How to choose a gas boiler: basic criteria

Recommendation points

• Condensing (condensing) gas boiler
• Where does the “extra” efficiency come from for condensing boilers?
• Characteristics of condensing boilers
• Comparative overview of gas boilers

Today in our article we will talk about condensing boilers. Consider the design, the principle of operation, their pros and cons, by what parameters, in addition to the price, such boilers should be chosen. Let’s review three brands of gas boilers with the definition of the best.

Why are gas boilers most popular with homeowners? The answer is simple – there is no more profitable source of energy used for home heating and heating water for domestic needs than main natural gas today. So, it remains only 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 a conventional 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 are about 10% of the total productivity of a gas boiler per unit of time.

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

The water vapor contained in the flue gases condenses at certain coolant temperatures – less than 57 ° C. The lower the temperature of the coolant, the more intensive the condensation takes place, and accordingly, 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 heating medium 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 a boiler efficiency of about 107% is 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, two types of heat exchangers 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 their small size and rather 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 thermal energy, and the resulting liquid condensate accumulates on the walls of the heat exchanger and flows into a special tray, 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).

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 by the higher heat, taking into account the heat of condensation.

In fact (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 double-circuit system;
• emissions of thermal energy and harmful substances into the atmosphere are minimized;
• the use of high-quality materials in the design 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 large buildings – from 200 m². 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 in relation to the outlet of the chimney channel, and not only for floor models, but also for wall 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 boiler prices are double the cost of conventional boilers.

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 described models 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, m2	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	not	not
Ignition
Ignition type	electric	piezoelectric element	piezoelectric element
Energy consumption
Power supply frequency, Hz	50	–	–
Consumed electricity, 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 (methane gas), mbar	20	13–20	6.5-18
Inlet pressure of fuel G30 (liquefied butane), mbar	thirty	29	20-36
Maximum fuel consumption G20, m3/ 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	not	not
Complete hydraulic instruments (pump, expansion tank, pressure gauge)	there is	not	not
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	13	13.5
Dimensions HxWxD, cm	85x35x52	88x42x60	105x42x48
Weight (without water), kg	89	90	49
Brand owner	BAXI S.p.A., Italy	Protherm Production s.r.o., 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	not	not
Timer	additional option	not	not
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” is equipped with a cheaper and less reliable steel one;
• ignition of the reduced boilers brands Protherm 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 produced 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, and 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 domestic-made automatic equipment set;
• 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 the use of 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 model of the “comfort” class, and not the cheaper “economy” or “standard” (the price difference is about 3000 –4000 rubles), equipped with German automatics. The boilers of the 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 decide in advance on the type and source of fuel – main or autonomous gas supply, the type of heating system – normal (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.