Condensing gas boiler: specifics of operation, pros and cons + difference from classic models

Sellers of condensing-type heat generators claim that the efficiency of the innovative equipment offered to us exceeds 100%. But you must admit, this slightly contradicts the law of conservation of energy, familiar to all of us from our school physics course. So what's the mystery?

On the one hand, such statements are a ploy by marketers. However, on the other hand, there is a grain of truth in their assurances that convince the buyer. We will analyze in detail how a condensing boiler works: the advantages and disadvantages, its specific operation and design deserve detailed study.

In order to get a complete understanding of the condensing type of equipment, let’s compare it with the classic type of thermal energy generator. Here are the features of its connection and operation. Let's reveal the secrets of ultra-high performance.

Gas condensing boiler

The high efficiency of a condensing gas heat generator is ensured by the presence of an additional heat exchanger in its design. The first standard heat exchange unit for all heating boilers transfers the energy of the burned fuel to the coolant. And the second one adds to this the heat from exhaust gas recovery.

Condensing boilers operate on “blue fuel”:

• main (mixtures of gases with a predominance of methane);
• gas tank or cylinder (a mixture of propane and butane with a predominance of either the first or second component).

It is acceptable to use any type of gas.The main thing is that the burner is designed to work with one type of fuel or another.

Condensing gas boilers are more expensive than conventional convection models, but they beat them in fuel costs by reducing gas consumption by 20–30%

The condensation heat generator shows the best efficiency when burning methane. The propane-butane mixture is slightly inferior here. Moreover, the higher the proportion of propane, the better.

In this regard, “winter” gas for a gas tank gives a slightly higher output efficiency than “summer” gas, since the propane component in the first case is higher.

Unlike a condensing gas boiler, in a convection boiler, part of the thermal energy goes into the chimney along with combustion products. Therefore, classical designs have an efficiency of around 90%. It is possible to raise it higher, but it is technically too difficult.

Economically this is not justified. But in condensers, the heat obtained from gas combustion is used more rationally and completely, since the heat released during steam processing is accumulated and transferred heating system. This additionally heats up the coolant, which makes it possible to reduce fuel consumption per 1 kW of heat received.

Design and principle of operation

The design of a condensing boiler is in many ways similar to its convection counterpart with a closed combustion chamber. Only inside it is supplemented with a secondary heat exchanger and a recovery unit.

The main features of the condensation heat generator design are the presence of a second heat exchanger and a closed combustion chamber with a fan

A gas condensing boiler consists of:

• closed combustion chambers with modulating burner;
• primary heat exchanger No. 1;
• exhaust gas cooling chambers up to +56–57 ⁰C (dew point);
• secondary condensation heat exchanger No. 2;
• chimney;
• air supply fan;
• condensate tank and its drainage system.

The equipment in question is almost always equipped with a built-in circulation pump for coolant. The usual option with a natural flow of water through heating pipes is of little use here. If the pump is not included in the kit, then it will definitely need to be provided when preparing the boiler piping project.

Additional percentages of efficiency in a condensing boiler are formed as a result of heating the return by cooling the exhaust gases in the chimney

Condensing boilers on sale include single-circuit and double-circuit, as well as in floor and wall versions. In this they are no different from classical convection models.

The operating principle of a condensing gas boiler is as follows:

1. The heated water receives the main heat in heat exchanger No. 1 from gas combustion.
2. Then the coolant passes through the heating circuit, cools and enters the secondary heat exchange unit.
3. As a result of condensation of combustion products in heat exchanger No. 2, the cooled water is heated up using recovered heat (saving up to 30% of fuel) and goes back to No. 1 in a new circulation cycle.

To precisely control the flue gas temperature, condensing boilers are always equipped with a modulating burner with a power range from 20 to 100% and an air supply fan.

Nuances of operation: condensate and chimney

In a convection boiler, the combustion products of natural gas CO₂, nitrogen oxides and steam are cooled only to 140–160 ⁰C. If you cool them below, the draft in the chimney will drop, aggressive condensation will begin to form and the burner will go out.

In this development of the situation, all manufacturers classic gas heat generators strive to avoid in order to maximize operational safety, as well as extend the life of their equipment.

In a condensing boiler, the temperature of the gases in the chimney fluctuates around 40 ⁰C. On the one hand, this reduces the requirements for heat resistance of the material chimney pipe, but on the other hand, it imposes restrictions on its choice in terms of resistance to acids.

When cooled, exhaust gases from a gas boiler form aggressive, highly acidic condensate, which easily corrodes even steel

Heat exchangers in condensing heat generators are made from:

• stainless steel;
• silumin (aluminum with silicon).

Both of these materials have enhanced acid-resistant characteristics. Cast iron and ordinary steel are completely unsuitable for condenser tanks.

The chimney pipe for a condensing boiler may only be installed from stainless steel or acid-resistant plastic. Brick, iron and other chimneys are not suitable for such equipment.

During recovery, condensate is formed in the secondary heat exchanger, which is a weak acid solution and must be removed from the water heater

When operating a condensing boiler with a power of 35–40 kW, about 4–6 liters of condensate are formed. Simplified, it comes out to about 0.14–0.15 liters per 1 kW of thermal energy.

In fact, this is a weak acid, which is prohibited from being poured into an autonomous sewer system, as it will destroy the bacteria involved in waste processing. And before discharging into a centralized system, it is recommended to first dilute with water in a ratio of up to 25:1.And then you can remove it without fear of destroying the pipe.

If the boiler is installed in a cottage with a septic tank or VOC, then the condensate must first be neutralized. Otherwise, it will kill all microflora in the autonomous treatment system.

The “neutralizer” is made in the form of a container with marble chips with a total weight of 20–40 kg. As the condensate from the boiler passes through the marble, its pH increases. The liquid becomes neutral or low-alkaline, no longer dangerous for bacteria in the septic tank and for the material of the sump itself. The filler in such a neutralizer needs to be changed every 4–6 months.

Where does the efficiency above 100% come from?

When indicating the operating efficiency of a gas boiler, manufacturers take as a basis the indicator of the lower calorific value of gas without taking into account the heat generated during condensation of water vapor. In a convection heat generator, the latter, together with approximately 10% of the thermal energy, is completely lost to chimney pipe, so it is not taken into account.

However, if you add up the condensation secondary heat and the main heat from the burned natural gas, you will get just over 100% efficiency. No scam, just a little tricky numbers.

When calculating the efficiency based on the higher calorific value of a convection boiler, it will be around 83–85%, and for a condensing boiler it will be about 95–97%

In essence, the “wrong” efficiency above 100% arises from the desire of manufacturers of heat generating equipment to compare comparable indicators.

It’s just that in a convection device “water vapor” is not considered at all, but in a condensation device it must be taken into account. Hence the slight discrepancies with the logic of basic physics taught at school.

Pros and cons of a condensing heater

The advantages of a condensing boiler include:

1. Reduction of harmful emissions by 60–70% (most of the carbon dioxide and nitrogen oxides go into condensate).
2. Compared to convection models, savings of up to 30% of gas fuel per 1 kW generated.
3. Smaller dimensions of gas heating equipment with the same power.
4. Low temperature of combustion products in the chimney (only about 40 ⁰WITH).
5. Possibility of installing a cascade of several boilers.
6. Versatility (suitable for both heating radiators and “warm floors”).
7. The presence of smart automation and complete autonomy of the gas heat generator without human intervention.

A cascade system of two or three heat generators allows you to install low-power boilers that make less noise and vibrate during operation than more powerful models.

This simplifies the installation of the entire heating system and allows for reduced dimensions. home boiler room. Plus, thanks to the possibility of more flexible regulation of the heat generation process, the overall efficiency of using heat-generating equipment increases.

The costs of a condensing boiler, compared to a conventional convection boiler, are recouped in 5–6 years due to savings on natural gas

The disadvantages of condensing heat generators include:

1. High price tag for equipment (1.5–2 times higher than classical convection-type models of similar power).
2. Problems with condensate disposal.
3. Reduced efficiency when using the boiler in high-temperature heating systems.
4. Energy dependence - the fan, automation and circulation pump require electricity to operate.
5. Prohibited use with antifreeze.

Despite the significant initial costs, a condensing boiler is quite justified from an economic point of view. During operation, it more than returns all the money initially spent.

In Russia, such equipment is still not widespread. A gas boiler with recovery is still too unusual and little studied in our market. But interest in such heat generators is gradually growing.

Conclusions and useful video on the topic

How does a condensing heat generator work:

Construction of gas boilers with water vapor recovery:

All the advantages of condensing boilers:

If you carefully understand how and by what principles a gas condensing boiler works, then at first glance the “incorrect” 108–110% efficiency becomes quite understandable and justified by the figures.

A heat generator with exhaust gas recovery is indeed more efficient compared to the classic design. Its only serious drawback is highly acidic condensate, which must be disposed of somewhere.

Please write comments in the block form below. It is possible that you have information that can supplement the stock of information presented in the article. Ask questions, share your own experience in choosing and operating condensing-type boilers, post photos on the topic of the article.