Indirect heating boiler: device, operating principle and connection diagrams

In private homes, cottages, sports complexes and hotels, an indirect heating boiler is often used - the water heater operates without being connected to a centralized water supply. This equipment copes with heating large volumes of water, easily maintains the desired temperature and ensures an uninterrupted supply of hot flow.

In a word, if you are looking for a budget water heater to go with a single-circuit boiler, consider installing a BKN in your home. And to simplify the task of choosing, we suggest that you familiarize yourself with the criteria for purchasing a boiler, the principle of its operation and connection diagrams.

Design features and operating principle of BKN

An indirect heating boiler can only operate on resources from an external source, but to service the system in the warm season, you can connect a heating element.

To ensure uninterrupted supply, the system includes a recirculation circuit - water will constantly move through the pipes, and when the tap is opened, a hot stream will flow at the water collection points connected to the circuit.

An indirect heating boiler is usually installed next to a heating boiler, and this “duet” occupies a fairly large area, especially if both devices are floor-standing.

Thus, the device allows you to significantly save on energy resources, but at the same time receive no less comfort than when using a centralized hot water supply system.

How does an indirect heating boiler work?

A device that heats water using an indirect principle. - This is an insulated storage tank, inside of which a heating element is installed, powered by an external coolant.

The tank itself can be made in the shape of a rectangle or a cylinder, and its walls have pipes for the circulation of hot water from the heating system and the inlet/outlet of water supply pipes.

The basis of the design is a metal or plastic container with a capacity of ten to several thousand liters. The inside of the tank can be coated with enamel, ceramic or porcelain glass, which helps maintain proper water quality and avoid corrosion.

The housing is protected from heat loss by an insulating layer made of polyurethane, foam rubber or mineral wool.

The coolant coil can be evenly positioned throughout the entire container, or placed tightly at the bottom to quickly heat the cold water entering there (+)

To prevent scale, a magnesium or titanium anode is installed in the upper part of the container, which softens the water, protects the metal from galvanic corrosion and increases the “life” of the device.

The device is also equipped with a thermostat that controls heating to a certain temperature, and safety valvesincluded in the protection group.

Pros and cons of BKN

The main advantage of an indirect heating boiler is energy savings. To operate it, you do not need to connect a gas burner or use electricity, as a direct heating device requires.

The heating system provides all the necessary resources, so hot water is an almost free “bonus” that will help significantly save the family budget during the cold season.

Most indirect heating boilers are additionally equipped with heating elements with a thermostat, so that the device can be used during the warm period when the heating is turned off

Other advantages of BKN:

• High performance – a tank with a volume of 100 liters is capable of delivering about 400 liters of hot water per hour (provided that the boiler is powerful enough).
• Long service life — the coolant does not come into contact with running water, as in similar direct heating devices.
• Does not overload the power grid — works from an external energy source.
• Instant hot water supply – no need to wait for a cold stream to flow, as when using geysers, or for a spent tank to heat up, as with electric boiler.
• Affordable price – if desired, the system can be assembled with your own hands.
• Possibility of connecting to multiple energy sources, for example, to a vacuum solar collector, boiler, geothermal heat pump.

Of course, there are some downsides here too. Despite all the advantages, the speed of heating water in the device leaves much to be desired - even modern equipment will spend at least 15-20 minutes to process 100 liters, but homemade systems may not be able to cope even in an hour.

Also, indirect heating boilers are quite bulky, so they take up an impressive amount of free space. Large tanks may require a separate room.

An indirect heating boiler can also be made from scrap materials, using a bent copper pipe for the coil, a capacious barrel with a lid, or a gas cylinder for the tank:

Another version of a homemade indirect heating boiler is made using a housing from a conventional storage water heater:

In addition, this device will be truly economical only during the heating season, and in the warm season you will have to either turn on the boiler or provide alternative heat sources - solar panels, heating element. And the price of a high-quality “indirect” heater is higher than analogues with direct heating.

Types and model varieties of equipment

The most common type of device is a storage tank, inside of which steel or brass pipes (coil) are placed with circulating coolant.

The rate of heating of water depends on the number of turns on its spiral. The operating principle of this design of an indirect heating boiler is extremely simple: cold water enters the tank, and the coolant, moving along the coil, warms it up to the desired temperature.

But there are also devices designed according to the “tank-in-tank” scheme, where instead of a spiral pipeline, two containers of different diameters are used.

The system works like this: cold water enters a smaller tank, which is heated by hot coolant circulating between the walls of the tanks.

In such devices, water becomes warm in a matter of minutes - a large heating area allows the equipment to operate effectively in flow mode, guaranteeing an uninterrupted supply of hot flow.

Boilers operating on a “tank in tank” system not only quickly heat water to 90⁰C, but also have a self-cleaning function (+)

A combined BKN for heating water can use energy from several sources at once or be equipped with a built-in heating element.

Types of KN boiler

By location:

• Wall – usually, this is a small device with a displacement of up to 200 liters. It is attached using special brackets to any vertical surface that is strong enough to support the weight of a fully filled tank (plasterboard partitions are definitely not suitable). It can be located high enough and not occupy the usable area of ​​the room.
• Floor – a spacious boiler designed for a large number of consumers. However, for a device with a capacity exceeding 1000 liters, it is recommended to allocate a separate room - arrange a boiler room. But such a system is usually installed to serve large cottages, enterprises, hotels and other institutions; for family use, you can get by with a 250-300 liter device.

According to the shape of the tank:

• Horizontal – takes up a lot of space, but it is easier to independently maintain the required water level without resorting to connecting pumps.
• Vertical – saves free space, but is very limited in capacity.

Depending on the nuances of use, layout features and availability of free space, you can choose the optimal BKN model that will organically fit into the design of the room and provide your home with an uninterrupted supply of hot water.

What to look for when choosing BKN

One of the main parameters that should be the decisive argument when buying a boiler is its capacity. To find out the required tank capacity, we advise you to focus on the number of people in your family.

Recommendations for displacement:

• 80-100 l — 2 consumers;
• 100-120 l - 3 persons;
• 120-150 l — 4 users;
• 150-200 l — 5 consumers.

It is important to separate the concepts of “total tank capacity” and “working capacity”, because the spiral pipe located inside the boiler occupies a significant area. Therefore, be sure to check when purchasing how much water actually fits in the device. This nuance should be indicated in the technical specifications.

Also, in addition to the “universal” recalculation of potential consumers, it is necessary to take into account both the frequency and volumes of water use. For example, if your family likes to soak in a warm bath rather than take a quick shower, the working capacity of the tank should be appropriate - at least 120 liters.

It is beneficial to use BKN in conjunction with a solid fuel or single-circuit gas boiler, but if the water flow rate is less than 1 l/min, a double-circuit boiler will be cheaper, which will take up much less space than a system with indirect heating

Other important parameters:

1. Power – the greater the water consumption, the longer the device’s lifespan should be. But at the same time, it is important that the power of the “indirect” does not exceed the capabilities of the heating system (or other external energy source). For example, if the volume of the storage tank varies between 120-150 liters, the boiler power should be at least 23 kW, and for 160-200 liters 31-39 kW will be needed.
2. Heating time – a parameter depending on the volume of the tank and the number of turns on the coil (large or combined containers can be equipped with several spirals).
3. Tank material – for long-term use, boilers made of stainless steel or medical steel are best suited.
4. Thermal insulation – cheap models use foam rubber, which wears out quickly and allows heat to pass through, so it is better to purchase a more expensive device that uses polyurethane.
5. Control – the device will be able to operate in automatic mode, turning off and starting the water flow as necessary, and control heating using a temperature sensor.

When choosing the shape and size of the tank, it is also necessary to take into account that although theoretically the boiler can be installed in any room where there is access to the heating main, its optimal location is next to the boiler. This is how heat transfer is most efficient.

You can make an indirect heating boiler yourself. Instructions for manufacturing the unit are described in this article.

Nuances and boiler connection diagrams

As already mentioned, BKN uses energy from an external source to heat water. Therefore, before connecting to the coolant, it is important to select a suitable diagram for installing the device. Let's look at the most common options.

General principles for installing the device

The boiler must be mounted on a prepared, flat surface in close proximity to the boiler. Hanging models are mounted on a concrete or brick wall, at the same level or slightly above the heating boiler.

For a floor-standing device, the area allocated for placing the tank should be leveled (if the floor is critically uneven, you can make a stand in the form of a podium).

Herself BKN strapping designed to connect the device to two systems - water supply and heating. For this purpose, there are special pipes on the body.

The safety group of the indirect heating boiler includes check and relief valves that protect the system from high pressure and leakage of hot water

Joining principles:

1. Cold water from the drainage system should flow into the lower part of the tank, and hot water should come out of the top.
2. At the cold water inlet it is necessary to install check valve, which will prevent leakage of hot flow from the boiler in the event of a decrease in pressure in the system.
3. A point for supplying recirculation can be located in the middle of the device.
4. When the heating circuit is turned on, the coolant must move from top to bottom. That is, the pipe with hot water goes into the upper pipe, and the pipe with cooled water comes out from the bottom.
5. All pipes must be equipped with taps with union nuts to ensure the ability to turn off the boiler, for example, in case of replacement of the device or repair work.

Thanks to this scheme, maximum efficiency of the device is achieved, since the coolant circulating in the coil heats the water in the upper part of the tank, transfers the remaining heat to the low-temperature layers and enters the boiler for new heating.

Schemes for connecting the BKN to the source

In order to choose the correct connection scheme for the boiler to the heating system, it is necessary to take into account the capabilities of the existing wiring and the installation location of the heating device. There are several solutions.

Option 1. The most common scheme for constant use of hot water is connection through a three-way valve. It assumes the presence of two heating circuits - the main one and a separate one for the BKN.

To do this, cut into the hot water supply circulation pump, and after it a three-way valve is installed, which is controlled by the boiler thermostat.

The scheme works like this: when the water temperature in the boiler drops below a set point, based on a signal from the thermostat, the valve switches the system to heating the storage tank, and after reaching the desired value, it sends the flow to circulate through the general house circuit.

For correct operation of the three-way circuit, it is important to adjust the thermostat data so that the temperature programmed for the boiler is not set lower than in the boiler thermostat (+)

Option 2. Installations of a two-pump system - it is advisable to use the scheme if there is no need for constant and large volumes of hot water. For example, a boiler is installed in a country house and is used only on weekends.

In this case, the BKN and the heating boiler are connected in parallel, and the coolant flows will move along two lines. To force the movement of water, two pumps are installed: on the heating circuit and the supply pipe for the boiler.

Heating is carried out using a thermostat, which temporarily turns off the radiator heating, directing the entire resource of the system to the boiler.

According to the two-pipe scheme, the heating circuit will be turned off while the boiler is operating, but during the 30-60 minutes that are necessary to heat the tank “from scratch”, the radiators will not have time to cool down critically (+)

Option 3. For rooms where a multi-circuit heating system is used, for example, in addition to radiators, warm floor, a hydraulic distributor is used.

Hydraulic arrows redistribute pressure in individual sections of the circuit and ensure uninterrupted movement of coolant even without the help of circulation pumps.

Without engineering experience, it is unlikely that you can make such a complex circuit yourself - in order to provide for a lot of nuances and ensure smooth operation of the system, it is better to turn to professional designers (+)

Without a hydraulic collector in a multi-circuit system, pumping equipment may fail and even thermal shock may occur with damage to radiators.

Return circulation system

If the boiler has a third input, a recirculation system can be connected to it. To do this, a looped circuit is inserted into the recirculation point on the device body.

The water flow is in constant motion thanks to the operation of the circulation pump, therefore it instantly delivers warm water to the point of consumption (+)

To implement the scheme, you will need to mount an additional circuit and install the following elements:

1. Check valve at the inlet to the heater.
2. Automatic air vent, protecting the pump from air penetration before starting.
3. Safety valve, which will protect against pressure changes.
4. Diaphragm expansion tank, into which excess water will flow when the pressure in the system increases. Please note that the capacity of the reserve tank must be at least 1/10 of the volume of the BKN itself.

If the indirect heating boiler does not have a pipe for the recirculation circuit, you can insert the return line into the cold water pipe and install a pump. Then the connection is made according to the diagram below.

The choice of a piping scheme with or without the possibility of recirculation should be based on the design of the BKN and heating system, as well as the power of the equipment (+)

Connection via a return circuit ensures an uninterrupted supply of hot water - that is, there is no need to wait until the coolant warms up the water in the tank.

Conclusions and useful video on the topic

Before you decide to purchase and install a new water heater in your home, we suggest you study video materials that clearly demonstrate the advantages of the “indirect” and the nuances of its connection.

In this video you can find out how indirect heating boilers with coolant located inside the coil work:

All the nuances of the internal structure and principles of operation of various types of indirect heating boilers in a detailed video educational program from a practicing master:

Overview and connection of a 300-liter device, which runs on two coolants - a boiler and solar panels:

Detailed analysis of the BKN device and its connection diagrams. Practical advice and tips from the master:

As for maintenance, an indirect heating boiler is a fairly unpretentious piece of equipment. With proper installation and smooth operation of the system, you may only need to replace the magnesium anode every 6-12 months and preventative flushing of the tank.

The frequency of maintenance depends on the quality of tap water and intensity of use. But if you have a device installed that uses “tank-in-tank” technology and has a self-cleaning function, you will not need to change the consumables.

However, you can always agree on regular equipment maintenance with specialists who specialize in the installation and repair of water supply systems.

Any questions? Or would you like to share your personal experience in choosing, connecting and using an indirect heating boiler? Please leave comments on this article.