Gas condensing boiler — why these boilers are the future

A gas boiler is one of the most efficient ways to provide home heating, so boilers of this type are always popular with consumers. The coefficient of performance (COP) of a conventional gas boiler, on average, is 90% or more.

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• Why is the efficiency of a gas condensing boiler higher than 100%

• The device of conventional and condensing gas boilers

• The principle of operation of a condensing boiler

• The choice of material for the secondary heat exchanger

• Features of operation of condensing boilers

• Operating modes of a condensing gas boiler

• Cascade boiler plants

• Why condensing gas boilers are the future

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This is a very good indicator — in terms of efficiency, a gas heating boiler is second only to heat pumps.

But there is a way to increase this efficiency even more — use a condensing type gas boiler or a condensing gas boiler.

Unlike conventional boilers, which are also called convection boilers, condensing boilers allow you to achieve very high efficiency of the useful use of heat from fuel combustion — in certain operating modes, the efficiency of the heat from the combustion gas of a condensing gas boiler exceeds 100% and reaches 108% and even 109%.

Why is the efficiency of a gas condensing boiler higher than 100%

Of course, according to the law of conservation and transformation of energy, an efficiency above 100% is unattainable in principle — additional energy cannot be taken from nowhere. However, as is the case with heat pumps, there are devices that have an efficiency above 100%, and the condensing gas boiler is one of them.

Where does the additional heat come from in a condensing boiler, and why does it not appear in a convection boiler? The answer lies in the name itself — a condensing boiler uses not only the energy “embedded” in the fuel, in this case, in gas, but also the energy that is released in the process of moisture condensation, which is one of the products of gas combustion. A conventional boiler simply releases this additional energy into the atmosphere along with smoke, but a condensing boiler directs it to heat the house.

Thus, if a convection boiler can spend only 100% of the energy generated during the combustion of gas on heating a house, then a condensing boiler has a solid advantage already at the start — in addition to these 100%, it also uses additional energy released during the process of water condensation, which , in the case of burning natural gas in a boiler, is equal to 11%.

Once again, for understanding — this additional 11% of energy is NOT contained in the fuel burned, so the efficiency of a condensing gas boiler is actually less than 100%, if only the gas combustion process and the heat generation due to this are taken into account. The condensing boiler for heating only uses two successive heat generation processes, and not one like a conventional boiler, and the efficiency of each of them is less than 100%.

But for an ordinary consumer, whose main concern is the cost of consumed gas, the origin of this additional 11% of energy does not matter, therefore, with good reason, gas equipment manufacturers talk about the efficiency of a condensing boiler at the level of 106-108%.

During the operation of any type of boiler, losses inevitably occur, but even in this case, a condensing type boiler has an advantage — losses in it, even without taking into account condensation, are usually lower. And these losses depend on the mode of operation of the boiler, we will talk about this below.

The device of conventional and condensing gas boilers

Ordinary gas boilers, in turn, are divided into boilers with an atmospheric burner (or with an open combustion chamber) and boilers with a ventilation burner (with a closed combustion chamber).

The simplest device is gas boilers with an atmospheric burner. The central element of such a boiler is a gas burner. It heats the heat exchanger through which water (or coolant) flows, supplied to the heating system, by burning gas. The products of combustion are discharged through the chimney due to natural draft, which occurs due to the temperature difference between the exhaust gases and the outside air temperature. From the room to the boiler there is a constant flow of air necessary for burning gas. The flow of air also occurs naturally. The removal of combustion products is provided by stationary chimneys.

The situation is different for boilers equipped with a ventilation burner. In such boilers, air is taken in by means of a fan, which pumps the air necessary for combustion into the furnace. And the removal of gases is carried out using coaxial chimneys. The latter differ from the stationary ones in that the gas exhaust pipe is laid in the center of the pipe supplying air to the furnace (this design is also called a “pipe in a pipe”).

Today, wall-mounted gas boilers and floor gas boilers are presented on the market in various modifications. As the name already implies, wall-mounted gas boilers are installed in a hinged version, which saves space, and floor-mounted gas boilers are installed on a horizontal surface. The consumer needs to know that wall-mounted gas boilers, on average, have less power than floor-standing ones.

In addition, there are single-circuit gas boilers and double-circuit gas boilers. Double-circuit gas boilers, unlike single-circuit ones, allow not only to provide heating at home, but also to supply it with hot water from a tap. Of course, it is more profitable to use a gas boiler for both heating and hot water production, therefore double-circuit gas boilers are more popular. While we were talking about convection gas boilers.

In the case of condensing gas boilers, manufacturers also offer both wall and floor models. Moreover, recently the condensing wall-mounted boiler has taken unofficial leadership. This is due to the possibility of cascade installation of gas condensing wall-mounted boilers.

The condensing gas boiler has a more complex device than the traditional one. Firstly, the combustion products in such a boiler have a much lower temperature than in a conventional one, therefore, in this case, one cannot do without a fan and a coaxial chimney. Secondly, two heat exchangers are installed in the condensing type boiler — primary and secondary. It is the secondary heat exchanger that makes it possible to literally «pick up» the heat released during the condensation of water vapor and transfer it to the heat carrier, i.e. dispose of. Recall that water vapor is one of the products of combustion, i.e. formed during the combustion of gas. Thirdly, a condensate collection and removal device should be provided in the condensing boiler.

The principle of operation of a condensing boiler

The operation of a condensing boiler at first is no different from the usual one. The heat carrier in the primary heat exchanger is heated using a gas burner, but then, having worked out the main process of fuel combustion and transferring a significant part of the heat generated in this case to the primary heat exchanger, the condensing boiler does not immediately emit gaseous combustion products with a temperature of 150-250 ° C into the pipe, but cools them up to 50-60°C.

It is at this temperature that the transition of gaseous water vapor to a liquid state begins. This transition is called moisture condensation. During condensation, additional thermal energy is released, which at one time was spent on the evaporation of water and in conventional gas boilers is simply thrown into the pipe along with other combustion products. This energy is transferred through the secondary heat exchanger to the so-called return, i.e. water entering the boiler after passing through all the radiators. Note that the movement of the coolant in the condensing boiler is carried out in the direction opposite to the movement of exhaust gases.

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And the condensate flows down the heat exchanger and is collected in a special tank. And here the problems begin. The fact is that the chemical composition of the condensate is far from pure water. It contains, albeit in a diluted form, several inorganic acids: nitric, carbonic, sulfuric. The temperature of the condensed moisture is quite high, which makes it very aggressive, and above all, to the heat exchanger on which it condenses. Therefore, the material from which the heat exchanger is made must be resistant to condensate.

Most often, the secondary heat exchanger is made of silumin, which is an aluminum-silicon alloy, or stainless steel.

The choice of material for the secondary heat exchanger

Silumin heat exchanger is characterized by lighter weight and better thermal conductivity. These qualities of the material make it possible to produce compact heat exchangers of high power from silumin. The heat exchanger, as a rule, is made cast, since the welds are a weak point — it is there that the process of corrosion destruction of the material begins in the first place.

Since silumin heat exchangers are assembled from cast sections, they are easier to manufacture and more technologically advanced, which allows experimenting with their shape to optimize the movement of flue gases and coolant, in order to further increase the efficiency of the boiler. The high anticorrosive properties of silumin are due to aluminum, which, when combined with oxygen, forms a layer of aluminum oxide on the metal surface, which reliably protects the heat exchanger from corrosion.

Moreover, silumin heat exchangers have a higher water content and lower hydraulic resistance. As a result, they are more hydraulically stable and do not impose such high demands on the circulation of the heat carrier as stainless steel heat exchangers. However, condensing boilers with silumin heat exchangers are more expensive.

Features of operation of condensing boilers

During the operation of condensing boilers, acid condensate is formed. Of course, condensate is also formed during the operation of conventional boilers, but most often the owners of the equipment do not know or do not think about its existence, because. almost all of it flies into the pipe. Although it would not hurt to think about it — this condensate, getting into the atmosphere, leads to the formation of acid rain in places where conventional boilers are concentrated.

But the owners of condensing boilers willy-nilly have to think about what to do with condensate. And, by the way, there is a lot of it. So a condensing boiler with a power of less than 30 kW collects about 30 liters of condensate for a period of operation equal to 1 day. Most often, the accumulated condensate is simply drained into the sewer. According to the rules for operating boilers, it is possible to drain condensate without neutralizing it from boilers with a power of up to 150 kW, i.e. for most household models. Otherwise, you need to install a neutralizer (a container with alkaline materials, such as magnesium and potassium granules) and periodically change it, which, of course, causes some inconvenience, but it is quite within the power of everyone.

By the way, conventional boilers also solve the problem of condensate, only differently than condensing ones. In convection boilers, everything is done to bring as much condensate as possible into the pipe so that it does not accumulate on the heat exchanger and does not destroy it. To do this, the exhaust gases are released «into the pipe» at a temperature of 140-160 ° C, when they are cooled to lower temperatures in the chimney, the draft decreases, and condensate begins to accumulate in the boiler, for which a conventional boiler is absolutely not ready.

The temperature of the gases at the outlet of the condensing boiler is much lower (only 10-15°C higher than the coolant temperature), because almost all of the heat energy is utilized in the boiler. Therefore, exhaust gases from it can be removed not only up to the roof, but also through the wall, and the chimney itself can be made of a smaller diameter and made of plastic.

It should be remembered that the choice of material for the chimney of a condensing boiler must be stricter than for a conventional one. After all, all the moisture that has not condensed on the heat exchanger in the boiler will definitely condense in the chimney and begin to corrode it. Therefore, the chimney for a condensing-type boiler is made of an acid-resistant material — stainless steel or plastic, and its horizontal sections are given a slight slope so that the water formed during the condensation of the steam that enters the chimney drains back into the boiler under the action of gravity.

Another significant feature of the condensing boiler is its increased sensitivity to the quality of the air entering the combustion chamber. The presence of a noticeable amount of dust in the air leads to rapid wear of the fan. By the way, this is typical for all boilers with a closed burner, not only the condensing type. But not only the dust content of the air affects the efficiency of the gas boiler, but also its temperature.

As practice shows, in cold weather, the air supply channel can freeze over, since the temperature of the exhaust gases in the condensing boiler is quite low, and they are not able to sufficiently warm the walls of the chimney. This leads to a decrease in the supply of oxygen necessary for the combustion of fuel, and, as a result, to a decrease in the efficiency of the boiler itself. Therefore, not all models of condensing boilers will be able to work in particularly harsh climatic conditions.

As you can see, designing a heating system using a gas condensing boiler is not an easy task, it definitely requires the help of specialists.

Operating modes of a condensing gas boiler

Only specialists will be able to design home heating systems in such a way that it works in the most efficient way. First of all, it is necessary to select the operating mode of the boiler, on which almost all parameters of the heating system will depend.

At different times of the day, a different amount of heat is required from the heating boiler, which can be regulated using a gas burner. The burners of most modern condensing gas boilers are modulating, i.e. with the ability to smoothly change the power of the boiler during operation. Burners, as a rule, have a wide range of modulation of thermal power within 20-100%. This makes it easy to maintain the required temperature in the house and save expensive fuel.

It should be noted that modulated gas burners are also installed on many models of conventional boilers.

A convection gas boiler usually operates in 80/60 or 75/60 ​​mode. Where 75 is the temperature of the coolant at the outlet of the boiler, equal to 75 ° C, and 60 is the temperature of the coolant 60 ° C entering the boiler after passing through all the radiators (the so-called return flow). If the return temperature is lower, condensate will begin to accumulate in the boiler, for which a conventional boiler is not designed at all, because. the dew point (moisture condensation temperature) is around 58°C.

Another thing is a condensing type boiler. It is most effective just at low-temperature operating conditions, tk. condensation in it is only welcome. In condensing boilers, the temperature of the coolant is specially lowered to the 50/30 mode (output — 50 ° C, return — 30 ° C). It is in this mode that the maximum extraction of heat from the combustion products of gas and heat occurs due to the condensation of water vapor. And here the question arises of how to design a heating system so that the boiler can operate in a mode close to 50/30, while the owners receive as much heat as they need.

It is clear that when using a conventional boiler, it is best to heat the house with radiators. But for a condensing boiler, many suggest using a “water-heated floor” system. It is with the help of water-heated floors that it is possible to make a condensing-type boiler use energy from the condensation of water vapor throughout the entire heating period and ensure maximum gas savings.

But if for some reason you cannot install a water heated floor, a condensing boiler will still be a good solution for arranging a heating system. Let’s try to prove it. In most of the country’s territory and adjacent states, severe cold lasts for a relatively short time in winter — up to 10 days. Yes, at this time you will have to “drive” your condensing boiler in the usual high-temperature mode, without condensation, which is quite acceptable, because. The efficiency of this boiler will still be at the level of 93-97%. This is more than the best models of traditional boilers. But in the rest of the winter, the condensing boiler will be able to operate at a lower temperature regime and in partial condensation mode.

Experts offer little tricks that allow you to adjust the operating mode of the condensing boiler so that the efficiency is kept above 100%, and save on fuel, and provide heat in the house. For example, initially design a heating system not for the optimal 50/30 mode, but for the 60/40 mode. Then the gas consumption will be a little more, but you won’t have to freeze in the cold either.

There are other options — for example, install additional radiators in the house, with a useful surface area increased by 2.5-3 times, designed for a coolant temperature of no higher than 50 ° C. But in this case, capital costs will increase. Although operating costs for fuel will decrease. Which is better is up to you.

Cascade boiler plants

Often, the consumer faces a difficult choice — to purchase one high-power heating boiler or install several low-power boilers interconnected (the so-called cascade). Recently, it is the latter option that has become popular. The fact is that cascade installations have a number of advantages:

• with their help, a high level of redundancy of the heating system is provided — in case of a malfunction of one or even several boilers;

• with the help of a cascade, an increase in the power modulation range of the entire installation is achieved, which allows it to be tuned to optimal operating modes.

Today, installations of several wall-mounted condensing gas boilers in a cascade can be found more and more often. Especially in the segment of medium and large capacity equipment, where condensing boilers successfully compete with traditional boilers. Most often, cascade units are used for heating commercial premises, residential rooftop boiler rooms and other facilities where it is important to provide the highest output of the heat source in the smallest space.

Why condensing gas boilers are the future

In our age, when the cost of fuel is increasing every year, the future lies in energy-efficient solutions, and here gas condensing boilers are at their best — both in terms of their efficiency above 100%, and in many other characteristics. Of course, like everything else in this world, there are also disadvantages to condensing gas boilers, which include the higher price of these devices compared to convection models. But the advantages of a condensing gas boiler are so great that they easily outshine all the disadvantages. Although once again we will make a reservation that conventional gas boilers are also a very energy-efficient and modern solution. Here are just a few advantages of condensing gas boilers over conventional gas boilers:

• Great fuel economy — up to 35% (more than a third) in 1 season. And this is not the limit, because. technology does not stand still. Therefore, condensing boilers are especially effective when using liquefied propane-butane gas, and not main gas.

• Even under the most unfavorable high-temperature conditions, the efficiency of condensing boilers remains higher than that of conventional ones.

• Emissions of harmful carbon dioxide and nitrogen compounds from a condensing boiler, on average, are 5-7 times less than those from a conventional one.

• For the arrangement of boiler rooms equipped with condensing boilers, such large areas are not needed as in the case of traditional boilers. And the total weight of the equipment for the «condensing» boiler room is lower.

• Compared to traditional blast boilers, condensing boilers have lower noise levels and less vibration.

• As a rule, with the correct setting of the boiler burner, the heat exchanger does not need to be cleaned. In addition, most of the surface of the heat exchanger is self-cleaned by the flowing condensate.

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Source: TopClimat.ru