Condensing gas boiler - how does it work?

How does a condensing boiler work? How does a gas condensing boiler work and what does the word "condensing" mean?

From the beginning... in the 20th century, gas heating was not yet popular, among other things, due to the lack of transmission infrastructure. Despite this, there was technology for gas boilers, which were used and improved. Their principle of operation is obvious: heat energy was generated from the combustion of gas, which, like any fuel, has a certain calorific value. The heat energy generated in this way heated water through a heat exchanger, which then reached our heat emitters - radiators, less commonly underfloor heating, which only became popular later. Even then, gas heating was considered to be significantly less maintenance-intensive or maintenance-free, as well as cleaner and more environmentally friendly compared to boilers using traditional fuels. However, these were not condensing boilers, and the phenomenon of "condensation" did not occur in them.

At the beginning of the 21st century, the technology of gas boilers was improved by adding the condensation phenomenon - making them much more efficient and cheaper to operate (they could consume less fuel while achieving higher efficiency). In a traditional gas boiler, heat energy was generated by burning gas, after which the gas fumes traveled into the atmosphere. They were themselves also a heat carrier because they often had a temperature higher than 100-110°C. The development of gas technology aimed to recover at least partially the heat energy that was escaping into the atmosphere in this way. Thus, condensation technology was created, which consists of condensing flue gases within a single heating device, a boiler of small dimensions and affordable for a larger part of the population, in addition to burning gas (and thus obtaining heat energy), condensation of flue gases occurs with the help of water, which also increases its temperature thanks to this process. In this way, heat exchange occurs between water and gas flue, as a result of which the flue gases no longer have such a high temperature as in traditional boilers. In condensing boilers, flue gases have only between 35 and 50°C (making them more environmentally friendly, which will certainly please supporters of the thesis that humans contribute to climate warming). However, thanks to recovering heat from the flue gases, the boiler consumes less gas to achieve the same results. This can be felt in the wallet. This is what condensation is all about.

Some numbers for comparing condensing and traditional gas boilers

Traditional boilers, in the best, textbook conditions, were supposed to achieve efficiency that was conventionally defined as 100% at the end of the 20th century. However, in practice, their efficiency rarely approached these 100% and ranged from 80-90%. Over time, with the operation of the device, it decreased even further, resulting in increased fuel consumption. How did the new generation boiler, i.e., condensing, compare to this? It usually exceeded this 100%, and often the efficiency reached even 110-111%.
In practice, compared to an old-type boiler, fuel consumption is about 15-20% lower. Considering that old gas boilers are today mostly worn-out devices, replacing them with a new condensing boiler can reduce gas consumption by almost 1/3.

Comparison of boiler prices

Of course, newer technology always means higher costs, and unfortunately, a condensing boiler is about 40-60% more expensive. It is increasingly difficult to find today conventional, non-condensing gas boilers (i.e., boilers with an open combustion chamber), but it is possible - their price starts from about 2200-2300 PLN gross. However, given today's gas prices, it is worth considering whether this is really a saving since using such a boiler will annually consume 15-20% more fuel. In some countries, due to uneconomical gas/fuel consumption, the installation of outdated gas boilers has already been banned - in Poland, they can still be offered, but this only applies to earlier production. It is no longer allowed to introduce new models to the market, so it can be said that this technology, as uneconomical, has been de facto blocked by the state (the impulse came from the European Union, which issued a special directive in this matter).

Ba-12-30

NT Ba-12-30

74,99 €

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Ba-12-40

NT Ba-12-40

89,99 €

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Ba-23-30

NT Ba-23-30

104,99 €

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Combustion chamber in a gas boiler

Boilers with the possibility of condensation, unlike older counterparts, have a so-called closed combustion chamber. This means that air (necessary for the combustion process) cannot be drawn from the room in which they are located - which many consider an advantage. An advantage - because from the point of view of air in the house, such a boiler is neutral, it does not affect its quality or composition. However, this requires bringing air into the chamber from outside the building, which is usually done through ventilation ducts. This means that in the place where we plan to place the boiler, this possibility must be anticipated. Additionally, the boiler must dispose of residual flue gases by connecting to the chimney. Residue condensation products will be discharged directly into the sewer system.

Does a modern gas boiler room require ventilation?

Definitely, regardless of the type of gas boiler, yes. It is a matter of safety. Boilers with a closed combustion chamber are indeed considered safer, but as a precautionary measure, ventilation should be applied. In the event of any installation leaks or other failures (we must think in terms of years), this will protect against high gas concentrations in the boiler room (which could even lead to an explosion).

What if I have an old coal or solid fuel boiler that I want to keep - and besides that, add a gas boiler? In an open or closed system?

Very common decision - we want maintenance-free heating, but we are afraid that during severe winters it will cost us a lot. Gas is a fuel susceptible to political tensions, as shown by the recent price jumps.
In such a case, the system is often divided into 2 parts, creating the so-called open and closed system. Briefly explaining, the open system is the part of our heating system that has contact with air at a certain point because it has an open expansion tank at its highest point. This tank is filled with water from the installation but not to the top, and in case of increased pressure, the water level rises in it (but it is designed in such a way that water is only released in case of overheating of the system, a dangerous pressure spike in the installation, which is neutralized in the open tank). The remaining part of the system - i.e., the closed system - is literally closed, without an open element. Instead of an open expansion tank, it has a closed pressure vessel, and for the protection of the installation - a safety valve, protecting against high pressures.

In such a case, it is advisable to leave the coal or solid fuel boiler in the open system because these heat sources, for various reasons, work more safely and better with an open-type expansion tank. Similarly, it is often recommended for the open part of the installation to work with a fireplace (if connecting a gas boiler with a fireplace with a water jacket). Why specifically in the open system? Because these heat sources are less controllable, less manageable. It is not possible to suddenly disconnect them from work and quickly cool them down like, for example, a gas boiler. They remain warm, so in case of overheating of the system and a dangerous pressure increase in the installation, the open tank will discharge excess water into the sewer and equalize the pressure in the installation, preventing its damage. 

Gas boilers, on the other hand, are recommended to operate in closed systems, just like electric boilers. 

How to connect both systems? How to connect the open system with the closed system within one central heating installation?

Systems of this type are connected by a so-called PHE, i.e., plate heat exchanger. This is a device that will transfer heat from the gas boiler system to the other side - without mixing the systems. Additionally, it will shorten the gas circuit, which will balance the pressures. This will positively affect the operation of the system. The correct selection of the heat exchanger is a prerequisite for its proper operation.

Should gas boilers always operate in closed systems?

No. This is only a fundamental principle resulting from practice in our country, but it has very sensible grounds, as mentioned above. The basic criterion for deciding whether a particular boiler should operate in a closed or open system is the manufacturer's recommendation. If we have specific plans regarding the location of the boiler in the installation - it is worth checking its specification before purchase. There are gas boilers designed for open systems and there are coal boilers designed for closed systems. However, this is less common in practice.

An additional advantage of a closed system for gas is the fact that contact between water in the installation and air over time contributes to corrosion. It is considered that a closed system with an expansion vessel is safer for delicate gas boilers.

Photo - Heat exchanger with thermal insulation designed for connecting closed and open systems. Photo - property of Nordic Tec

Old installation - and a condensing gas boiler

It can be done - although the saying "gas only in new construction" holds some truth. Gas is a relatively economical fuel only in new or modernized systems. It works best in insulated houses in new construction, as they require less thermal energy for heating (and have lower heat losses). Additionally, it is often said that gas boilers are super economically efficient in installations based on underfloor heating (also fully, without radiators), as in this case the medium is heated only to temperatures around 30-35°C, not to 50-60°C as in the case of radiators. There is a lot of truth in this. To the point.

An old installation, if we cannot modernize it at the moment, is often a source of various types of impurities and boiler scale. This is not good for the lifespan of our boiler. We recommend that in such a situation, you decide to separate the installation from the gas boiler by allocating a shorter circuit to it in a closed system, connected to the installation with a plate heat exchanger. This will protect our boiler and at the same time balance the pressures in the boiler operation (which will work somewhat similarly to a hydraulic clutch).

Hydraulic clutch - is it necessary if everything will work on one system?

Not always, often it is not necessary. Most boilers have a built-in pump that generates circulation. Despite the presence of circulation pumps in the installation, in the case of extensive systems, the pressures in individual parts of the system are not similar and interfere with the operation of a condensing boiler. The water circulation caused by the boiler in the case of too extensive installations does not solve the problem. Therefore, if such a problem occurs, you need to balance the operation of the system by allocating a short circuit for the boiler. Theoretically, this can be done, as described above, using a heat exchanger - but in this case, two separate systems that do not mix will be created. If there are no specific indications to install a plate heat exchanger (e.g., the desire to separate the boiler from an old, potentially not very clean installation, or dividing the system because of the intention to leave an old coal or solid fuel boiler in the heating installation) - you can then use a hydraulic clutch, which will divide the system but allow the water to mix. This will create a shorter quasi-circuit for the boiler, in which the pressure will balance, resulting in our boiler working efficiently and effectively.

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We recommend Nordic Tec heat exchangers, among which you will find a range of suitable models for connecting a gas boiler to the installation. For technical questions regarding the use of the heat exchanger in connection with a gas boiler, contact us.