The difference between a single and two-circuit boiler

What are the differences

Single-circuit and two-circuit boilers have common elements. But if the piping or gas burners are almost the same, the heat exchanger is different in design.

In single-circuit devices a simple type, representing a pipe, is installed. In two-circuit models are often used thermal systems. That is, the pipe is placed in another pipe with a larger diameter. This makes it possible to create 2 circuits needed to perform the functions.

difference, single, two-circuit, boiler

Three-way valve is considered to be different. Basically, they are installed only in two-circuit models. Three-way valves are designed to change the direction of hot water. Therefore, when using the liquid for domestic needs and stops heating the room. The valve closes the supply of coolant to the radiators.

Design differences between single and dual circuit units

What is the difference between the design of units with 1 circuit and with 2 circuits? If we talk about the basic units, the basic principle of operation is the same in both types of equipment:

  • piping through which water from the well (water supply) is supplied;
  • expansion tank;
  • flue or coaxial tube and burner.

One of the main things that distinguishes a single-circuit gas boiler from a 2-circuit boiler is the design of the heat exchanger.

If the 1-circuit is a pipe through which by exchanging heat from the gas burner is heating water, the 2-circuit is a dual heat exchanger of pipes and plates or a bithermal (to be clearer this pipe inside the pipe).

However, the key difference is the 3-way valve, which is responsible for the direction of water. When the heating circuit is heated, the valve is in the normal position. However, when you open the tap of the DHW, due to the pressure drop, the automatic device triggers. A valve cuts off the hot water supply to the heating circuit. Then the coolant begins to circulate inside the boiler. At this point, the room is no longer heated.

Differences of boilers by main parameters

The difference between the boiler with 1 circuit from its counterpart with 2 circuits in operation can be traced to the following range of characteristics:

Single-circuit boiler is designed initially only for heating and can not supply the house with hot water without a boiler, while the two-circuit can do both in the basic set. It does not need to install additional equipment.

But the unit with 2 heat exchangers gives priority to hot water supply. When the taps are turned on, the supply to the radiators is shut off. This means that if water is consumed for a long time during the heating period, the house can get noticeably colder. Only installation of a storage boiler can save the situation.

2-circuit unit does not allow for an even distribution of hot water at the same time intake from two or more taps. For example, if a kitchen sink faucet is opened while showering, the water temperature from the shower head will quickly drop, and the user will be in discomfort.

Avoid this without installing a storage tank allows only the original choice of a more powerful unit. However, if the boiler capacity will be significantly higher than the required value for the area of a particular room, the equipment will quickly fall into disrepair. because of the frequent switching on and off.

In this case, a single-circuit boiler is better than a two-circuit or a popular gas boiler, also working on a flow type, as it allows you to select and connect the appropriate under the conditions of the boiler wall or floor installation. However, as the volume of the boiler increases, so do the requirements for installation. So if the calculation gives a value over 200 liters, it is better to choose a model with two coils.

2-circuit models are already equipped with all the necessary devices for efficient operation. 3-way mechanism, pump, membrane tank, fuse. And everything is in one unit. Therefore, to operate it is only required to install the device and connect the gas.

1-circuit models require all necessary equipment to be installed separately. In addition to the complexity of connecting the components, space must be taken into account. For this reason, high-capacity units require a separate room.

The 2-circuit models are the most compact. Everything you need for efficient operation is located in the boiler body. For it remains only to choose a place for installation in accordance with the technical requirements. 1-circuit models equipped with a boiler need more space, and therefore may require additional costs for the arrangement of a separate room.

With the average area of the heated space of 70-140 m 2 and using only one crane optimum power of the 2-circuit unit is 17-19 kW. With the simultaneous use of 2 taps, the figure should increase to 23-29 kW. If you need instantaneous hot water intake from 3 or more taps at once, it is better to install a single-circuit boiler with a boiler for indirect heating.

Two-circuit boiler

Pros of

  • The ability of the boiler to operate on two circuits completely solves the problem of heating and DHW in the home without the purchase of additional equipment.
  • Complete set allows you to place the device more rationally in the building. Wall-mounted models are mainly mounted in kitchens, which makes it possible to install them in apartments.
  • Convenient operation. Hot water is prepared by flowing method, as required. Therefore, it is not necessary to run the boiler at a higher mode specifically for heating the liquid in the boiler. This indirectly affects the fuel economy.


  • The device is more complicated than their single-circuit counterparts.
  • structural elements (a second heat exchanger, three-way valve) increases the risk of breakage of boiler equipment.
  • Inertia. Hot water will not come out of the faucet immediately. you will have to wait until its temperature rises to the set value. And in the meantime, the HTW meter will “overcharge”.
  • Special requirements for the heating medium. If it is of poor quality, the internal surfaces of the secondary heat exchanger quickly become contaminated with salt deposits.

Descaling cleaning gives a short-term effect, in addition, the degree of hot water heating gradually decreases. If the heat exchanger is bimetallic, the experts do not recommend its maintenance. the costs are not justified by expectations. That is why the device is immediately replaced, and it costs quite expensive:

  • Instability of operation. With simultaneous water withdrawal from several points, pressure jumps and temperature drops are possible.
  • The price is higher than for single-circuit models. But this is true only for the boiler, without taking into account the cost of the boiler and other equipment.

Single-circuit or double-circuit heating boilers. Comparison and choice

Owners of private houses have the eternal dilemma: what is better: single-circuit boiler or double-circuit? To understand this, you need to remember what a circuit is. In the world of gas boilers, there are only 2 circuits. All that transmits heat from the boiler to the house is a heating circuit. These can be radiators, underfloor heating systems or baseboard heating. Regardless of how many circuits you have in your heating field, it will still only be one heating circuit. In turn, anything that brings hot water or cold water into the house is considered a hot water circuit.

So we determined that there can only be two circuits. If the boiler inside itself can heat both the heating fluid and hot water, the boiler is called double-circuit, that is, there are two circuits inside the boiler. At first glance it may seem that such a boiler is ideal for our home. Because inside one unit we get both heating and hot water preparation. In addition, as a rule, two-circuit gas boilers are cheaper, but we will not jump to conclusions. First you need to understand the inner workings of the two-circuit boiler.

Single-circuit or two-circuit gas boiler

Single-circuit or two-circuit: what gas boiler to choose and why?

Let’s look at the main differences and summarize.

Before choosing a gas boiler, it is worth getting acquainted with the options for heating water for household needs. Water can be heated in the gas boiler, or heat the tank, and then distribute to consumers.You can not apply one device on two types of heating, this is why there are two types of boiler: single-circuit. works for heating and two-circuit. heating of d.h.w.

Diagram Connection Dual Circuit and single-circuit boiler with a boiler indirect heating.

Single-circuit gas boiler is designed for space heating, hot water supply is an option here, it can only be used jointly with a boiler of indirect heating.

Two-circuit gas boiler carries out the heating of the liquid in the device itself.How it happens?The boiler has two circuits, for switching between which the three-way valve is responsible. This is essentially the same single-circuit system with a boiler, only in a reduced form, but the principle of heating will be the same.The cooled water from the radiator enters the heat exchanger located above the gas burner, is heated and moves on its heating circuit until the tap opens in the bathroom. Three-way changes its position immediately and the water instead of the radiator enters the secondary plate heat exchanger, where through the thin walls, heat exchange with water from the well takes place. Having given the heat back to the main heat exchanger for heating and will continue to do so until the tap is closed.The role of plate heat exchanger in single-circuit models performs indirect heating boiler with a built-in coil.

A huge problem with dual-circuit boilers occurs when using two taps at the same time, such as in the kitchen and bathroom. If you look at the forums or read reviews, through one will be the phrase “contrast shower”. Unfortunately such a problem is valid and it is due to the small flow. If you still dare to buy such a device, you should know that the minimum capacity of a gas boiler for cooking hot bath must be at least 18 kW. If the shower, you will need a device not less than 16 kW in power. 28 kW works more confidently with two water points, but when used in small rooms because of the frequent switching on and off the service life will be reduced.

In a system with a single-circuit boiler problem with a lack of hot water is solved by replacing the water heater.If you live 4 people and a boiler of 80 liters is not enough for you, change it to 140 or 200 liters. If 200 liters was not enough, in this case, buy a water heater with two coils and the problem will be solved. The main thing to consider when choosing the capacity of the device and the coil. All about boilers can be read in the article “Selecting a boiler indirect heating, a comparison, prices”. In this article you will find a lot of useful information about popular water heaters in 2022.

In two-contour built-in heat exchanger, and single-circuit it will need to buy in the form of a boiler indirectly heated and additionally connect their temperature sensor.This is the main difference, the cost of such a boiler is not small. Some models can go up to the cost of the boiler.

The size of the boilers will be the same. But occupy space in conjunction with additional devices will be different.

In the absence of space for the boiler room is a popular place to install two-circuit models kitchen. Not only compactness is important, but also aesthetic appearance. With two devices will be more complicated, for such a system requires a separate space. There are several options for installing the tank:

So which is better, a single-circuit or two-circuit boiler?

Single-circuit boiler is good, there is no water stoppage, no contrast shower, you can perform recirculation (the constant movement of hot water along the circuit). But these boilers are more expensive, the installation is more complex, just a barrel costs from 1000. this is like half the cost of the boiler. It is not always possible when building a house to lay extra in the budget, as a rule, all trying to save money. But remember, you will be saving money on yourself.

While heat exchangers are clean and not scale will be stable and work with two loops, but in a few years these devices can have problems, especially if the annual maintenance is not carried out.Two-circuit, of course, are more affordable, installation is not particularly difficult. And if you do not have high expectations for the flow, these boilers perfectly cope with two tasks at once.In conclusion, we have compiled a rating of two-circuit boilers for the year 2022.

What is an indirect heating boiler

Today, more and more people resort to using indirect heating boilers. This useful device allows you to use the heat of the gas boiler more efficiently. In winter, when the heating system is on, the indirect heating boiler works from this system, and in summer, when the boiler does not work for heating, such a boiler works from an electric heating element or the boiler is switched to summer mode (if the settings allow).

Heating water in an indirect heating boiler is not a direct way, but as if by. Inside such a boiler is a fairly large coil, which circulates coolant from the heating system. Thus, the gas boiler heats the water in the indirect coil, which by means of a pump circulates in the heating system, and already the heat exchanger heats the water in the boiler. This heating of water for DHW line is recognized the most cost-effective.

The casing of this boiler is mostly made of steel sheet, while the inside is made of stainless steel. Between the external and internal parts of the boiler indirect heating is a special material that almost does not conduct heat. The design of such a boiler is very similar to that of a thermos.

As a rule, the indirect heating boiler is installed near a heating boiler. Through the boiler heated coolant must pass in the first place. Water in such boiler will always be hot, and to this boiler it is possible to connect simultaneously some points of selection of hot water. Heating of hot water in a boiler of indirect heating is carried out without prejudice to the heating system.

In most cases, such a boiler is equipped with a highly efficient heat exchanger, which is made of special steel. expensive indirect heating boilers are equipped with heat exchangers that are made of brass. Boiler heat exchanger has a long spiral shape, which allows you to quickly heat water to a temperature that currently has the water in the heating system.

How to choose a boiler of appropriate power

If the choice in favor of the one-or two-circuit boiler is already made, it is necessary to select the appropriate boiler capacity. In order that the temperature in the rooms was comfortable and hot water temperature of about 50 degrees Celsius, you can make a simple calculation of capacity of the boiler to be installed. To do this, sum up the area of all heated rooms and divide by 10. This is an approximate load for heating. Increasing it by about 15-20%, we have the total capacity of the boiler including hot water supply. However, as professionals advise, the boiler should be purchased with a slightly higher capacity than that obtained by calculation. The point is that over time, the productivity of the equipment decreases, reduces the heat transfer from fuel to heat transfer medium, due to contamination of heating surfaces. In addition, when selecting equipment, you should consider the value of heat loss from the pipes, possible voltage drop.

What is the difference between a two-circuit boiler and a single-circuit

Gas heating equipment can be classified according to the following features:

  • The number of heating circuits provided;
  • The thermal power that the unit can produce;
  • equipment level;
  • Purpose. household or industrial appliances.

All these units have one thing in common. they work by burning natural gas.

Hot water supply and heating

Systems in which only one heat exchanger, designed to connect the lines of radiators, is called a single-circuit. This type of products has a mechanically simple and reliable design.

CENTRAL HEATING SYSTEMS EXPLAINED. S Plan, Y Plan, One pipe, Two Pipe Underfloor Heating

Two-circuit units are designed in addition to heating to perform another task. to provide the house with hot water. If inside there is only one heat exchanger, redistribution of coolant flow between the circuits is achieved by operating a three-way valve. The priority here is the heating of the liquid, which is used for domestic needs. Systems with two heat exchangers allow each of them to connect their own circuit.

System dimensions and capacity

The more powerful the device, the greater will be its linear dimensions. Thermal output of single-circuit units are chosen based on the area of the building, which is equipped with this type of heating. Two-circuit heaters have the peculiarity of poor heat output in the case of hot water extraction from two points. Therefore, to avoid the appearance of cold water in the hot faucet at the time of opening the second, it is necessary:

  • Install a boiler with a capacity greater than 16 kilowatts, if the house has a shower room;
  • to use units with a heat output of at least 18 kilowatts for apartments with bathrooms;
  • To use boilers with the power not less than 28 kilowatts for the apartments with two or more hot water taps.

The area of space for 130-160 squares suggests that the two-circuit or single-circuit gas boiler can often be turned on, risking rapid failure of heat exchangers.

Complete set

A complete dual-circuit gas heater involves:

  • valves to protect against excessive pressure;
  • three-way valve;
  • of the circulating pump;
  • expansion tank of membrane type.

Some manufacturers additionally equip boilers with heating and hot water filters.

Single-circuit type units are generally not supplied with associated equipment.

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