Wall-hung boilers are divided into standard efficiency and condensing wall-hung boilers in respect of thermal efficiency.

Standard Efficiency Boiler

The classification of standard efficiency wall-hung boilers is also very diverse. If there is no fan, it is called an atmospheric natural exhaust type wall-hung boiler. This type of wall-hung boiler does not have forced exhaust and air intake by the fan, and relies on the natural convection circulation of the flue, which are not allowed to use in China by law since they are considered unsafe under some circumstance. In fact, it is a safety problem caused by improper installation. If installed properly, it is still very safe. The biggest feature of this kind of combi is that it is cheap and not bad in efficiency.

If equipped with a fan for forced exhaust, it is called an atmospheric forced exhaust wall-hung boiler. This type of wall-hung boiler is divided into European models and Japanese/ Korean models. The representative is European standard efficiency wall-hung boilers. The fan is located on the top of the boiler. It is an AC shaded pole motor. On Japanese/Korean models, the fan is located at the bottom left of the unit, and the fan is generally a DC inverter fan. These type of models are represented by Korean companies such as Kyungdong, Gidulami, Dacheng, and Rinnai wall-hung boilers in Japan. If components of this type of combi boilers are reliable, objectively speaking,  Japanese/Korean styles have higher combustion efficiency. Why? Because with fans at the bottom, air and gas are premixed to some extent before delivered for burning. The fans on Japanese/Korean models are usually DC frequency conversion fans which have more delicate air volume adjustment, making efficiency at small loads is higher than the European models. At present, this combustion method is commonly applied in large capacity gas water heaters.

As shown in figure below, the blower is on the bottom instead of on the top, and the DHW of this kind of boiler is produced through plate heat exchanger.

The boiler in the picture above show clearly that the fan is at the bottom, and Japanese/Korean boilers have this structure.

European models have also changed their fans to speed-regulating fans in recent years, but because the fan use shaded-pole motors, the adjustment range is not large. It is just a simple step-by-step adjustment, and cannot be adjusted infinitely variable speeds like Japanese and Korean units.

The figure below is a typical structure of a European-style combi.

The advantages of standard efficiency wall-hung boilers are simple structure, relatively simple combustion technology, high technological maturity. European-style combi boilers are divided into plate heat exchanger type and bithermal heat exchanger type in respect of method to heat DHW. The first type uses a plate heat exchanger to heat the DHW, while the second type allows the DHW pipe to be sheathed in the heating heat exchanger, and heats the DHW in the inner pipe by heating the water in the outer pipe .

Their structure diagram is as follows:

The picture below is a combi with bithermal heat exchanger.

The difference between the two is that the DHW heating method is different. Combis with plate heat exchanger have a plate heat exchanger and a motorized three-way valve. The water circuit structure is also a little more complicated, but the bithermal heat exchanger type boilers do not have these parts.

Comparison of two types combi

On boilers with bithermal heat exchangers, during heating season when the hot water is just coming out, it will be very hot, because DHW has been heated to a high temperature during the winter heating. This results in insufficient bathing comfort. While on boilers with plate heat exchanger pre-heated bath water, no preheated DHW, hot water for shower felt with more comforts.

In areas water with high calcium content, combi with bithermal heat exchanger is more likely to be blocked by scaling due to long-term high temperature heating, while units with plate type heat exchangers are less likely to scale because the bath water temperature is not too high.

Due to the addition of plate type exchanger and motorized three-way valve, their cost are higher. Therefore, many low cost combination boiler on Chinese market are with bithermal heat exchanger. At present, major European manufacturers have basically stopped making combi with bithermal heat exchanger. Some Italian companies and a German company continue to produce them for low-price competition in the Chinese market. Many domestic manufacturer also make boilers with bithermal heat exchangers for some limited budget projects. Combis with double heat exchangers mostly are sold in the retail market in order to provide users with a good bathing experience. Professionals agree that they should be priced higher than boilers with bithermal heat exchangers. However, in the year of 2021, the copper cost had skyrocketed to 76000RMB/ton, the cost between the two become less significant.

On standard efficiency boilers, in order to prevent the flue gas temperature from being too low and thus causing the condensed water to flow back and corrode the heat exchanger in winter, the flue gas temperature must be forced to above 150℃, or even to about 200℃. High temperature flue gas is emitted ,which apparently waste energy. Generally, the maximum efficiency for standard efficiency boiler is about 90% (at max.heat load). At low load, it is only 80% under low load. They are labeled National Energy Efficiency Class II.

Condensing boiler

What is a condensing wall-hung boiler?

Condensing combination boiler releases and re-use heat from high-temperature flue gas in the process of Condensing. Condensing technology is to fully collect this heat to increase heat. The energy recovery in the exhaust gas is realized through the condensation of the flue gas, and the net heating value is increased, thereby realizing a thermal efficiency of 107%. According to the user's heating demand, the temperature sensors monitors the return and outgoing water temperature, indoor temperature and indoor/outdoor temperature difference, it intelligently selects the most energy-saving and comfortable heating power and heating temperature, making the boilers run with low energy consumption and high efficiency. It can save energy by up to 20% compared with regular wall-hung boilers.

The condensing gas wall-hung boiler fully absorbs the sensible heat in the flue gas and the latent heat of water vapor through two heat exchangers. The calorific value of gas refers to the heat released by the complete combustion of 1Nm³ gas. The calorific value is divided into high calorific value and low calorific value. High calorific value refers to the heat released when 1Nm3 gas is completely burned, and the flue gas is cooled to the original temperature, and the water vapor in it is discharged in a condensed state; low calorific value refers to the complete 1Nm3 gas After combustion, the flue gas is all cooled to the original temperature, and the water vapor in it is still the heat released when it is discharged in the steam state. It can be seen that the difference between the high calorific value and the low calorific value of gas is the latent heat of vaporization of water vapor.

The core of the condensing wall-hung boiler is to recycle the high-temperature flue gas that the non-condensing wall-hung boiler will emit into the atmosphere so as to release the energy contained, so that the efficiency will be higher.

According to the different combustion structure, the condensing wall-hung boiler is divided into two modes of secondary heat recovery condensation and premixed combustion and condensation.

Secondary heat recovery condensing boiler

The picture below is a secondary heat recovery condensing wall-hung boiler. A stainless steel heat recovery device is added to the upper right of the boiler. With this device, the efficiency will increase by about 15%. Due to the use of atmospheric forced exhaust combustion, the combustion is simple , The efficiency is better than that of ordinary boilers. The only problem with this kind of boiler is the secondary heat recovery device. Due to processing quality problems, this device is prone to water leakage. There are people in the market slandering this low-cost condensing boiler as fake condensing (most of them come from European brand agents or manufacturers). I don’t agree with this term.Although I don’t think this type of boiler would become dominant in the market, it is indeed a boiler with a condensing structure, and the efficiency has also been improved. It is in line with the Chinese national energy efficiency Class I standard. As for the shortcomings of this kind of boiler, we will compare them later.

Full premixed condensing boiler

The following two types of boilers are called fully premixed condensing wall-hung boilers. The heat exchangers of the following boilers are stainless steel heat exchangers. Generally, this kind of stainless steel heat exchangers were originally made of 316 stainless steel, which is relatively expensive. With the advancement of technology, almost all manufacturers currently use 441 or 304 stainless steel, both of which can meet the design service life of 15 years. 441 has simpler production process requirements, and 304 stainless steel is more complicated.

This is also a full premixed combi boiler and its heat exchanger is made from aluminum-silicon alloy.

Let me summarize the advantages and disadvantages of the above three types of condensing wall-hung boilers:

Secondary waste heat recovery type: the structure is simple, so is the technology, and it is easy to manufacture. The early condensing wall-hung boilers in Europe adopted this structure.

In Europe, some manufacturers still produce condensing wall-hung boilers of this structure. The disadvantage is that the condensing effect is slightly inferior than that of a fully premixed unit, by roughly 5-6%. Some boilers on the market have ever used enlarged secondary heat exchanger, which is said to achieve 107% efficiency. As the boiler uses atmospheric combustion, NOx emission is high. Some manufacturers use low-nitrogen burner to solve this problem, which can barely meet the Beijing emission standards. Another advantage of this boiler is the price is  lower than fully premixed unit. How to tell whether the condensing boiler is of this structure? Generally, on the promotional materials, no wording of full premixed would appear.

Full premixed condensing boiler:  the combustion adopts the pre-mixed method of gas and air, and uses high-speed DC frequency conversion fan, which increases the exhaust pressure, and the exhaust temperature is only about 50-60℃. The technical content is relatively high. The parts (heat exchangers, burners, fans and gas valves) are all imported pushing the cost rather high. At present, dominant boilers on the market use pneumatic proportional valves for the cooperation of gas valves and fans. The pressure of the fan is used to control the opening of the proportional valve, and the Venturi structure is used to mechanically fix the air-fuel ratio. This method is called pneumatic proportionall structure. Its shortcoming is that it cannot adapt to the quality of the gas and can only burn in a fixed proportion. However, the gas quality varies tremendously across China are very different. It needs to be well adjusted with a flue gas analyzer during installation. In case gas quality and pressure change after installation, its adaptability is not good enough and needs to be maintained every year.

At present, self-adaptive gas combustion has begun to appear on the market. The main feature is that the ratio of the gas valve is changed from pneumatic connection to electrical control, so that the flow of gas and air are automatically matched according to the combustion conditions. This is  high-tech combustion. Ferroli high-end condensing boiler, Viessmann 200W series, Vaillant ecotec exclusive series, Riello latest condensing units adopt self-adaptive combustion, which can automatically match and regulate combustion according to gas quality, pressure, and external wind resistance, making the units maintain optimum combustion at all time. The condensing wall-hung boiler with this structure can achieve high efficiency at all times, and reduce the problems of carbon deposits, efficiency decline caused by insufficient combustion. It is the most updated combustion method at present and developing trend for condensing wall-hung boilers. It appears a small change, but actually the technical complexity several times that of the pneumatic proportional structure, and it requires superb software, hardware, and algorithm innovation.

The fully premixed wall-hung boiler is currently the most advanced combustion method, with efficiency of up to 107%, and the emission of nitrogen oxides as low as 30mg/kwh. It is the best choice in respect of energy saving and environment protection.

Comparison of the advantages and disadvantages of stainless steel heat exchangers and aluminum-silicon heat exchangers: both are similar in terms of energy efficiency and Nox emissions. Both have some advantages and disadvantages in application. In simple terms, stainless steel heat exchangers with small cross-sectional area, and the flow rate is not as large as that of aluminum-silicon heat exchangers. However, the use of aluminum-silicon heat exchangers is prone to clogging of the flow passages, but it is still reliable in use. At present, condensing units with stainless steel heat exchangers on the market account for as high as 70%, and the proportion of aluminum-silicon is relatively low. The choices on the market also represent different genres and technological developments. It is not objective to say that a certain technology is best.