Dual circuit boiler operation for heating only

Working principle

Two-circuit gas boiler can operate in two modes of heating and water heating. The boiler itself has two loops, through which the coolant flows. One of them with a primary heat exchanger works for heating, the second with a plate heat exchanger for DHW treatment. Switching is performed by three-way valve.

The exact mode of operation of two-circuit boiler laid down by the manufacturer, but usually the mechanism of operation for heating is as follows:

  • When the temperature in the room drops below a certain level, the automation sends a signal to turn on the circulation pump. The coolant begins to move, creating a vacuum in the return pipe. Simultaneously turns on the burner.
  • Flowing through the heat exchanger, the coolant is heated, distributing heat through the system.
  • Bypassing the radiators, it cools down and returns to the boiler inlet with a lower temperature.
  • Heats up again as it passes through the heat exchanger.
  • When reaching the set temperature (there is control by the coolant temperature, there is a temperature of the room) the burner is off, the coolant still circulates for some time, driven by the circulating pump (post-circulation mode). When the heat exchanger temperature drops to a certain level, the circulation pump stops. The boiler does not work as long as the temperature is within the limit. As soon as it becomes 1°C below the set limit, the boiler turns on again.

With some variations this algorithm is repeated in different boilers. When heating domestic hot water, the process is similar, but the signal to turn on the burner is the water flow in the circuit. That is, you open the hot water tap, the burner ignites. Only in this operating mode does the three-way valve switch and close the heating medium inside the boiler. The secondary heat exchanger is heated by the hot fluid and the flowing water is heated by the secondary heat exchanger. Heating stops when the water overheats (when the threshold value is reached) or after the tap is closed. The burner goes out, the circulation pump works until the heat exchanger cools, then it shuts off.

Can a two-circuit boiler be used as a single-circuit boiler

During the installation of heating in a private home, many users face the question of choosing the boiler. Some property owners buy a unit for two-circuit heating, and then try to understand whether this device can be used as a single-circuit boiler. In our article we will try to answer this question in detail.

Pay attention! The main difference between the two-circuit equipment and the device for single-circuit heating is the ability to heat the coolant for the heating system and heating fluid for hot water.

Water in the cottage or cottage can also be heated when using a single-circuit solid fuel or pellet boiler. In this case, the boiler is connected to the pipeline in the immediate vicinity of the heating unit. In today’s construction market of two-circuit models, only gas units are found, so the problem with the choice of device exists only for those people who plan to use gas for heating a one-story or two-story country house. Below we will get acquainted with the advantages and disadvantages of each model in more detail.

Can a two-circuit boiler be used as a single-circuit boiler for hot water

During the installation of heating of a private house, many users face the question of choosing a boiler. Some property owners buy a unit for two-circuit heating, and then try to figure out whether this device can be used as a single-circuit boiler. In our article we will try to answer this question in detail.

Please note! The main difference between the two-circuit equipment and the device for single-circuit heating is the ability to heat the coolant for the heating system and heating fluid for hot water.

Water in the cottage or cottage can also be heated by using a single-circuit solid fuel or pellet boiler. In this case, the boiler is connected to the pipeline in the immediate vicinity of the heating unit. In today’s construction market of two-circuit models there are only gas units, so the problem with the choice of device is only for those people who plan to use gas for heating a one-story or two-story country house. Next, let’s get acquainted with the advantages and disadvantages of each model in more detail.

Single-circuit boiler

Such boiler equipment is designed to heat the heating medium. Further transport of the hot heating medium occurs naturally when the water expands during heating. After the installation of a circulating pump in the heating system there is a more even heating of the living rooms. Pumping equipment creates pressure and directs the flow of the coolant to the radiators. Here, the liquid gives up some of the heat, is cooled and returns to the heating device via the return pipe.

The main advantages of liquid fuel heating boilers are:

  • Comparative cheapness in relation to the two-circuit units, but to heat water for the needs of consumers buy an additional water heater, boiler;
  • Easy installation of autonomous heating of a private home;
  • Single-circuit models of solid fuel heating boilers for heating a private home heat the entire volume of water in the boiler, so the user at the opening of the faucet immediately receives hot water.

Important! The liquid in the water heater gradually cools over time, it must be periodically heated, which is the main cause of heat loss.

Features of double-circuit boilers models

Unlike a single-circuit boiler in the specified unit there is an additional circuit for hot water heating. The heat exchanger of the unit from the water supply system is fed cold water, then the liquid is heated and pumped through the booster pump to the consumer who opened the faucet in the kitchen or bathroom.

The three-way valve is an important element of the equipment under consideration. This device is connected to the boiler automation, its main task is to supply coolant to the desired circuit. Pay attention to the fact that the heating system does not work with the turned on taps. In this case, preference is given to DHW, this property is commonly referred to as the priority of hot water.

The main advantages of the two-circuit boiler are considered:

  • Small cost compared to buying a set of boiler and single-circuit boiler;
  • compact dimensions;
  • the equipment heats the required amount of water, but not the entire volume.

DHW with a flow heater does not use a circulating pump. If the faucet is far away from the boiler, the user will have to wait some time until hot water is available. In this case, the temperature of the fluid may vary at the same time turning on the second faucet or changing the water flow.

Each boiler model has its own specifics, but any unit should be installed by an experienced master plumber. In general, a two-circuit boiler can be used as a single-circuit. If you have any questions about the proper connection or installation of heating equipment, please leave a request on our website or call us at

Standards for the installation of a gas boiler

Gas is still the cheapest type of fuel. Accordingly, the cheapest heating is obtained exactly on the natural gas. However, the installation of gas boiler associated with certain complexities of the premises must comply with fire safety standards.

To install powerful gas boilers requires a separate room

Combi Boiler with Two Heating Zones, 230V Switching

Additional recommendations on installation

You can not categorically reduce the diameter of the inner cross section of the pipes to be connected. The optimum value for a boiler to 28 kW is not less than 20 mm for heating and not less than 15 mm for water supply.

If the boiler is not included in the scheme, it is better to install the device close to the hot water tap, which is often used.

In the case of connecting the underfloor heating, the system is supplemented by a separate branch with a manifold and circulation pump.

The scheme of a two-circuit boiler

The presence of DHW system complicates the scheme of work of the gas boiler of two-circuit type. The coolant heated by the burner moving through the heat exchanger heats the plate heat exchanger through which the tap water flows.

Simultaneous use of a two-circuit model in the heating and hot water supply is impossible. When the hot water tap is activated due to the three-way thermostatic valve the heating medium circulation through the heating system is stopped. Boiler starts moving water through the loop with a plate heat exchanger, which heats water for domestic needs.

At considerable consumption of hot water for a long time the boiler operation focusing on heating can be paralyzed. The problem can be solved in two ways. to provide for the installation of a more powerful heater or include in the scheme of the arrangement of the boiler of indirect heating.

When actively using the DHW system is possible to install a dual boiler with a built-in boiler. In this case the fuel consumption slightly increases due to the fact that in the pause between work cycles of the heating system the burner energy is used to maintain the water temperature in an additional gas water heater.

A certain amount of hot water in the built-in boiler allows the domestic hot water system to be used without shutting off the heating circuit. As a result, both systems operate alternately, there is no overheating of the liquid and the life expectancy of the heat exchanger is extended.

Built-in standard boiler allows you to get hot water at the desired temperature at any time, the stock of which is provided in automatic mode. Whereas a flow-through DHW system requires a few minutes to heat water to the desired temperature.

What are the heat exchangers of gas boilers

The heat exchanger in the gas boiler is responsible for obtaining heat in order to transfer it to the water. If we are talking about a two-circuit boiler and its principle of operation, there is a primary and a secondary heat exchanger. The first one is placed above the burner and is represented by a tube with ribs, curved in a snake. Water heated to the desired temperature in the heat exchanger advances in the three-way valve, and then enters the heating system. The secondary heat exchanger is presented by the whole system of the plates bent in waves, all of them are combined into a single unit, which also has 4 holes. Two of them carry out the flow of water, two more are responsible for the movement of the coolant, which is fed into the heating circuit.

The system of two heat exchangers is called a double one. On the market, there are heating appliances that use a bithermal heat exchanger. It is characterized by a complex configuration. Copper is used for its manufacture, the element itself is represented by tubes arranged one inside the other: the outer one carries the heat carrier, the inner one serves for water movement, with the help of which hot water supply is provided.

Gas boilers equipped with such heat exchangers are difficult to operate, the latter is difficult to remove limescale. However, such heating devices are in demand, because they are small in size and very quickly heat water.

Use an expansion tank

In winter, when the boiler has a heating circuit, any hot water connection causes the heating circuit to turn off so that the hot water circuit can turn on. In summer, when the boiler is not involved in the heating circuit, each time you turn on the hot water causes the inclusion of a gas boiler.

Often the user opens the hot water tap just to rinse his hands. The boiler turns on or switches on, hot water runs through the pipes. But the user may simply not wait for it, washing their hands with cold water.

Meanwhile, the frequent and useless switching and switching of the boiler “eats up” its service life. Possible way out. installation of a small expansion vessel. It is installed in front of the water heater. With such a tank, for the first time after turning on the faucet hot water flows into the pipe solely from the water heater due to increased pressure in the tank. Thus, in the case of short-term use of hot water, there is no need to turn on and switch the boiler.

It is recommended to use a tank designed for hot water supply. Using a tank designed for heating, will lead to odor, because its membrane is made of rubber.

Design of the structure

The design of the device includes the following elements:

  • Burner. Is the main module of the heating boiler. The place of its placement is the combustion chamber. The task of the burner is to heat the coolant and release heat energy for the DHW circuit. To maintain the exact temperature required, the element is equipped with an automatic combustion control system.
  • combustion chamber. Burner location. Available in open or closed type. Enclosed versions are equipped with a fan that produces air flow and smoke evacuation. It achieves the effect of silent operation of the equipment.
  • Circulation pump. Is responsible for the creation of forced circulation of the coolant within the heating pipes and for the efficiency of the DHW. In contrast to a fan, the pump barely makes any noise at all when running.
  • Three-way valve. Thanks to this device the boiler has the ability to switch to a function of heating water for hot water supply.
  • Main heat exchanger. The design of double wall gas boiler provides for its location over the burner, inside the combustion chamber. This provides heating of the water that subsequently enters the heating pipes or DHW system.
  • Secondary heat exchanger. Responsible for heating water for DHW.
  • Automatic devices. Provide control equipment operation parameters, watching the level of coolant and water heating. This makes it possible to make adjustments to the burner, control various components, maintain the flame, record malfunctions, etc.

The space at the bottom of the casing is intended for the installation of heating circuit switching outlets, hot and cold pipes and the gas supply. Some modifications of gas two-circuit boilers are equipped with dual heat exchangers: the principle of operation of a two-circuit boiler in this case does not change.

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