How To Choose A Gas Boiler: Basic Criteria

Condensing (condensing) gas boiler
Where does the "extra" efficiency come from for condensing boilers?
Characteristics of condensing boilers
Comparative overview of gas boilers
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Why are gas boilers most popular with homeowners? The answer is simple - there is no more profitable source of energy used for heating a house and heating water for domestic needs than mains natural gas today. So, all that remains is to choose a gas boiler, guided by … what? Price, admonitions of sellers, motley recommendations of relatives and friends, brand recognition? Description of condensing boilers, recommendations for choosing a conventional gas boiler, a comparative analysis of several models in order to determine the best among them - in this article.

Condensing (condensing) gas boiler

We will return to the choice of conventional gas boilers later, but for now we will consider one interesting subspecies of them - condensing boilers, whose manufacturers promise an efficiency of over 100%. How is it even possible - the efficiency is more than 100%, because it contradicts the fundamentals of physics?

An ordinary gas boiler, the models of which are widespread in the CIS, transfers the thermal energy obtained as a result of burning natural gas to the heat carrier of the heating system through a steel or cast iron heat exchanger. However, the resulting flue gases and water vapor, whose temperature is about 120 ° C, are not used for heating purposes. Heat from steam in an ordinary boiler cannot be transferred to the coolant, since the moisture contained in it is extremely harmful to its structure, since when it cools and condenses, corrosive wear is inevitable, a significant deterioration in the quality of gas combustion and condensate deposition on the surfaces of the chimney. Losses of thermal energy escaping along with steam account for about 10% of the total productivity of a gas boiler per unit of time.

The principle of operation of a gas boiler

The principle of operation of a conventional gas boiler: 1 - gas supply; 2 - atmospheric burner; 3 - coolant return; 4 - outlet of the heated coolant; 5 - products of combustion

Water vapor contained in flue gases condenses at certain coolant temperatures - less than 57 ° C. The lower the temperature of the coolant, the more intense the condensation takes place, respectively, the more heat energy is taken from the water vapor. The maximum efficiency of a condensing gas boiler is achieved at the lowest temperature of the coolant in the return circuit of the heating system. For example, with a temperature difference between supply and return of 15 ° C (supply 80 ° C, return 65 ° C), the efficiency of a condensing boiler will be no more than 97%, that is, a particularly noticeable difference with the efficiency of a conventional gas boiler (93–95 %) will not be. To achieve the highest performance, the condensing boiler must be installed in low-temperature heating systems, i.e. the optimal temperature difference between supply and return is 50 and 30 ° C, respectively. Only in this case is a boiler efficiency of about 107% achieved with a simultaneous saving of natural gas of about 17% due to an atmospheric burner.

Condensing boiler operation diagram: 1 - burner air; 2 - gas supply; 3 - coolant outlet; 4 - primary heat exchanger; 5 - recuperative heat exchanger; 6 - coolant supply; 7 - smoke exhauster; 8 - condensate drain

Condensing gas boilers, capable of extracting heat energy from water vapor, are equipped with a special heat exchanger-recuperator, in which this vapor condenses and gives off heat to the water circulating in the heating system. In the design of such boilers, heat exchangers of two types are used - plate (finned) and tubular.

The plate heat exchanger is a honeycomb structure formed by thin metal panels welded together with a 90 ° rotation between each pair of panels. Plate recuperators are usually large in size, therefore they are most often used in large-sized condensing boilers for industrial and domestic use. A tubular recuperator consists of several twisted tubes with thin walls - in each of them a vortex flow of gases is formed, pressing against the walls, which makes it possible to increase the intensity of heat exchange and simultaneously reduce the resistance of the channel through which the flue gases pass. By the way, it is tubular recuperators that are used in supply valves built into airtight plastic window frames. Due to its small size and sufficiently high efficiency,tubular heat exchangers are widely used in the construction of domestic condensing boilers.

Condensing tube heat exchanger

The design of the condensing boiler includes two heat exchangers - primary and recuperative. The primary heat exchanger is made of cast iron or steel, it is identical to the heat exchangers of conventional gas boilers, it is able to take away most of the heat generated by the burner flame. A plate or tubular recuperator is installed along the path of flue gases inside the boiler body - it extracts the remaining heat energy by condensing water vapor at a coolant temperature below the dew point. The coolant passing through the pipes inside the recuperator absorbs heat energy, and the resulting liquid condensate accumulates on the walls of the heat exchanger and flows into a special pan, from there - through a plastic tube into the sewer. Due to the high chemical aggression of condensate, recuperators and trays are made of materials resistant to chemical corrosion, such as stainless steel and silumin (an aluminum-silicon alloy).

Floor standing gas condensing boiler design

The daily volume of released condensate approximately corresponds to the capacity of a condensing boiler - for example, a 20 kW boiler will release about 20 liters of condensate per day. In European countries, strict standards apply to such condensate, due to its chemical aggression, it is necessary to install neutralizing filters between the boiler drain and the sewage system containing calcium or magnesium granules. At the moment, in Russia, there are no requirements for the discharge of condensate from condensing gas boilers.

Where does the "extra" efficiency come from for condensing boilers?

Thermal energy generated by gas boilers was traditionally calculated from the combustion of fuel (lower heat), i.e. the heat that goes away with water vapor was not taken into account, since it was impossible to extract it before the appearance of condensing boilers. The difference in efficiency between conventional and condensing boilers is approximately 10–15% - hence the advantages of the latter type of boilers, the efficiency of which is calculated from the higher heat, taking into account the heat of condensation.

In reality (from the point of view of higher heat) the efficiency of conventional gas boilers does not exceed 80%.

Characteristics of condensing boilers

There are many positive aspects of gas boilers of this type:

high power in a compact size. For example, wall-mounted gas boilers of a conventional design cannot have a higher power than 35 kW, while wall-mounted condensing boilers are limited to 120 kW;
heat loss does not exceed 2% of the total generated heat energy;
a burner of a special design allows you to more accurately control the modes of activation and shutdown of the boiler, as a result of which fuel economy is achieved;
low operating temperature allows them to be used in heating systems such as underfloor heating and baseboard heating - condensing boilers are ideal for a two-circuit system;
emissions of thermal energy and harmful substances into the atmosphere are minimized;
the use of high-quality materials in the construction of heat exchangers doubles the service life of such boilers compared to conventional ones;
at the same capacity, condensing boilers take up less space than conventional gas boilers.

The highest efficiency of condensing boilers is observed in heating systems of buildings with a large area - from 200 m ². To heat such a house, a powerful boiler with a high consumption of natural gas is required, and in this case a condensing boiler with its high efficiency and economical fuel consumption is simply irreplaceable.

Disadvantages of condensing boilers:

the need for a sealed chimney with forced exhaust. At the same time, it should be noted the flexibility of boilers of this design with respect to the outlet of the chimney channel, and not only for floor-standing models, but also for wall-mounted ones. In conventional gas boilers, the chimney outlet is limited to a strictly defined point;
dependence on electricity - in its absence, the boiler will automatically turn off, stopping the burner and the supply of natural gas;
the highest efficiency of a condensing boiler is achieved in low temperature heating systems. In conventional heating systems, with a coolant temperature within 80 ° C, the efficiency of such a boiler is no different from that of a conventional gas boiler, since there will be no secondary heat intake;
high cost due to expensive materials used in the structure (cheaper materials cannot withstand chemical corrosion caused by condensation). Condensing boilers are priced at double that of conventional boilers.

Private house boiler room with gas condensing boiler

Condensing gas boilers are not produced in Russia, mainly European products of Italian companies Baxi, Ferroli and Hermann, German Vaillant, Slovak Protherm are presented.

Comparative overview of gas boilers

Let's consider three models of gas boilers of the "usual" type and choose the best boiler for purchase. Below we will analyze the characteristics of floor-standing boilers of three manufacturers - Baxi (Italy), Protherm (Slovakia) and JSC "ZhMZ" (Russia). The thermal power of the models described is approximately 16 kW.

Specifications Baxi Slim 1.150 i Protherm Bear 20 TLO Zhukovsky AOGV-17.4-3 "Comfort" Thermal performance Rated power, kW 14.9 17 17.4 Heated area, m ² 130 140 140 Efficiency,% 90 90 88 Maximum water temperature in the heat exchanger, ° C 85 85 90 Operating temperature ranges First stage (radiator heating), ° C 30–85 up to 85 up to 90 Second stage (warm floor), ° C 30–45 no no Ignition Ignition type electric piezoelectric element piezoelectric element Energy consumption Power supply frequency, Hz 50 - - Power consumption, W 120 - - Control Working thermostat electronic mechanical mechanical Safety switch there is there is there is Automatic shutdown of the gas supply when the flame is extinguished there is there is there is Automatic shutdown when overheating there is there is there is Traction sensor there is there is there is General characteristics Type of shell vertical monoblock vertical monoblock vertical monoblock Body material cast iron and steel cast iron and steel steel Body shape rectangular rectangular rounded type of instalation floor floor floor Combustion chamber type open open open Heat exchanger material cast iron cast iron steel Burner type atmospheric atmospheric atmospheric Fuel type (with replacement of a burner with a fan) G20 / G30 G20 / G30 G20 / G30 Fuel inlet pressure G20 (gaseous methane), mbar 20 13–20 6.5-18 Inlet pressure of fuel G30 (liquefied butane), mbar thirty 29 20-36 Maximum fuel consumption G20, m ³ / h 1.74 2.14 1.87 Fuel consumption G30, kg / h 1.12 1.8 1.3 Number of contours one one one Heat carrier water water water / antifreeze Electronic flame modulation there is no no Complete hydraulic instruments (pump, expansion tank, pressure gauge) there is no no Heat exchanger capacity, l nine 9.1 - Noise level at a distance of a meter from the boiler, dB - no more than 55 - Storage boiler capacity, l additional option additional option 64 Flue gas discharge method natural draft chimney natural draft chimney natural draft chimney Chimney diameter, cm eleven thirteen 13.5 Dimensions HxWxD, cm 85x35x52 88x42x60 105x42x48 Weight (without water), kg 89 90 49 Brand owner BAXI SpA, Italy Protherm Production sro, Slovakia JSC "Zhukovsky Machine-Building Plant", Russia Country of origin Italy Slovakia Russia Warranty, years 2 2 3 Approximate service life, years fourteen fourteen fourteen Other options Frost protection there is no no Timer additional option no no Average cost, rub. 40,000 38,000 17,000

Baxi Slim 1.150 i

Let's analyze the data collected in the table:

boilers of the Baxi and Protherm brands are equipped with a reliable and durable cast-iron heat exchanger, the boiler of JSC "ZhMZ" - a cheaper and less reliable steel one;
The ignition of the reduced boilers of the Protherm brands and JSC "ZhMZ" is carried out by a piezoelectric element, that is, without electricity consumption;
the electronic control of the Baxi boiler is able to independently and accurately adjust the operation of the atmospheric burner, adjusting to the data of temperature sensors in the room and outside the building. For this reason, this boiler has the lowest gas consumption among the models described;
among these models, only the boiler manufactured by JSC "ZhMZ" has a built-in boiler for heating water for domestic needs;
the automatic mode of turning on and off the heating boiler is present only in Baxi, two other boilers with piezo ignition can only turn off when the pressure in the gas supply drops, but they cannot turn on independently;
the cost of the Zhukovsky Machine-Building Plant boiler is half that of the models of European manufacturers, moreover, the AOGV-17.4-3 Comfort boiler is equipped with a Honeywell mechanical automation system made in Germany, which will last much longer and more efficiently than a set of domestically produced automatic equipment;
each of the boiler models presented in the review is capable of operating both on gaseous fuel (methane) and liquid (propane, butane), however, for the second case, a fan burner will be required, the cost of which, depending on the power, will be at least 15,000 rubles;
The gas boiler of ZhMZ OJSC presented in the table overview is capable of operating at half the pressure in the natural gas supply network - the situation with a pressure drop in the gas pipeline is common in Russia. While imported boilers will stop working when the pressure in the gas pipeline drops below 13 mbar;
only the AOGV-17.4-3 "Comfort" boiler allows using two types of coolant in the heating system - water or antifreeze (however, there are a number of factory requirements for antifreeze brands).

Protherm Bear 20 TLO

Conclusion: according to the results of the review, the AOGV-17.4-3 "Comfort" model produced by JSC "ZhMZ" has an undoubted advantage - moreover, it is the "comfort" class model, and not the cheaper "economy" or "standard" (the price difference is about 3000-4000 rubles), equipped with German automatics. The boilers of Zhukovsky Machine-Building Plant, although not equipped with a cast-iron heat exchanger, are most adapted to unstable pressure in the gas pipeline, they are more suitable for use in heating systems with a coolant temperature of 80 to 90 ° C.

Zhukovsky AOGV-17.4-3 "Comfort"

While natural gas is relatively cheap and available in Russia, gas boilers are and will be in constant demand. When choosing one or another boiler model, it is necessary to determine in advance the type and source of fuel - main or autonomous gas supply, the type of heating system - conventional (heating the coolant to 90 ° C) or low-temperature (with water heating no more than 45 ° C). An equally important initial value for choosing a boiler will be the heating area - if it exceeds two hundred square meters, then it is more profitable to purchase an economical and high-performance condensing boiler, albeit at a higher price than conventional gas boilers.