Indirect heating boiler

Recommendation points

• Why do you need an indirect heating boiler
• Device and principle of operation
• Indirect heating boiler – pros and cons
• Manufacturers and prices of indirect heating boilers
• How to choose an indirect heating boiler
• Connecting an indirect heating boiler
• How to descale an indirect boiler

In this article: assigning an indirect heating boiler; device and principle of operation; positive and negative characteristics; prices and manufacturers; Criterias of choice; how to connect an indirect heating boiler; how to remove scale and dirt.

The degree of comfort in our life is firmly connected with such an integral element of civilization as the constant availability of hot water – to wash the dishes, wash, wash ourselves. In urban conditions, hot water is more or less present in taps, but in the vast expanses of rural areas there are problems with hot water supply – it goes only to heating houses, circulating in heating systems with single-circuit boilers. The option of providing households with hot water using electric heating boilers is not very attractive – electricity is not cheap, gas boilers or, more commonly, gas water heaters have impressive dimensions and are not able to maintain a constant water temperature. The best option is an indirect heating boiler.

Why do you need an indirect heating boiler

An indirect heating boiler is purchased together with a single-circuit boiler – together this equipment takes up more space and is more expensive than a double-circuit boiler. However, their important advantage over a double-circuit boiler is the ability to provide households with a significant amount of hot water practically without interruption, while its temperature will remain unchanged..

Any water heating system based on heating running water has two negative factors – its performance is sufficient for only one or two points of water consumption, while the water temperature is not constant. For example, households will not be able to wash the dishes and, say, take a shower at the same time, without experiencing a lack of water and a sharp drop in its temperature, which is very unpleasant. We have to develop a certain internal schedule for the consumption of hot water – if someone intends to take a shower, then he has to inform the other residents of the house about this and ask them not to turn on the hot water tap and cause unexpected discomfort to the swimmer.

In families with a number of households of 2 or more people, instantaneous water heating systems will be ineffective, a more optimal solution is an indirect heating boiler.

When using an indirect heating boiler, the number of simultaneously used hot water points is limited only by the capacity of the boiler – hot water is already collected in it and has the same temperature throughout the volume, that is, you do not have to wait until the water circulation when opening the tap delivers hot water of the required temperature.

Device and principle of operation

Externally, the indirect heating boiler resembles a metal barrel for collecting water and has a cylindrical shape, it can hold either tens or hundreds of liters of water – the exact volume depends on the specific model. There are boilers for both vertical and horizontal installation – the first type is attached to the wall at a certain height from the floor level and is preferable if the boiler room is small in area. The body of the storage tank is made of painted enamel steel, plastic and stainless steel – the latter two materials provide a significantly longer service life, since they are not subject to corrosion.

The device of an indirect heating boiler with a coil: 1 – cold water inlet; 2 – hot water outlet; 3 – protective anode; 4 – coolant inlet; 5 – thermal insulation; 6 – tubular heat exchanger; 7 – coolant outlet

The serpentine pipe of a heat exchanger made of steel or brass, installed inside the tank of an indirect heating boiler, often has a complex shape, which allows the coolant circulating in it to heat the water better and faster. The coils of the heat exchanger can be located closer to the lower part of the boiler tank, where a colder layer of water accumulates, or they can be evenly distributed throughout the entire volume of the tank – according to the manufacturers, such an arrangement allows for better heating of water, achieving a uniform temperature. There are models of boilers with two tubular heat exchangers, the first of which is designed for the circulation of the coolant from the heating system, the second for the coolant from other sources, for example, a solar collector or a heat pump.

In addition to boilers with built-in coils, models are produced without heat exchange tubes – such boilers consist of two tanks installed one into the other, the inner one is made of stainless steel, and the coolant circulates between the walls of the outer and inner tanks.

Boiler of indirect heating “tank in tank”: 1 – cold water inlet; 2 – hot water outlet; 3 – coolant inlet; 4 – inner tank made of stainless steel; 5 – coolant outlet

Inside the boiler, in a tank filled with water, there is a magnesium anode. It is designed to protect a metal container from galvanic corrosion – its electric potential is lower than that of metals in the boiler structure, therefore corrosion will affect and destroy the magnesium anode. Requires periodic replacement due to wear – corrosion “eats” it.

Most indirect heating boilers are equipped with an electric heating element, which is used in the warm season, when the heating boiler is turned off..

Indirect heating boilers work according to a fairly simple scheme – cold water enters through the inlet into the boiler tank, a coolant circulates through the coil of the heat exchanger or between the walls of the double housing, heated by the heating boiler, which in turn heats the water in the boiler and maintains its constant temperature. Each indirect heating boiler has input and output nozzles that connect it to the heating boiler, and heated water enters the points of its consumption through a special outlet nozzle. To reduce heat loss in contact with the internal atmosphere of the boiler room and eliminate the threat of damage to the boiler due to constant changes in internal and external temperatures, the outside of its container is closed with a heat-insulating material made of polyurethane foam, expanded polystyrene or mineral wool. The external body of the indirect heating boiler is in the form of a cylinder or an oval rectangle.

With a low water consumption, i.e. not higher than 1.3 l / min, the installation of an indirect heating boiler and a single-circuit boiler is not profitable, it is easier and cheaper to install a double-circuit boiler. It is not profitable to use such a boiler even when heating the coolant with an electric heating boiler, when generating electricity by a diesel generator. In the case when hot water is needed to fill the bath and the heating boiler runs on solid fuel or natural gas – there is no way to do without an indirect heating boiler!

Indirect heating boiler – pros and cons

Positive characteristics:

• does not increase the load on the power grid during the cold season, since it does not use electricity to heat water during this season;
• high performance, provided that a single-circuit boiler is equipped with a heat exchanger of sufficient diameter and sufficient power;
• the absence of direct contact of the coolant with the heated water, that is, the inner surface of the coil in the indirect heating boiler is in contact only with prepared water containing a small amount of salts;
• providing consumers with hot water without its preliminary draining at the point of consumption, achieved by recirculation, inaccessible to other water heating devices without their increased wear;
• it is permissible to connect various sources of thermal energy, as well as a seasonal transition from one to another source of heat carrier.

Negative characteristics:

• the high cost of a set of equipment (indirect heating boiler + single-circuit boiler), compared to an electric boiler or a double-circuit boiler;
• it takes several hours to heat cold water in containers with a volume of 100 liters; during the heating period, the intensity of heating of residential premises decreases;
• the capacity of a boiler, especially a large one, takes up a lot of space, which creates difficulties when placing in a boiler room and requires careful planning.

Let’s compare the indirect heating boiler and other types of water heaters:

• Gas water heater– its installation requires a project, construction of a chimney and preliminary approval from government agencies. A maintenance contract for the gas water heater is mandatory. The main disadvantage of this type of water heater is the inability to maintain a stable water temperature.
• Gas heater, storage– similar in most characteristics to an indirect heating boiler, but requires the same preliminary measures as installing a gas water heater.
• Instantaneous electric heater– fast heating of water, but low productivity due to lower power than an indirect heating boiler.
• Storage electric heater– as in the case of a flow-through electric heater, it has a lower power, which is why it is not able to provide heating of a new portion of water in a short time. A serious minus of any electric heaters is a high power consumption.
• Double-circuit boiler – the performance of this water heater is low, and the temperature of the heated water is not constant. The main disadvantage in solving the DHW problem with the help of double-circuit boilers is the overgrowing of the coil in which the water circulates with lime deposits (scale). As a result, the double-circuit boiler loses power, since scale impedes heat transfer, the section of the coil tube decreases, which is why water flows in a weak stream when the tap is opened – it is extremely difficult to completely remove scale inside the coil.

Manufacturers and prices of indirect heating boilers

A boiler with a water tank volume of 80 liters costs about 14,000 rubles, and one of the most capacious – 900 liters – on average 165,000 rubles.

In Russia, boilers with indirect water heating are represented by a wide range of brands and volumes – German Buderus, Junkers Bosch Gruppe, Reflex, Viessmann, Wolf and Bosch, French Saunier Duval, Czech Protherm, Mora and Drazice, Slovenian Gorenje, Polish Galmet, Italian Beretta and Belgian ACV. By the way, boilers without a coil – in a double casing – are produced only by Belgians..

How to choose an indirect heating boiler

The choice begins with the selection of the optimal volume of the boiler capacity. The average consumption of hot water per person per day is as follows: for washing – 6–17 liters; for washing dishes – 20-25 liters; shower – 60-90 liters; for taking a bath – 160-180 liters. Based on these data, the approximate needs of households are calculated based on water consumption, rounded up and the required capacity is determined. Keep in mind that a 20 liter boiler will heat the water to the optimum temperature in 40 minutes, while a 200 liter boiler will take over 6 hours..

Having decided on the volume of the boiler and proceeding to inspect the models offered by the stores, pay attention to the materials from which the coil tube, the internal water tank and the thermal insulation between it and the outer casing are made.

Models of the lower price range are equipped with a steel coil welded to the boiler tank, more expensive models have a brass coil, in some models – with fins, fixed with a flange connection. The removable brass coil is more efficient in heating water and is easy to remove and descale, which cannot be done with a welded steel coil.

In inexpensive boilers with indirect heating, the water tank is protected from corrosion by a layer of enamel or glass-ceramic, but such a coating will not last long – temperature drops will cause the formation of microcracks in which corrosion processes begin. Therefore, the best choice would be boilers with stainless steel tanks. Please note: if the boiler tank is made of stainless steel, then the warranty for it will be more than one year, if not – up to one year.

Make sure that the insulation between the boiler bodies is made of polyurethane – in the cheaper models, the insulation is built on quick-wear foam rubber.

Each indirectly heated boiler is equipped with a cathodic protection that absorbs the effects of galvanic corrosion. There are two ways to protect against such corrosion: a magnesium anode, which is replaced every six months or a year; cathodic protection with an external power supply – it does not need to be changed, but it must be connected to the power supply.

An important selection criterion will be the capabilities of a single-circuit boiler, to which an indirect heating boiler will be connected – if the boiler passport states the flow rate of the coolant passing through the heat exchanger coil corresponding to 45-50% of the flow rate of the heating boiler installed in your house, then the performance of the heating system of the house will significantly decrease … In addition, the DHW heating time will increase..

Check for the presence of a thermostat and safety valve, usually included with the boiler – they are necessary when using the appliance.

Indirect heating boilers differ in the installation method: with a volume of up to 200 liters, they can be wall-mounted and floor-standing, with a larger volume, only floor-standing.

Connecting an indirect heating boiler

It is carried out by a circuit parallel to the heating one, in which a circulation pump is built in – thanks to this, the independence of the hot water supply from the heating system is achieved. In addition, such a connection will allow, if necessary, to completely disconnect the heating circuit and direct the coolant only to the boiler..

Example of piping for an indirect heating boiler: 1 – heating boiler; 2 – boiler safety group; 3 – shut-off valves; 4 – boiler safety group; 5 – expansion tank of the DHW system; 6 – hot water supply to consumers; 7 – boiler thermostat; 8 – heating radiators; 9 – indirect heating boiler; 10 – boiler circulation pump; 11 – heating system circulation pump; 12 – check valve; 13 – coarse mesh filter; 14 – cold water supply

Installation of an indirect heating boiler in a gravitational heating system is permissible; it must be introduced into the heating circuit in series through the bypass, after the boiler and before the heating devices.

The scheme and procedure for installing the boiler are given in its passport. If you purchased a wall-mounted appliance, then you need to raise it to such a height so that the bottom is higher than the top of the heating boiler – with this arrangement, the water in it will heat up faster.

How to descale an indirect boiler

According to the instructions attached to the boiler by the manufacturer, its preventive maintenance is performed once every six months or a year, it is better to carry it out twice a year – before and after the heating season..

Manufacturers of indirect heating boilers, however, like any other heating devices, do not recommend carrying out its prevention on their own, only in specialized service centers, therefore they do not give any information on how to open access to the internal capacity of the boiler. You can remove and replace only the magnesium cathode, while reducing its length by half. But it is impossible to remove scale through the small holes of the flanges without special equipment. What can be done in a domestic environment, that is, independently?

First of all, determine the degree of limescale build-up on the heat exchanger, for this you need to measure the temperature of the coolant at the inlet and outlet of the boiler, to determine the heat loss. During normal operation of the boiler, the temperature difference at the inlet and outlet should not exceed 15 ° C, i.e. if the incoming heat carrier has a temperature of 90 ° C, and at the outlet 75 ° C, everything is in order. Lime scale, as it is deposited on the outer surface of the heat exchanger coil, prevents heat transfer, and if at 90 ° C at the inlet the water temperature at the outlet from the boiler is 80-85 ° C – descaling is necessary, otherwise the water heating period will increase many times.

If the scale layer on the heat exchanger is small, that is, the temperature difference at the input-output is at least 14–12 ° С, it can be removed using the “heat stroke” method. To do this, completely drain the water from the boiler and let the coolant from the operating boiler through the coil for 10 minutes in order to warm up the heat exchanger as much as possible in the empty tank of the boiler. Then fill the boiler with cold water – it should fill the container in 3-5 minutes. As a result of the greater difference between the temperature of the coil and the surrounding water, limescale deposits will lag behind the tube – rinse and repeat the descaling with “heatstroke” again.

If at the input-output of the coolant the temperature difference is less than 12 ° C, the “thermal shock” method will be ineffective – the scale build-up on the coil is too large. You will need to use chemicals for descaling, ideally those recommended by the manufacturer of this boiler model. It is not recommended to use any “home” products and compositions for removing limescale – the capacity of the boiler may be damaged. The descaling chemicals specifically for indirect boilers are diluted in water – the temperature and dilution ratios are indicated on the cleaning agent packaging. The prepared mixture is poured into the tank of the boiler with the condition that its level completely covers the heat exchanger coil. The cleaning time is given by the manufacturer of the chemical, as a rule, it is from 4 to 8 hours and depends on the volume of the boiler. At the end of the cleaning period, the contents of the boiler are completely removed, then it must be filled twice with cold water and drained – this completes the descaling.