closer the heating device is placed with respect to the main vertical pipeline, the shorter the length of the circulational loop.

It is difficult in «dead-end» systems to achieve equal resistances in short and farther circulational loops. Therefore, the heating devices which are placed close to the main vertical pipeline will be heated much better than those which are further from the main vertical pipeline. And when the circulational loops which are closest to the main vertical pipeline do not have a big warmth load (warmth output to give to the room), balancing of the circulational loops becomes even more difficult.

In the heating systems with «continuing flow» motion of water, all circulational loops have the same length. Therefore, the vertical pipelines and heating devices work in equal conditions. In such systems, independent of location of the heating devices horizontally with respect to the main vertical pipeline, heat will be the same. However, this type of heating system is of limited use because often, during the design of actual systems which consider the layout of the house, it is seen that such systems will require more pipes than for the «dead-end» systems. But in the case where balancing of «dead-end» systems is impossible, then the «continuous flow» system is used.

To make «dead-end» systems more widely used, the lengths of the main pipelines are decreased and instead of one long contour, there are two or more shorter contours. In such cases, better horizontal balancing of the system occurs. Balancing of the heating loops of the contour should be the starting point of the design of the system. To make the system work evenly all the loops of the contour must have approximately equal hydraulic resistances. In other words, a loop which is placed close to the main vertical pipeline must have almost the same resistance as a loop which is further away from the main vertical pipeline and the sum of the hydraulic resistances of all the loops should not be more than the circulational head; otherwise the water in the system will stop. Such systems are called «clamped».

Let’s imagine that the heating contour is in the shape of a closed road (e.g. racetrack) on which six trucks start simultaneously, beside one another, loaded with hot water. Let’s look at their motion under the condition that all six trucks move with the same speed and they cannot move ahead or behind one another. The task for the trucks is to reach the radiators, unload, and go back to the start to get a new supply of hot water.

Illustration of Motion of water on the Contour of the Heating System
Figure 6: Illustration of Motion of water on the Contour of the Heating System

It is obvious that for the trucks to start simultaneously there must be a six-lane road. That will be the main vertical pipeline of the system, which has the largest pipe diameter. Let’s assume that we are in a two-loop heating system. Therefore, after the start on our road, there is a T-shaped crossroad (tee-fitting in the heating system). The trucks divide into two groups: one group turns to the left, and the other to the right. While turning, the trucks which are closer to the center line turn on a further radius: they go a longer distance, and coming out from the turn, they are a little behind the trucks which turned on the closer radius. The first energy loss has occurred. In the heating system, the water molecules which are situated closer to the centre of the pipe are luckier than the ones which are close to the walls of the pipe. Losses of hydraulic pressure are happening in this tee-fitting.

Look further. Six trucks came to the T-shaped crossroad and six of them must leave it. (The volume of water which arrives at the fitting equals the volume of water which leaves. That is an axiom.) For the three trucks which turn to the left, we do not need a six-lane road; three lanes is enough. Therefore the cross-sectional area of the pipe can be half. Notice, we decrease the area by half, but not the diameter. They are different quantities. So three trucks are left which go on three lanes. Make the first branch from the main pipeline to the first unloading place with a width of one lane. (We are putting one more tee-fitting on the pipeline.) Three trucks come to the newly-created crossroad. One of them notices the branch in the road and makes a turn. The other two go on because there was only one lane available in that branch. The second pressure loss has happened in the tee fitting at the turn. The water passing by the turn experiences almost no pressure loss. For the water which is passing by, there must be a further decrease in cross-sectional area and diameter of the pipe; in this case, with a 2:1 ratio, for the two- and one-way movement of the trucks. The truck which turned into the branch is almost as its goal: it is running towards its unloading place. The other two continue moving forward on the road.

Let’s make one more branch in the road (putting a tee-fitting) and divide the trucks. One of them goes to its unloading place; the other continues along the main road. It is obvious that from this crossroad, one lane is enough for each of the trucks, making the cross-sectional area of each pipe the same. There is no need to make another crossroad because the last truck will turn to its unloading place. There is no further unloading place on the main road. The heating ability of the boiler to provide warmth has been used up completely; a further increase in the length of the pipe will not accomplish anything.

But let’s go back to the truck which took the first turn. It unloaded a long time ago (gave out its warmth) and has gone back to the loading place and at the same time the second truck is just arriving at it place of unloading and the third one is still on the highway. We see a unbalance in the heating system. While the third truck arrives at its unloading place, the first one is able to make one more circle and deliver one more portion of hot water. Therefore it is necessary to make the first truck go slower: to put bumps on the road (decrease the cross-sectional area of the pipe) or put a traffic policeman (regulator of changes in the amount of hot water — in other words, a valve). The policeman can stop it and force it to unload manually instead of automatically. We can put the same kind of control in the way of the second truck and while the first two are busy unloading, the third truck can get to its destination and unload automatically. In the «same way» motion, regulators are not necessary because of the lengths of all circulational loops are equal.

As a result of decreasing the diameters of the pipes leading to the radiators or the installation of valves on them (manual or automatic thermoregulators), it is possible to achieve a situation when all three trucks travelling along this contour arrive simultaneously at the meeting place with the three trucks which came from the other contour. Here they are again united in one flow on the six-lane main road and they go back to the loading place to start again. This system can now be considered balanced.

The balancing of the system with the help of valves is done after the heating system has started. Someone must go into each room, one by one, write down the temperatures in each, and close the valves leading to the radiators. The procedure must be repeated many times until an even balance of heat is obtained. If you use thermostatic valves, then the process is easier. The desired air temperature is set on the valve’s handle, and then the valve automatically opens and closes the hot water being supplied to the radiator.

It is to be noted that in going different distances, the trucks expend different quantities of energy: the ones which a long way burn more fuel and encounter more obstacles. While going along a straight line, the water overcomes the hydraulic resistance of friction of the walls of the pipes: steel pipes have a larger pipes, and polymer ones have less. All tee-fittings, cross-fittings and turns also have resistances. The sum of all resistances should not be more than the circulational head. And what will happen if we decide to decrease the number of lanes for the six trucks from six lanes to two (in other words, increase hydraulic resistance)? The result is known — there will be a traffic jam. The flow will not be stopped completely, but it could not be called a movement. So, to avoid the effect of a «clamped» heating system, the cross-sectional areas of pipelines must correspond with the flow of the hot water.

The hot water in the pipe must move with a certain speed because in each second a sufficient volume of hot water must go into the radiators, achieving the necessary supply of heat. This volume is called «Supply of Hot Water».

The higher the speed of the water, the greater its usage. But if the speed is increased, there is an increase of resistance (friction) in the pipe. In other words, with an increase in usage of the hot water, the system resistance increases. If you use a larger diameter pipe, the resistance decreases and vice versa — if you use a smaller diameter pipe, the resistance increases.

With pipes that are too thin, because of the increase in the force of friction (hydraulic resistance), the hot water usage goes down and the boiler gets overheated more often, but the heating devices stay cold because the hot water doesn’t go into them in a necessary volume.

Calculations for a heating system is done by heating engineers and is too complicated to be put on this website. However, for gravity systems with horizontal length of steel pipelines no greater than 20 metres, these calculations have been done thousands of times and therefore we can use this previous experience.

Usually from the boiler the vertical pipeline has a diameter of 50mm (2 inches). The pipe which supplies or collects water from one or several radiators with more than 35 sections in total should have a diameter of 2 inches; with 25–35 cast iron sections, the diameter should be 1.5 inches; for 10–25 sections, 1 inch; and for less than 10 sections, 3/4-inch. For a length of pipe without radiators of more than 10 metres, one should add 1/2 inch to the sizes listed above to decrease the resistance of the motion of the water in the pipes.

To choose the power of radiators for the climate in the Moscow region, it is possible to follow a simple rule: for heating ten square metres of living space in a room with a height of 2.5 metres, with one external wall and one window, it is enough to use 1 KW; if the room has two external walls and one window, then 1.2 KW is sufficient for heating; if the room has two external walls and two windows, then you will need 1.3 KW. You simply need to know the area of each heated room and calculate the necessary radiator power. Usually, the power of one section of a radiator is given in the store on the price tag. The power of the boiler should provide the total power of all the sections of all the radiators.

While choosing the pipe material, the power of radiators, and the boiler, it is better to design a heating system with more power than you will need than less. For example, polymer pipes have less hydraulic resistance than steel pipes and you can choose a smaller diameter. However, it is better not to decrease the diameter but to make a system with the same diameter as for steel pipes. Similarly for the radiators and boiler. The reason is that regulators can decrease power but not increase it.

Here I must explain something. In heating engineering, there are two ways to regulate a warmth system, quality and quantity, which change the warmth head and therefore the speed of the water, temperature, and volume of liquid in the system, according to certain cross-sectional area of the pipes per time unit.

Quantity regulation is achieved by different types of valves which you open or close. Quality regulation is done by changes in the heat of the water in the system (by regulation of flame in the boiler), and therefore its density which leads to changes in volume, head, and temperature.