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How to choose an indirect heating boiler

Using an indirect heating boiler is an effective way to organize hot water supply in a private house. Proper selection will allow not only to get hot water at the lowest price, but also to take advantage of the additional benefits available only to this type of boiler. More details in this article.

The principle of operation of indirect heating boilers
Unlike other types of water heaters (geysers and boilers, electric storage and flow), an indirect heating boiler does not heat the water itself. To prepare hot water, it uses heat from an external source – most often a heating furnace. Inside the indirect heating boiler there is a heat exchanger through which the coolant circulates – distilled water, propylene glycol, and so on.

The coolant is heated in an external source and gives off heat to the water in the boiler.

Features of indirect heating boilers
An indirect heating boiler is not the only way to organize hot water. Installing a double-circuit boiler, electric water heater or column may seem like an easier option. But in each of these cases, an indirect heating boiler has its own advantages.

Compared to a double-circuit boiler, the scheme with the installation of an indirect heating boiler is much more reliable, especially if the water is hard. With direct heating of water, scale accumulates in the heat exchanger, which can damage the boiler. In winter, this will not only deprive you of hot water, but also leave the house without heating.

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In front of a separate column, an indirect heating boiler has the same advantages – due to the low temperature of the coolant, scale in the boiler accumulates much more slowly. The column, on the other hand, is overgrown with scale very quickly, and in general it is much more complicated and prone to breakage.

Compared to electric water heaters, the “indirect” is many times more economical. Although it costs more, water from a boiler will cost you 3-7 times cheaper than heated water.

In addition, indirect heating boilers have additional advantages that distinguish it from other water heating devices:

It is convenient for organizing hot water supply with recirculation. In a conventional system, hot water in the pipes stands and cools. To wait until the water gets warmer, sometimes you have to empty a few liters. In a recirculation system, hot water constantly circulates in the pipes and appears at the tap immediately after it is opened.
Suitable for use in systems with alternative heat sources – solar panels, heat pumps.

However, the indirect heating boiler also has disadvantages:

Limited volume. When the supply of hot water runs out, you will have to wait for some time until the water heats up again.
Low heating rate. Due to the low temperature of the coolant, heating is slower than that of columns and electric water heaters.
High price.
Large dimensions.
Special requirements for the boiler: not everyone has the ability to connect an indirect heating boiler.
Characteristics of indirect heating boilers
The useful volume of the tank determines the amount of hot water available. The parameter should be selected based on the size of the family and the number of points of consumption. The maximum temperature in an indirect heating boiler is lower than that of an electric one, so the volume required for it is larger.

 

But the larger the volume of the boiler, the longer it will take to heat it up.

Thermal power determines the heating rate – the higher the thermal power, the faster the tank will warm up. The exact duration of heating can be calculated by the formula:

 

where T is the heating time in hours, V is the useful volume of the tank, Dt is the difference between the temperatures of cold and hot water, W is the heat output of the boiler.

But keep in mind that the thermal power of the boiler, given in the characteristics, is achieved at the maximum temperature of the coolant and the maximum flow rate. If the actual values ​​are lower, it will not be possible to achieve the nameplate heat output, and the heating rate will be lower. The coolant temperature and flow rate are usually indicated in the boiler manual, be guided by these data when choosing a circulation pump.

The heat output of the boiler depends on the available boiler output. So, if you spend 20 kW on heating, and the boiler is installed with a capacity of 30 kW, then only 10 remains for the indirect heating boiler. Even if the nominal thermal power of the boiler is much larger, its real value will not exceed 10 kW.

Heating method
Some indirect heating boilers, in addition to the main – indirect – heating method, can also use electric. For this, a heating element is installed in them. Some models of heating elements do not have, but they have the possibility of installing it – in a revision or a specially designed hole.

 

The heater allows you to use the boiler when the external heat source is turned off – for example, for repairs or maintenance. Also, the heating element can be used to heat water if an external source cannot heat the coolant to the desired temperature. This is possible when using solar panels or a heat pump.

It is not recommended to use the electric heating method as the main one – all the advantages of an indirect heating boiler are lost. It turns into an uneconomical electric storage water heater, which costs several times more than a conventional one.

The power of the heating element affects the rate of water heating when using the electric heating method. To determine the warm-up time of the entire volume, you can use the above formula. Typically, the power of the heating element is relatively small and does not exceed 2 kW. More powerful heating elements are installed in large (200 liters and more) boilers, but, in any case, their power is much lower than the thermal power of the boiler when using heat from an external source. If it takes a boiler from 15 minutes to heat the entire volume of water from the boiler, then when using a heating element, this period increases to several hours.

The maximum water heating temperature for different models can vary from 65 to 90 ºС. When choosing this option, consider the following considerations:

– At a temperature of 40-60 ºС there is a rapid reproduction of bacteria. To disinfect water, it is advisable to warm up the boiler to 80 ºС at least once a week or constantly maintain the temperature at least 70 ºС.

– At temperatures above 55 ºС, the deposition of hardness salts (scale) increases. Because of this, the heating rate will decrease.

– The higher the temperature, the more efficiently the volume of the tank is used.

– At temperatures above 70 ºC, there is a risk of water burns if only the hot tap is opened.

– In any case, the temperature of the water in the boiler will not be higher than the temperature of the heating medium.

The area of ​​the heat exchanger affects the rate of heat exchange and the heat output. When selecting, it is better to rely primarily on thermal power, but when choosing among two boilers with the same power, the one with the larger heat exchanger area should be preferred. To achieve the same heat output, it will require a lower flow rate, which reduces the price and power of the circulation pump.

The coating of the inner tank affects its ability to resist corrosion. Stainless steel protects well against corrosion and is not afraid of mechanical stress, but it is expensive. Enamel, glass porcelain and glass ceramics are approximately on the same level, although it is believed that glass porcelain and glass ceramics are more reliable than enamel.

But this greatly depends on compliance with the application technology, so the coating of the inner tank of the boiler of a trusted manufacturer, regardless of the name, will be more reliable and durable than the glass-ceramics of the budget “Chinese”. A common disadvantage of these types of coatings is brittleness. Under mechanical stress, cracks appear on them, in which corrosion develops.

 

The magnesium anode is also used to protect the boiler tank from corrosion. Magnesium reacts with oxygen dissolved in water. Water becomes less corrosive, which preserves the metal of the tank, but the anode breaks down over time, and it must be changed periodically. On hard water – 1-2 times a year, on soft water – once every 2-3 years.

Recycling . Some boilers have an additional inlet for use in recirculation systems. Hot water is disassembled through the usual – upper – outlet, and the recirculation return enters the additional – middle – inlet, where it mixes with already warm water. The lower inlet used to supply cold water cannot be recirculated.

 

The maximum pressure in the tank should be 1.3-1.5 times greater than the cold water pressure in the system. Usually in private houses, the pressure of cold water is set to 3-4 bar, so the maximum pressure in the boiler tank of 6-8 bar is enough. But in houses with centralized water supply, the pressure of cold water may be higher.

Maximum pressure in the heat exchanger. An indirect heating boiler usually uses a heating medium from a heating system, and in it the pressure rarely exceeds 2 bar. The maximum pressure in the heat exchanger for boilers is never lower than 6 bar. However, if your system pressure is higher, pay attention to this parameter. In steam heating systems, the pressure is 10 bar or more.

Options for choosing indirect heating boilers
To create a recirculating hot water system, you will need an indirect heating boiler with the appropriate option . And you no longer have to wait for hot water to flow from the tap.

 

If you want to achieve maximum savings when heating hot water and are going to use solar panels, choose among indirect heating boilers with a heating element . Such a boiler will allow you to warm up the water to the required temperature on a cloudy day.

A boiler with an inspection hole is easier to clean from scale, and in addition, a heating element can be installed in it if the need arises.

 

The magnesium anode will protect the inner tank of the boiler from corrosion.

An indirect heating boiler with a volume of 75-100 liters will be enough for a small family of 2-3 people. For a family of 3-4 people, it is better to choose among models with a useful volume of 150-200 liters . For a large family or for a house for several families , a boiler with a volume of 250 liters or more is required .

 

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