Double-circuit gas boiler: types, principle of operation, choice

To date, gas heating is still the cheapest. Therefore, if there is a main gas pipeline and technical capabilities nearby, it makes sense to install a double-circuit gas boiler. Why dual circuit? Because one device will provide both heat and hot water. 

When choosing a double-circuit boiler, be sure to pay attention to several parameters:

• installation method – floor wall;
• power;
• type of combustion chamber (open, closed);
• type of heat exchanger and the material from which it is made;
• set of service functions.

There are many other points, but these are the main ones. Without them, it is impossible to choose a double-circuit gas boiler, and we will talk about them further. And first of all, let’s get acquainted with the structure of this equipment and the principle of its operation. Then all the stages of choosing a gas boiler for a house, apartment or cottage will be clear.

Structure and main differences

The gas boiler consists of three main modules – a burner, a heat exchanger, and control automation. The burner is located in the combustion chamber, a heat exchanger is located above it, in which the coolant is heated. The whole process is controlled by automation. It provides security and changes the modes of operation of the equipment.

Heat exchanger types

A double-circuit boiler is distinguished by the fact that it can heat both water for heating and for water supply. This must be done separately, because heat exchangers need special ones. They are of two types:

• Dual heat exchanger. Consists of two separate modules – primary and lamellar. In the primary, the coolant from the heating system is heated, in the secondary – lamellar – water for domestic needs. The primary heat exchanger is a tube with fins, the secondary is a set of plates. They are located in different parts of the boiler – at the top – primary, lamellar at the bottom, but they are interconnected because they are read as a single piece.
• Bithermic heat exchanger. It consists of two metal tubes of different diameters, inserted one into the other. In the inner tube, water is heated for hot water supply, in the outer tube – for the heating system.

More reliable is a system with a dual heat exchanger. Since heating is a closed system and the coolant circulates in a circle, little scale is formed. When heating water for hot water supply, the situation is reversed – running water is heated, which means that there is a lot of scale. This part of the heat exchanger periodically requires repair or replacement. If in a dual heat exchanger it is possible to replace only the part that heats water for domestic needs, then in a bithermic separation it is not provided, you have to change the entire device, and this is much more expensive. There is one more thing: a double-circuit gas boiler with a dual heat exchanger, while working normally for heating, with a bithermic situation is different – it does not work at all.

Material of heat exchangers

The choice of a double-circuit gas boiler can also be influenced by the material from which the heat exchanger is made. It could be:

• Stainless steel. A good option for durability, but it has a rather low heat transfer, which reduces the efficiency of the boiler. However, the price of such a heat exchanger is high.
• Cink Steel. This is the cheapest type of heat exchangers, but also the most short-lived.
• Copper. An excellent option for heat transfer (boilers have high efficiency) and durability, but the price is high. But this scares few people – it outweighs the ability to pay less for gas during operation. It is for this reason that most good double-circuit gas boilers are equipped with a copper heat exchanger.
  

Choosing a double-circuit gas boiler for this parameter is not so difficult. Copper seems to be the best option. Not without drawbacks – high chemical activity and low melting point – but they have long been learned to compensate. The automation of the boiler ensures that there is no overheating. Chemical activity is neutralized by the use of chemically neutral materials in the heating system – polymer pipes are used – polypropylene or cross-linked polyethylene.

Types of burners for gas boilers

Atmospheric gas burners are installed in wall-mounted double-circuit gas boilers. According to the method of controlling the flame, they are:

• Single stage. The burner either burns or it doesn’t. There are no intermediate provisions. To maintain the required heating mode, the burner either lights up or goes out. Not the best algorithm.
• Two-stage. There are two modes of operation – at 100% power and at 50-60%. Automation itself regulates the required power. Such burners are more economical, allow you to maintain the temperature more accurately. By reducing the number of on / off.
  

• Modulated. The most expensive type of burners, but also the most economical. It is controlled by automation, the power is set to the one that is needed at a given time from 10% to 100%. A double-circuit gas boiler with such a burner has a minimum of on / off cycles, which has a positive effect on durability. In addition, fuel consumption under equal conditions is 10-15% less.

If we talk about the optimal choice, then these are modulating burners. They allow not only to maintain the heating temperature accurately, but also heat the water exactly to the specified parameters. If you want to choose an economical double-circuit gas boiler, it must have a modulating burner.

Automation

Automation in gas boilers is a must – it ensures safety, switches operating modes. There are three main parameters that are constantly monitored:

• the presence of draft in the chimney;
• gas pressure;
• flame control.

These are key points that simply need to be controlled. If at least one of these parameters is not normal, the boiler does not turn on. In addition, there is an extensive list of additional functions:

• Maintaining a stable temperature. There are two types of automation: one controls the temperature of the coolant, the second – the air in the room. More comfortable conditions are created when the air is controlled – the weather changes, but regardless of this, the set mode is maintained in the room.
  

• post-circulation mode. After the burner is switched off, the heat exchanger has a very high temperature. If the circulation pump is turned off immediately, the water in the heat exchanger may boil, which will lead to its failure. To avoid such a situation, the pump drives the coolant for some more time and turns off only when the temperature becomes acceptable.
• Antifrost. If a double-circuit gas boiler is located in a country house where you live only periodically, you can set the economy mode. The boiler will maintain a stable temperature of +5°C. The same function can be used at the time of departure.
• Summer mode. In summer, the heating does not turn on, the boiler works only for heating water. To prevent stagnation in the system at this time, so that the elements do not clog, the automation once a day for a while turns on the circulation of the coolant.

These are the functions that are often encountered, but there are also specific ones: the ability to connect (and control) solar panels, underfloor heating systems. There is a weather-dependent automatic. In this case, there are remote sensors that are installed on the street. According to their data, the operation of the boiler is corrected.

All these functions are embedded in the microprocessor, which manages everything. The average user of all automation is only faced with a remote thermostat, which can be installed in any room and according to the readings of which the temperature can be regulated (another additional option). Basically, all interaction with the boiler and its automation is limited to a small panel. All necessary information is displayed on the screen. There are also buttons with which you change modes, set the temperature.

Principle of operation

A double-circuit gas boiler can operate in two modes – heating and water heating. In the boiler itself there are two circuits along which the coolant moves. One of them – with a primary heat exchanger – works for heating, the second – with a plate heat exchanger – for preparing DHW water. Switching takes place using a three-way valve.

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

• When the room temperature 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 pipeline. The burner turns on at the same time.
• Flowing through the heat exchanger, the coolant heats up, spreading heat throughout the system.
• Running around the radiators, it cools down and returns to the boiler inlet with a lower temperature.
• It heats up again, passing through the heat exchanger.
• When the set temperature is reached (there is control by the temperature of the coolant, there is control by the temperature of the room), the burner turns off, the coolant circulates for some time, driven by the circulation pump (post-circulation mode). When the heat exchanger temperature drops to a certain level, the circulation pump stops. The boiler does not work until the temperature is within the normal range. As soon as it falls 1°C below the set limit, the boiler turns on again.
  

With some variations, this operation algorithm is repeated in different boilers. When heating water for domestic needs, everything happens in much the same way, only the signal to turn on the burner is the appearance of water flow in the circuit. That is, you open the hot water tap, the burner lights up. Only in this mode of operation does the three-way valve switch and close the coolant inside the boiler. The secondary heat exchanger is heated from the hot coolant, and running water is heated from it. 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 down, then it turns off.

Installation method

According to the installation method, gas boilers are floor and wall. Wall-mounted – compact installations, the size of a small kitchen cabinet. They do not require the equipment of a separate room, they can be installed in the kitchen or in another suitable room. The maximum power of a wall-mounted gas boiler is 30-35 kW. This is usually enough to heat rooms with a total area of ​​250-350 square meters. m.

Floor gas boilers are more powerful, respectively, have large dimensions and weight. There are models that can stand in a residential area, others require a dedicated room – a boiler room. Installation requirements are prescribed by each manufacturer separately, but usually the distance from the top of the boiler to the ceiling, the volume of the room and the presence of ventilation are specified.

Regardless of the type, a certified project is required for the installation of gas boilers. A gas meter is necessarily present in the circuit, so if it is not there, it will need to be purchased. Connection work must be carried out by a company licensed for this type of activity. Only in this case the boiler will be put into operation.

Choosing a double-circuit gas boiler according to the installation method is simple. If there is enough power, they usually take a wall-mounted version, if not, a floor-mounted one.

Combustion chamber type

The gas burner is located in the combustion chamber. There are two types of it – open (atmospheric) and closed (with a turbine, forced). A double-circuit gas boiler with an open combustion chamber can only be installed in rooms with good ventilation. During operation, the oxygen necessary for combustion is taken from the room, and the products of combustion are discharged into the chimney with good draft. Therefore, a good air flow and a properly functioning exhaust ventilation duct are necessary.

Gas boilers with a closed combustion chamber are distinguished by the fact that at the outlet of the chamber there is a coaxial chimney (pipe in a pipe) equipped with a fan. They take the chimney out into the street, you can – into the wall near the boiler. Air is taken from the street through one pipe, combustion products are removed through the second, their movement is provided by a fan-turbine.

Which combustion chamber is better? A boiler with a closed combustion chamber works more stable – air enters directly into the combustion zone. But there is a minus in this: with a side wind, the air flow can be so strong that it blows out the burner, the boiler turns off. The second disadvantage of such a solution is freezing and ice formation in winter. Well, the third drawback is that such a boiler only works if there is electricity – it turns off without a turbine. Well, another small minus — the turbine is not silent. It is almost inaudible, but it is “almost”. Apparently for these reasons, if possible (serviceable ventilation duct), boilers with an open combustion chamber are installed. After all, it is much easier to ensure a constant flow of air.

Boiler power

One of the key points in choosing a heating boiler is determining the required power. If we approach this with full responsibility, it is necessary to consider the heat loss of each room, if we are talking about an apartment or a building as a whole, if the boiler is selected for heating a private house. The calculations take into account the materials of the walls, their thickness, the area of ​​windows and doors, the degree of their insulation, the presence / absence of an unheated room below / above, the type of roof and roofing material. Geographic location and a whole bunch of other factors are taken into account.

Such a calculation can be ordered from a specialized organization (at least in GorGaz or a design bureau), if desired, you can master it yourself, or you can take the path of least resistance – calculate based on average norms.

Based on the results of all calculations, the norm was derived: 10 kW of heating power is required for heating 1 square meters of area. This standard is suitable for rooms with ceilings of 2,5 m, with walls with an average degree of thermal insulation. If your room falls into this category, divide the total area that needs to be heated by 10. You get the required boiler output. Then you can make adjustments – increase or decrease the resulting figure, depending on the actual conditions. It is necessary to increase the power of the heating boiler in the following cases:

• The walls are made of a material with high thermal conductivity and are not insulated. Brick, concrete fall into this category for sure, the rest – according to the circumstances. If you are choosing a boiler for an apartment, you need to add power if the apartment is corner. For “internal” heat loss through them is not so terrible.
• Windows have a large area and do not provide tightness (old wooden frames).
• If the ceilings in the room are higher than 2,7 m.
• If in a private house the attic is not heated and poorly insulated.
• If the apartment is on the first or last floor.

The design power is reduced if the walls, roof, floor are well insulated, energy-saving double-glazed windows are installed on the windows. The resulting figure will be the required power of the boiler. When looking for a suitable model, make sure that the maximum power of the unit is not less than your figure.

With or without boiler

Let’s talk about how water is heated. A conventional double-circuit gas boiler acts as an instantaneous water heater. Hot water is often needed not constantly, but in small portions, which leads to frequent switching on / off of the boiler. This mode leads to rapid wear of the equipment, but it is too expensive to turn off the water. The solution to this problem is a double-circuit gas boiler with a boiler.

The boiler gas boiler has a small built-in storage in which a certain supply of heated water is stored. When the hot water tap opens, the flow comes from the tank, when the supply runs out, the burner turns on and continues to heat the water. After the tap is closed, the boiler works for some time, filling the boiler, then it turns off. This mode of operation leads to less wear on the equipment. The disadvantage of gas boilers with built-in boilers is their large size, because you still need to place the boiler somewhere. There are models with a remote boiler, then the tank is connected to the boiler, and the floor can be installed by the boiler or next to it.

Double-circuit gas boilers: manufacturers

Having decided on the technical characteristics, it remains to find a suitable model and choose a manufacturer. This is not at all easy – there are many firms on the market, prices differ decently. As usual, there are three segments – expensive, mid-priced and cheap.

Expensive – these are products of European manufacturers:

• Italian gas boilers – Ferroli (Ferroli), Beretta (Beretta), Ariston, Baxi (Baksi).
• German ones are not inferior to them in quality: Viessmann (Weissmann), Wolf (Wolf), Vaillant (Vialant).
• Korean Navien (Navien) make a worthy competition to the leaders.

This equipment works reliably and without failures, but only under certain conditions. The first is a stable power supply, without significant deviations in frequency and voltage. Our networks do not sin with stability, therefore a voltage stabilizer is needed, and better – electronic. The second condition for normal operation is a certain gas pressure in the line. Most of the German and Italian gas boilers operate if the gas pressure is 2 atm or more. The exception is the Ariston and Navien boilers.

Russian-made wall-mounted double-circuit gas boilers – Danko, Protherm (Proterm) have proven themselves well in the market. They have approximately the same functionality as the “Europeans”, but they react less sharply to deviations in the power supply, they work at low gas pressure. What does not please – the Russian “service”.

There are also Bosch boilers (Bosch). The company itself is German, but there are factories in Russia, so it is not easy to determine the geographical location of these boilers – some are produced in Russia, some at other plants in other countries. Bosh campaign specialists have developed a new boiler model adapted to our conditions – Gaz 6000 W.