Indirect heating boiler for a gas boiler: specifics of operation and connection

A powerful gas boiler installed in a cottage can easily solve the problem of heating and supplying the house with hot water. But the amount of heated liquid for hygienic and household needs is limited and does not always suit the owners. To compensate for its deficiency, they install a BKN - an indirect heating boiler for a gas boiler.

Let's look at the features and functional capabilities of the storage unit, and also find out how best to use it in tandem with a gas boiler so that the result is as efficient as possible.

Distinctive features of an indirect heating boiler

A boiler is a large barrel whose main function is storage. It comes in different volumes and shapes, but its purpose does not change. Without a boiler, a problem may arise when using, for example, two showers or a shower and a kitchen tap at once.

If a household 2-circuit boiler with a power of 24-28 kW produces only 12-13 l/min per flow, and one shower requires 15-17 l/min, then when any additional tap is turned on, there will be a shortage of water supply. The boiler simply does not have enough working capacity to provide hot water to several points.

If a large storage tank is installed in the house, even with several water points turned on at the same time, everyone will be provided with hot water

All storage boilers can be divided into 2 large categories:

• direct heating, creating a supply of hot water using a heating element - for example, an electric heating element;
• indirect heating, heating water with already hot coolant.

There are other types of boilers - for example, conventional storage water heaters. But only volumetric storage devices can indirectly receive energy and heat water.

BKN, unlike energy-dependent equipment running on electric, gas or solid fuel, uses the heat generated by the boiler. Simply put, it does not require additional energy to function.

BKN design. Inside the tank there is a coil - a steel, brass or copper tubular heat exchanger that acts as a heating element. Heat inside the tank is retained according to the principle of a thermos

The storage device fits easily into the DHW system and does not cause problems during operation.

Users see many advantages in using BKN:

• the unit does not require electrical power and benefits from the economic side;
• hot water is always “ready”, there is no need to let cold water pass through and wait for it to heat up;
• Several water distribution points can operate freely;
• stable water temperature that does not drop during consumption.

There are also disadvantages: the high cost of the unit and additional space in the boiler room.

The volume of the storage tank is chosen based on the number of people permanently residing in the house. The smallest boilers are designed for 2 consumers, so when choosing, you can start from a volume of 50 liters

According to all characteristics, BKN is suitable for use in conjunction with gas boiler. Moreover, this is one of the best solutions for equipping a hot water preparation system for a private house with a large number of residents.

But boilers are different, so we will consider both acceptable options and those where problems may arise.

Gas heating equipment + BKN

At a dacha for a summer holiday it is not necessary to install a complex communications system, whereas for a cottage of permanent residence it is simply necessary. There is no point in wondering whether an additional boiler is needed for a gas boiler - of course, it will turn out to be a very useful acquisition, significantly increasing the comfort of living in the house.

We will review the basic BKN connection diagrams in order to prevent errors that are possible when installing the equipment yourself.

Connection diagram to a 1-circuit boiler

Single-circuit units perform one of the functions declared by the manufacturer: either provide water heating for hot water supply, or heat the house - and the second option is used much more often in everyday life.

The combined solution - a 1-circuit gas boiler + BKN - is one of the best for a small cottage.

Connection diagram of BKN to a gas boiler. Heating of water for heating and hot water supply occurs synchronously, after which it is distributed into two branches - to the heating radiators and the boiler

The heating process occurs according to the following scheme:

• cold water enters the boiler, where it is heated to the required temperature (for example, +80°C) by a gas burner;
• the heated coolant enters the heating circuit - to the radiators - and into the heat exchanger storage water heater, i.e. boiler;
• By increasing the temperature of the heat exchanger, the water in the boiler is heated and when the water taps are turned on, it flows to the consumer.

When implementing this scheme of a gas 1-circuit boiler with an indirect boiler, the main importance is the technical data - power and coolant supply rate.

According to average indicators, the water in the boiler, if it has not previously functioned, heats up from zero to an acceptable temperature in 5-15 minutes, that is, the wait will not last long. Usually the unit is in working condition, so there is always access to heated water.

Let's look at the features of the strapping of the devices in question.

First, you need to choose the most convenient installation location - most often it is a boiler room, a separate non-residential premises. It is better to place the units at a close distance from each other - this way the heating process occurs faster, and less materials are consumed.

Floor mounting requires installation in such a way that the distance to the nearest obstacles or devices is at least 0.5 m - this will be required for maintenance or inspection of the unit

The piping occurs on two sides: loading and water supply.

At the boiler outlet it is recommended to install membrane expansion tank, compensating for thermal expansion and stabilizing the operation of the system. All circuits must be equipped with ball valves and check valves that regulate the direction of coolant flows.

It would not be superfluous to install filters - the water supplied to the system is different, and during an accident sand or other debris can get in, which can contaminate the coolant and damage the equipment.

A mandatory element is a circular pump that supplies water at the required pressure.It is mounted on a pipe between the boiler and the boiler, and it is controlled in one of the following ways: either by a storage thermostat, or by a temperature sensor in the boiler

Shut-off valves are installed on both sides of the pump. The same tap is at the cold water inlet to the boiler.

A tee with a drain valve is installed on the boiler pipe, and shut-off valves are installed on both pipes so that the unit can always be cut off from the boiler for cleaning or other maintenance. On the supply, in front of the shut-off valve, you need to place an air vent.

The best option is to connect using a three-way thermostatic valve, which organizes the coolant exiting the boiler and dividing it into two flows - into the boiler and the heating sector. With its help, you can regulate the temperature: if heating up to + 80-90 ° C is suitable for radiators, then for heated floors it is better to limit it to + 45 ° C.

If there is no three-way valve, then install two circulation pump, one of which serves the BKN, the other is intended for the heating branch.

Diagram of connecting the boiler to the solar system. Interaction with the solar collector requires the installation of a separate closed circuit with a heat exchanger and temperature sensors

Sometimes circuits with coolant recirculation are used - for example, to constantly maintain a heated towel rail in “working” condition. Heated water circulates in a closed circuit, preventing the pipe from cooling. A circulation pump is a mandatory element, but a check valve is not needed. The disadvantage of such a system in the summer is excessive energy consumption.

When the operation of the gas boiler switches to “summer mode,” the heating circuit is simply cut off - the gas burner heats the coolant for the boiler.But there is another way out - simply turn off the gas boiler and use only the boiler. This is possible if the storage device is additionally equipped with an autonomous heating source - a heating element.

Two application options with a 2-circuit boiler

Owners of gas heating equipment are also interested in how it works indirect heating boiler with a 2-circuit gas boiler. Experts believe that interaction between the units is possible, but the result depends on the connection diagram: with one of them they are simply not designed to work with each other.

Introduction of a boiler into the DHW circuit

First, let's consider the option when the boiler is integrated into the DHW circuit. From a hydraulic point of view everything looks correct. A three-contact mechanical thermostat, placed in the boiler body, closes the power supply circuit to the pump when the temperature drops.

That, in turn, begins to pump water, which circulates along the circuit between two heat exchangers: it is heated by a gas burner, and then moves to the BKN coil.

Simply put, both devices perform their functions: the boiler stops heating and begins to heat water for hot water supply, and the boiler tries to heat the contents of its “storage” due to the increased temperature of the hot water.

The problem arises precisely because of the mismatch of temperature parameters. Let's assume that the initial filling temperature of the boiler is +15˚С, and the recommended temperature for heating water in the boiler is +60˚С - the automatic limiter no longer allows.

The difference between the two set parameters of 45˚ is significant, so the heat exchange in the boiler occurs quite intensively. But the temperature begins to rise, and when it reaches +40˚С, the difference is much smaller - only 20˚.Accordingly, heat transfer slows down.

Don't forget that water continues to circulate between the two devices. The gas burner of the boiler begins to receive not a 15-degree coolant from the cold water system, as recommended by the manufacturer, but a 40- and then 50-degree heated liquid from the boiler.

As a result, the temperature of the liquid instantly soars to +60˚С, the sensor is triggered, the burner turns off, as it is programmed for parameters that are safe for consumers

The water in the boiler begins to cool - the sensor turns on again and the circulation process resumes. And so on all the time. This leads to the fact that the water in the boiler does not reach the required temperature, but remains insufficiently hot, which is not suitable for domestic hot water use.

The process of interaction between the two units could take place if the boiler burner heated the coolant to +80˚C, but this is prohibited by the instructions to protect users from burns.

Another reason not to use the BKN and DHW circuit of a gas boiler in steam lies in the inability to heat the water in the boiler to a temperature exceeding +60˚C. This is due to sanitary standards.

About once a week, the device is filled with hot water at about 70-75˚C, so that the legionella bacterium, whose colonies resemble mucus, does not multiply in the tank. A large number of bacteria in water leads to the development of allergies and other diseases

It can be concluded that due to only two, but significant reasons, the scheme of combining a gas boiler and BKN through a hot water supply circuit was recognized as ineffective and unsafe. If you already have a double-circuit boiler, just use it for its intended purpose: use one circuit for the heating system, the second for DHW.

Interaction of BKN with the heating circuit

The second option is the interaction of the BKN with the heating circuit. The technical solution works great if the performance of the gas boiler is not enough, and this is the only effective way to connect the BKN to a 2-circuit boiler.

We do not touch the hot water circuit, but use only the branch responsible for heating - that is, going to the radiators and the “warm floor” system. The pipe leading to the boiler must be connected immediately under the boiler, after the tap

With electronic control, you need to set the boiler to heat water +70°C - exactly this coolant will enter the boiler, where further temperature adjustment will take place. The thermostat located in the boiler will turn on the pump when the temperature drops, and turn it off when the required value is reached.

With mechanical control of a gas boiler, everything happens differently. A second thermostat, the boiler, is connected to the boiler thermostat, and then the first device can be controlled using the second. For example, if you set the temperature on the second one to +80°C, then the first one, the worker, will require heating the water to +80°C, regardless of what temperature it is set to.

When the water in the boiler heats up to the required temperature, the second thermostat breaks the circuit, and the first one, located on the boiler, again becomes the “main” thermostat. If at this moment the temperature is set to +40°C, then it will drop to 40.

How to make a harness without errors?

In order for both units - both the gas boiler and the BKN - to operate without failure and throughout their entire service life, it is important to correctly perform tying, that is, set the security group and other elements.

When installing coarse filters, be sure to check its location - there is an arrow on the body indicating the direction of water movement.

Such a filter cannot be installed vertically, since as a result, coarse particles accumulate in the pipe and not in the filter itself. Cleaning can only be done by pressure washing - this way you can dirty the entire boiler room.

The expansion tank must not be installed separately, but between the boiler and the check valve, otherwise it will be useless and will not perform its functions.

Conclusions and useful video on the topic

How to piping a 1-circuit boiler with BKN:

About the nuances of connecting an indirect heating boiler:

Connecting a gas boiler to the BKN in practice:

We recommend that you involve specialists in installing and connecting the boiler so that you do not encounter heating or hot water problems in the future only because of incorrect piping or the absence of important elements in the system. Don’t forget about the manufacturer’s instructions, which often indicate the nuances of how the unit works with a particular device.