Primary-Secondary Rings

Primary-Secondary Rings


Quite recently a new approach has appeared regarding the installing of complex heating system with a large number of heat consumers. Right after the boiler, at the edges of the floor, a short primary closed ring is created (see Figure 43) where the warmth carrier is carried by a pump. The circulating pump of the boiler transfers the warmth carrier only through this primary ring. The outlets for feeding the branches for the heat consumers are created in it: each floor branch with radiators, «warm floors» etc. are secondary rings. Each secondary ring has its own pump. Getting water and returning it must be situated close together, not more than 300 mm apart.

An example of a heating scheme with primary-secondary rings
Figure 43. An example of a heating scheme with primary-secondary rings

The secondary rings can be formed as independent heating systems by any of the schemes described above, and by any means of pipe connections: T-shape or collector. In other words, near the boiler, a circular ring is made which seems to work by itself and other absolutely independent rings are connected to it. The primary ring acts as a heat generator (boiler) for these rings. Also, the primary ring acts as a expansion tank for the secondary rings.

Let’s see the principle of how this system works. From the rules of traffic movement, you know about traffic circles. All the cars coming to this circle move in one direction. By going to the right lane, a car can turn into any of the roads connected to the ring, but if it continues moving along the ring, then it must give way to cars entering the ring. Everything is simple and logical (see Figure 44).

Automobile traffic circle
Figure 44. Automobile traffic circle

The circulational pump which pushes the water in the circle (see Figure 45, a) is installed in the primary ring of the heating system. The warmth carrier simply cannot go anywhere, driven by the pump; it goes around in circles ad infinitum without producing any useful work, just like a «ferris wheel» in children’s amusement parks. The cabins endlessly climb up but the same number go down as go up — the warmth carrier only circulates through the primary ring without increasing the height of the water.

Principal scheme of interior of primary-secondary rings
Figure 45. Principal scheme of interior of primary-secondary rings

Let’s connect one more ring (see Figure 45, b) to the primary ring. It is obvious that the water immediately fills it and stops. The secondary ring is longer than the section of the pipeline (between points A and B) of the primary ring between the outputs to the secondary ring. Therefore the hydraulic resistance of the secondary ring substantially exceeds the hydraulic resistance in the region between A and B. The warmth carrier always flows to the side of lowest hydraulic resistance — i.e. the circulation in the primary ring will continue and it will stop in the secondary one. In general, all the cars which come to the second ring will not be able to leave it. Nobody taught our warmth carrier traffic rules and that is why it doesn’t know the rules and doesn’t yield to the cars on the right. All the cars tend to drive through the traffic circle of the ring and those which are gathered at the side do not bother them at all.

In this heating scheme, we are trying to achieve this. We need the common ring to be always in a working state and the secondary rings not to be working. We will use them as needed. In fact, probably it is silly to run the whole complicated heating system if it is not all needed — for example, the system of heating the floors in the swimming pool. Once again, a heating system with primary-secondary rings is mainly used for complicated heating systems with a large number of consumers using different temperature regimes, but which work from one generator of warmth (boiler). In order to have the secondary ring in an inoperative state, it is necessary that hydraulic resistance at the points A and B be roughly equal. For this purpose, the maximum length of this section should not be more than four pipe diameters (4d). Usually for pipes of diameter 1.5–3 inches, this distance does not exceed these limits (6 to 12 inches (150–300 mm)). These limits are needed because the resistance between points A and B must be extremely low. Why should the warmth carrier flow into the secondary ring and overcome the hydraulic resistance and circulate? It will easily flow in the region A-B where the hydraulic resistance is almost zero.

The diameter of the pipes in the primary ring is determined based on the total supply of warmth carrier to all the secondary contours (see Table 1). Usually it is equal to the diameter of the boiler pipes which in turn is chosen according to the area of the heated space. The circulational pump of the primary ring is chosen based on the hydraulic resistance of this ring.  Because in the primary ring there is not a large number of tees and corners, therefore, as a rule, a rather weak pump is needed, which is installed without a foundation, directly into the pipeline.

For activating the secondary ring in the heating of the house, there are three possible variants (see Figure 46). The first is to install, in the section A–B, a pipe of smaller cross-section, a bypass. If we go back to the analogy of the traffic circle, then the installation in the section A–B of the pipe with the smaller cross-section causes a traffic jam in this region and some cars will try to go around using the secondary ring. The second variant is to install at the point B a three-way valve, a kind of barrier, which will partially or completely redirect the warmth flow into the secondary ring. Both methods require an accurate calculation of heating load and the variant with a three-way valve requires manual or automatic valve control.

Variants of including the circulation in the secondary heating ring
Figure 46. Variants of including the circulation in the secondary heating ring

Therefore it is easier to install its own circulational pump on the secondary ring; turning it on causes movement of the warmth carrier and turning it off stops the circulation and excludes the secondary ring from the heating system. It must be noted that modern circulational pumps are made with modes of speed control. They can have either two or three speeds. By setting the speed of work for the pump, we can manage the speed of circulation and therefore the temperature regime. By stopping the pump we can turn off the whole secondary circulational ring but the primary ring will still work as usual. And once again, the scheme of heating in the secondary ring can be made according to any of the schemes of pump circulation which have been shown in the preceding pages of the website, the only difference being that the placement of the boiler is occupied by the primary ring and the position of the expansion tank is occupied by the common section of the rings, A–B.

The circulational pump for the secondary ring is selected based on its hydraulic resistance, i.e. the primary ring is not taken into account, and the pump is chosen for the secondary ring as it is for an independent heating system. Here is that clever scheme: a lot of secondary rings are connected to the primary ring and all of them are considered as independent warmth systems with their consumers and pumps and thus turning off and on in one secondary ring has no effect on the other secondary rings.

But what will happen in the primary ring if circulational pumps of higher or lower power output than the one on the primary ring are installed on the secondary rings? Let’s try to figure this out by looking at examples (see Figure 47).

Examples of installation of circulational pumps of different power output in the primary and secondary heating rings
Figure 47. Examples of installation of circulational pumps of different power output in the primary and secondary heating rings
  1. For example, we have chosen both primary and secondary pumps with a capacity of 10 litres/min. When the secondary pump is not working, then the supply developed by the primary pump, ie. 10 litres/min, will circulate between the points B and A. There will be no circulation in the secondary ring. When the secondary pump is turned on, the whole supply of water will be taken, at the point B, from the primary ring to the secondary one. The supply of water through the common section of the pipeline A–B will be zero. Remember?  All the water coming into the tee must go out from it. In this case, the water has two ways of leaving the tee: to continue its way along the primary ring, or to turn to the secondary one. And the way it will go entirely depends on the fact of whether the secondary pump is turned on or not. If the secondary pump is turned on and if its power is the same as that of the primary pump, then there is no circulation in the section A–B, but it is completely resumed immediately after the point A — i.e. the turning on of the secondary pump does not affect the circulation (as a whole) in the primary ring.
  2. Let’s now change the conditions slightly. Assume the capacity of the primary pump is 10 litres/min and that of the secondary pump is 5 litres/min. When the secondary pump is not working, the whole flow of 10 litres/min from the primary pump will go through the common section A–B of the pipeline. Turning the secondary pump on will take 5 litres/min off through the tee at the point B. The other 5 litres will go through the common section and at the point A the same 5 litres/min which went along the secondary ring will join them again. By turning the secondary pump on, we divided the existing flow into two directions, but after passing the common section A–B, it joined together again and it didn’t affect, as a whole, the circulation of warmth carrier in the primary ring.
  3. Again, change the conditions. Install a pump with 10 litres/min capacity on the primary ring and a more powerful pump with capacity of 15 litres/min on the secondary ring. When the secondary pump is turned off, then the flow of liquid at 10 litres/min will pass through the section A–B as it is supposed to do. However when the secondary pump is turned on, it starts to demand 15 litres/min from the primary ring, but where will it get the missing 5 litres if the primary pump sends only 10 litres/min from the boiler to the point B? Everything is very simple: the secondary pump will pull the missing 5 litres from the opposite side of the tee from the section A–B. In other words, the pump will pull the same water which was pushed by it at the point A; i.e. on the tee at the point A, the warmth carrier splits in two: one part goes through the section A–B back to the secondary ring and the other part continues its movement along the primary ring. As we see, in general, installation of a powerful pump on the secondary ring doesn’t affect the circulation of warmth carrier in the primary ring at all.

Hence, it must be concluded that on the primary ring, it is possible to install pumps of power designed to overcome hydraulic resistance only of the primary ring.

But not all is so simple. On a secondary ring with a powerful pump, mixing of cooled water with hot water happens, and this affects the temperature regime of the whole secondary ring. And there where the heating engineer happily rubs his hands together because he was able to change the temperature of the warmth carrier by changing the power of the circulational pump, the common man will put his hands down. Not knowing the basics of heating engineering, you cannot make all the calculations necessary for a heating system. Therefore, generally speaking, there is a good chance for quality regulation of the heating system, but it will be missed by the non-specialist. When using a system with primary and secondary rings, you should install pumps on the primary ring which have equal or greater power than the most powerful pump on the secondary ring.

Regulation of the secondary ring by turning the circulational pump on or off
Figure 48. Regulation of the secondary ring by turning the circulational pump on or off

The easiest way to accomplish regulation of the warmth carrier in the secondary rings is to install two-positioned (on/off) switches, controlled by the room regulator (see Figure 48), on the secondary pumps. For example, if you set the temperature on the regulator at 21 degrees C, it will send the command to turn on the circulational pump when the temperature is less than 21,  or to turn it off if it is more. In other words, if it is cold in the house, then the sensor will turn the pump on and it will work until the air temperature reaches 21, and then the command for turning the pump off again will be followed. Thus, repeatedly turning the secondary pump on and off will align the temperature to the desired value. If it gets cold outside, then immediately the warmth will start to leave the house and the pump, obeying the command of the room controller, which is usually situated on an external wall, will immediately go into its working regime. In general, the heating system works like the common household refrigerator located in your kitchen: it turns itself on and it turns itself off.