Design And Principle Of Operation Of A Two-Circuit Gas Heating Boiler

All questions of the organization of autonomous heating and hot water preparation are solved by the purchase of a single boiler capable of serving both systems. Without knowing the principle of operation of a two-circuit gas heating boiler, it would be illogical not only to make a purchase, but also to operate the unit. Do you agree?

We will tell you about the scheme of operation of the heating device, consider all its weaknesses and strengths. Understanding the basis of operation of the equipment, you can easily enjoy all its benefits. And if necessary, it will be possible to detect malfunctions in time, to understand and eliminate the causes of their origin.

Content of the article:

  • Design of a boiler to serve two circuits
    • Types of gas burners for two-circuit boilers
    • Types of heat exchangers of gas units
    • Automatics or boiler control unit
  • The operating principle and specifics
  • The scheme of operation of a two-circuit boiler
  • Types of double-circuit gas boilers
  • Benefits and disadvantages of double-circuit appliances
  • Conclusions and useful video on the subject

Design of a double-circuit boiler

The double-circuit gas heat generator differs from its single-circuit analogue by The double-circuit gas heat generator differs from its single-circuit counterpart in that instead of one heat exchanger it has two, called in technical terminology primary and secondary heat exchangers.

The first, i.e. the primary heat exchanger, is located directly in the combustion zone of the flame. It has the task of heating the heat carrier for the operation of the heating system. The secondary exchanger is responsible for the operation of the DHW.

Design and principle of operation of a two-circuit gas heating boiler

Stable operation of the heater is possible if all its components work smoothly. Information on the main functional units helps to understand the principle of operation

The construction of any two circuits unit includes the following standard elements:

  • Combustion chamber with burner unit;
  • heat exchangers;
  • Control and protection devices.

To understand the peculiarities of the construction of gas boilers of the two-circuit variety, let us look in detail at each of its design elements.

Types of gas burners for two-circuit boilers

The burner of the gas boiler is responsible for obtaining sufficient heat necessary for the heating and hot water supply circuit functioning. The heat energy is obtained by burning the fuel. Place the burner in the combustion chamber, in which in addition to the gas is pumped air. It is necessary for the combustion process.

Depending on the operating modes, the burners can be classified into the following types:

  • Single level burner. A unit with such a burner can operate in only two modes, \"Stop\" and \"Start\". Such boilers, despite their low efficiency and reduced service life, are popular due to their simple construction and low cost.
  • Two-level burner. A heater with such a burner can operate at full and half power. Its advantages are noticeable in warm seasons when it is not necessary to operate the appliance at full power to heat water that is not too cold.
  • Modulated burner. A smart boiler system with such a burner allows adjustment and regulation of the output. Such a boiler is characterized by high durability and efficiency, but also costs an order of magnitude higher than units with one- and two-level burners.

Burners are divided into open and closed-type constructions. In an open type burner, the air required for combustion is drawn directly from the room in which the boiler is located. To discharge the products of combustion, a chimney is required, which must ensure sufficient natural draught.

Atmospheric heating units are equipped, as a rule, with an ordinary metal chimney, the turbine ones with a coaxial chimney. Depending on the technical conditions of the room, the flue duct is placed vertically or constructed at an angle. The corner versions are led through the wall into the street or connected to a public chimney.

Design and principle of operation of a two-circuit gas heating boiler

The gas burner is the main element of the two-circuit gas boiler, it is responsible for burning the fuel and producing thermal energy in the required quantity

Turbine boilers have closed combustion chambers, in which air cannot enter spontaneously. They are safer and more reliable in operation, but more expensive and difficult to operate. Boilers with closed burners, in addition to the chimney, need a channel through which oxygen required for combustion is supplied to the chamber.

Turbine boilers are equipped with coaxial pipes, because besides smoke exhaust, they also draw in fresh air from the street. It happens that for normal operation to the closed combustion chamber is connected two coaxial chimneys. In addition the whole construction is supplemented with a pipe for air supply.

All such models of boilers are equipped with fans, providing movement of smoke, multilevel protection systems, automation. For operation of these devices and systems it is necessary to have electric power. The disadvantage is the power dependence, increasing the operation costs.

Variants of heat exchangers for gas units

If the burner is used to burn fuel to get heat, the heat exchanger provides this heat for further transfer to water. As already mentioned, the two-circuit design includes a primary and a secondary heat exchanger.

The primary heat exchanger is located directly above the burner and is a finned tube curved in the shape of a snake. The flame heats the water in the heat exchanger and propels it via a three-way valve into the heating system.

The secondary heat exchanger is a system of corrugated plates which are assembled into a single unit with two pairs of holes. Each pair of holes has its own function.

Water from the plumbing flows through one of the pairs, and the heating medium entering the heating circuit flows through the other. Such a system of plate and tube heat exchangers is called a dual system.

Design and principle of operation of a two-circuit gas heating boiler

The primary and secondary heat exchanger are combined into one system, whose correct operation is ensured by a special three-way valve

There are heaters that use a bithermal heat exchanger of complex configuration instead of a dual system. Such a heat exchanger is made of copper and consists of a pair of tubes arranged one inside the other. The heat transfer medium flows through the outer tube and the water for DHW operation flows through the inner tube.

Design and principle of operation of a two-circuit gas heating boiler

The bithermal heat exchanger of the coolant and water heater features a complex configuration, where the tube of one circuit is placed into the tube of the other circuit

Beaters with a bithermal heat exchanger are more difficult to operate, since both heat exchangers are represented by a single unit, which makes it difficult to clean it from scaling. But such heating devices are in demand since they have small overall dimensions and high speed of water heating.

Automatics or boiler control unit

The boiler automatics is responsible for safe and stable operation. It controls the water temperature in the DHW components and maintains the temperature of the heating medium in the heating lines. The gas boiler automatics prevents the operation of the heater in case of dangerous situations.

The unit interrupts or does not switch on in the following cases:

  • Lack of pressure in the gas system;
  • Lack of draft;
  • Lack or critical overheating of the heating medium.

The control unit controlling the operation of the protection and process automation devices is represented by a set of switches, microcircuits or a combination of these. In addition to providing safety and temperature control, it monitors the operation of the circulation pump and fan.

Modern gas boilers feature intelligent control, with different operating modes present in the software.

Principle of operation and specifics

Many gas equipment owners do not even think about how a dual circuit gas boiler actually works. They mistakenly believe that heating of water and heating circuit occurs simultaneously. In fact, everything does not look so rosy.

Design and principle of operation of a two-circuit gas heating boiler

Two-circuit gas boiler cannot work in two modes simultaneously, providing heating and DHW systems. This is proven by the three-way valve in its construction

In normal mode, the boiler only works continuously to heat the heating medium circulating in the system. The switching frequency and the flame intensity are controlled by a temperature sensor. The circulation pump is started at the same time as the burner if the operation of the heating system is not based on natural circulation.

Design and principle of operation of a two-circuit gas heating boiler

The space in the bottom part of the wall mounted boiler is dedicated to gas supply, cold water connection as well as DHW and heating circuit outlets

In fact, when the heating fluid reaches a set point, the sensor sends a signal to reduce burner activity. Until the temperature drops to the set point, the boiler is in passive mode. Then the automatics receives again a command from the sensor to activate the fuel valve.

Circuit Boiler Operation Diagram

The presence of the DHW system makes the two-circuit gas boiler slightly more complicated. The coolant heated by the burner, moving through the heat exchanger, provides heating to the plate heat exchanger through which the water from the water supply moves.

Design and principle of operation of a two-circuit gas heating boiler

Design scheme of a dual circuit gas boiler with both bi-thermal and two conventional heat exchangers. In the first variant it is not necessary to use two heat exchangers

The simultaneous use of the two-circuit model in heating and hot water mode is not possible. When the hot water tap is activated, thanks to the three-way thermostatic valve, the heating medium circulation through the heating lines is stopped. The boiler switches to water circulation mode with a plate heat exchanger which heats the domestic hot water.

A considerable consumption of hot water for a long time can paralyze the boiler operation with heating orientation. The problem can be solved in two ways - to provide for the installation of a more powerful heater or to include in the arrangement of the boiler indirect heating.

When actively using the DHW system, it is possible to install a double-circuit boiler with a built-in boiler. In this case the fuel consumption increases slightly due to the fact that in the pauses between the heating system operating cycles the burner energy is used to maintain the water temperature in the additional gas water heater.

A certain reserve of hot water in the built-in boiler allows the DHW system to be used without switching off the heating circuit. As a result, both systems alternate, avoiding the overheating factor and extending the life of the heat exchanger.

Design and principle of operation of a two-circuit gas heating boiler

The boiler with integrated condensing boiler not only provides a sufficient reserve volume of hot water but also helps avoid long disconnections of the heating circuit

The integrated condensing boiler enables hot water at the desired temperature, automatically provided at all times. Whereas a flow-through DHW system takes several minutes to heat water to the desired temperature.

Types of execution of gas boilers for two circuits

The features of gas equipment operation are largely determined by the version of the heating device. Modern boilers are produced in two forms-factors - floor-standing and wall-mounted.

When choosing the version it is necessary to be guided by the size of the heated area, the activity of using the DHW system. It is necessary to realize that wall-hung boilers are more compact, but have much less capacity.

Design and principle of operation of a two-circuit gas heating boiler

Wall gas double circuit boiler is characterized by compact size and modern design, but is effective only when heating small areas with moderate consumption of hot water

The choice of a wall mounted double circuit boiler can be justified if the heated area does not exceed 200 sq. m, and the total capacity of the hot water system does not exceed 14 l/min.

Small size of the wall hung boiler, though it seems to be an advantage, actually hides many disadvantages. Compactness is achieved by using thinner heat exchanger tubes. In addition to the fact that they have a shorter service life, there is a probability of their clogging.

In floor installations are used more massive and reliable cast iron heat exchangers. This not only increases the degree of reliability of the heater, but also prolongs its service life.

Benefits and disadvantages of two-circuit appliances

The advantages of a two-circuit heating unit are:

  • Economic fuel consumption. The direction of comparison is to use a two-circuit boiler or a single-circuit boiler with an indirect boiler.
  • Compact size.The vast majority of two-circuit boilers are wall mounted heaters. They can be easily placed both in the utility rooms and in a small kitchen.
  • Universal.No need to buy additional equipment and to solve the problems of compatibility with the boiler.

The flow heater, the heater and the circulating pump are already successfully combined into one automated system in one unit.

Obviously, along with its advantages, there are also disadvantages:

  • The simultaneous operation of heating and DHW circuit.This makes the considerable consumption of hot water likely to cause a temperature drop in the house.
  • Limits the wall mounted models.Compact wall mounted boilers are not able to provide the required temperature regime at maximum head due to the minimum burner dimensions. A similar disadvantage can be seen in the case of remote water intake points.
  • Sensitivity to water quality.The secondary plate heat exchanger is very particular about the water quality of the consumed water. The presence of impurities becomes the reason for using the means to reduce water hardness and for cleaning the heat carrier.

Another criterion for evaluation of the two-circuit boiler is its cost. The price of a two-circuit heater is higher than that of its single-circuit analogue.

But if you consider the availability of DHW system and ways of solving the issue in case of a single-circuit boiler installation, the price of a two-circuit heater will be lower if the indirect heater is included in the scheme of assembly.

Conclusions and useful video on the topic

The design components and principle of operation of gas heating equipment will introduce the following roller:

Bundling of a two-circuit gas boiler will be presented by the author of the video:

A detailed introduction to the features and principle of operation of two-circuit gas units makes it possible to determine the advantages of their operation. The purchase of such heaters will help to save on the purchase of additional equipment needed to organize the DHW system.

When one of the circuits breaks down, it is possible to operate the other, and replacing the circuit will always be cheaper than repairing a separate heating unit. Two-circuit boiler can also be used in the warm season, exploiting it only in the mode of water heating for household needs, which is convenient and economical in comparison with the purchase of separate units.

Tell us about how you chose a two-circuit gas boiler for your own house/apartment/ cottage. What was the decisive criterion in your choice? Please share useful information on the subject, photos in the block below, ask questions.

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