By including in the heating scheme two or more boilers we cannot only expand the heating power but also reduce energy consumption. As has already been mentioned, the heating system is originally calculated to be sufficient for the five coldest consecutive days of the year; for the rest of the year, the boiler works at half strength. Assume that the power consumption of our heating system is 55 KW and you buy a boiler of this power. All the power of the boiler will be used on only a few days a year, and the rest of the time less power is needed for heating. Modern boilers are usually equipped with two-stage blower burners; so both of the stages of the burner will work on only several days per year, and the rest of the time only one stage will work, but even its capacity can be too much for «off season» (late fall and early spring). Therefore instead of one boiler with a capacity of 55KW, two boilers can be installed; for example, 25 and 30 KW; or three boilers: two of 20 KW and one of 15 KW. Then, on some days of the year the less powerful boilers can work, but at peak load, all of them will be turned on. If each of the boilers has a two-stage burner, then the adjusting of the operation of the boilers can be considerably more flexible: in the system, boilers on different regimes of burner operation can function simultaneously. This directly affects the efficiency of the system.
In addition, the installation of several boilers instead of one solves some more problems. Boilers of high power are heavy units which, for one thing, must be delivered and brought into the house. Using several small boilers significantly simplifies this task: a small boiler can easily get through the doors and is a lot lighter than a big one. If suddenly, during the usage of the system, one of the boilers fails (the boilers are extremely reliable but it can happen), then it is possible to separate it from the system and repair it without rushing, because at the same time the heating system continues to work. The remaining working boilers may not heat the rooms to full extent but you will not freeze; in any case, you will not need to empty the system.
Including several boilers in the heating system can be done according to a parallel scheme or on the scheme of primary-secondary rings.
When working in a parallel scheme (see Figure 63) with automatics turned off at one of the boilers, the water of the return pipeline still runs in the non-working boiler and so it overcomes the hydraulic resistance in the boiler’s contour, and the circulational pump uses electricity. Besides, the return pipeline (with cooled warmth carrier) which passes through the non-working boiler mixes with the supply water (heated warmth carrier) from the working boiler. This boiler needs to increase the heating of water in order to compensate mixing of return water from the non-working boiler. To prevent mixing of cold water from the non-working boiler with the hot water of the working boiler, you must manually close the pipelines with valves or else provide them with automatics and servodrives.
Installation of boilers on the scheme of primary-secondary rings (see Figure 64) doesn’t allow for this kind of automatics. When you turn off one of the boilers, the warmth carrier which goes in the primary ring simply doesn’t notice «the loss of a fighter». The hydraulic resistance in the area A — B where the boiler is installed is extremely small, so there is no need for the warmth carrier to go into the boiler’s contour and it goes calmly in the primary ring in such a way as if the gates are closed in the turned-off boiler, but of course there are no actual gates. In general in this scheme everything is happening the same way as in the scheme of connecting secondary heating rings, with the only difference that in this case the generators are «sitting» on the secondary rings, but the consumers of warmth are not. Practice shows that inclusion into the heating system of more than four boilers is not economically feasible.
Several typical schemes are designed by the company «Hydromontage» with usage of hydrocollectors «HydroLogo» for heating systems with two or more boilers (see Figures 65–67).
A universal scheme for any number of boilers (but not more than four) and practically an unlimited number of consumers is shown in Figure 67. In it, each of the boilers is connected to the distribution group which consists of two usual collectors or the collectors «HydroLogo» which are installed in parallel and close to the hot water supply boiler. On the collectors each ring from the boiler to the hot water heater has its common region. Small hydrocollectors of the type «Element-Micro» with miniature mixing units and circulational pumps are connected to the distribution group. The entire heating scheme from the boilers to the hydrocollectors «Element-Micro» is the usual classical heating scheme forming several (according to the number of hydrocollectors) primary rings. The secondary rings with warmth consumers are connected to the primary rings. Each of the rings which is located on the higher stage uses the lower ring as its own boiler and expansion tank — i.e. taking the warmth out of it and giving back the used water. This scheme of installation is becoming a widespread means of making «advanced» boiler rooms, both in small houses and in large buildings with a large number of heating contours, allowing the possibility of fine quality tuning of each contour.
To understand more clearly the universality of this scheme, let’s look at it in detail. What is a usual collector? On a large scale, it is a group of tees collected in one line. For example, there is one boiler in the heating scheme and the scheme is primarily for the preparation of hot water. Then the hot water coming out of the boiler is directed straight into the hot water heater, giving part of its warmth to the preparation of hot water and comes back to the boiler. Let’s add one more boiler to the scheme. Then on the pipelines of supply and return we must install two tees and connect the second boiler to them. And what if there are four boilers? Everything is easy; you simply need to install three more tees on the supply and return of the first boiler and connect the three extra boilers to these tees; or, do not install the tees into the scheme but replace them by collectors with four outlets. So we see that we connect all four boilers via the supply to one collector and to the other collector via the return. The collectors themselves are connected to the hot water heater. We have just made a heating ring with the common region using the collectors and the pipes of the boiler connection. Now we can safely disable or enable some of the boilers and the system will continue functioning; only the supply of warmth carrier will be changed.
However in our heating system not only heating of domestic water supply must be provided, but also radiator and «warm floors» heating systems. Therefore for each new heating contour, a tee must be installed on the supply and return pipelines and the number of tees must be equal to the number of heating contours. Why do we need so many tees? Isn’t it better to replace them with collectors? But we already have two collectors in the system; therefore we can simply expand them or else use collectors with enough outlets to be able to install boilers on the heating contours. Let’s buy collectors with the necessary number of outlets or assemble them from ready-made parts or use ready-made hydrocollectors. For the further expansion of the system if required we can install collectors with a larger number of outlets and temporarily cut them off with the help of ball valves or plugs. The classical collector heating scheme results, in which the supply ends with its own collector, the return with its own, and from each collector the pipes go to separate heating systems. We short circuit the collectors themselves with the boiler which, depending on the rate of turning on the circulational pump, can have hard or soft priority, or may not have priority since it is included in the chain in parallel with other heating contours.
Now it’s time to recall the heating system with primary-secondary rings. Let’s short circuit each pair of pipes coming out from the collectors of supply and return by a hydrocollector of the type «Element-Mini» (or other hydrocollectors) and we will get heating primary rings. Through pump-mixing units, we will connect the heating rings according to a primary-secondary scheme to these hydrocollectors — those rings which we consider to be necessary (radiators, warm floors, convectors) and in our required number. Notice that in the case of failure of warmth demand, even of all the secondary heating contours, the system continues to work because not only one primary ring is in it but several, according to the number of hydrocollectors. In each primary ring, the warmth carrier from the boiler (boilers) goes through the collector of the supply and from it it comes to the hydrocollector and returns to the collector of the return and to the boiler.
As it turns out, to make the heating system both with one boiler or with several and with any number of consumers is not that difficult. The main thing is to select a boiler (boilers) of the necessary capacity and choose the correct cross-section of hydrocollectors. But we have already talked about that in detail.