A condensing boiler uses the energy content of the fuel almost wholly. Thus, they achieve heat value-related efficiencies of over 100 percent. Physical cheating?
The difference to conventional boilers is that condensing boilers – regardless of which fuel – also use the condensation heat of the exhaust gas. For example, condensing boilers achieve temperature value-related efficiencies of over 100 percent. This physical impossibility is the result of a lot of cheating many years ago: at that time, boilers had efficiencies of around 70%. That sounds bad, and to pretend high quality to the customer, it was stated that a boiler could never have more than 90% efficiency. Based on this “sound limit,” then has a 70% boiler, the salable value of 70:90 x 100% = 78%.
Over time, the Boilermakers learned and eventually reached a physically correctly measured efficiency of 85%, which – calculated with the same formula – was sold as a 94% boiler. When condensing boilers were invented a few years ago, which also use the condensation heat of moisture in the exhaust gas, the physically correct efficiency increased to 95%. However, the manufacturers did not dare to say that they had cheated with the efficiencies so far and continued to use this formula. And this delivered 95:90 x 100% = 106%. And that was the “abuse” obvious. However, it makes more sense physically to refer to the calorific value: An ideal condensing boiler without losses achieves a calorific value-related efficiency/utilization rate of exactly 100 percent.
This is how the use of condensation heat works.
During the combustion of the (carbon) hydrogen-containing fuels, exhaust gas containing water vapor is produced. In condensing boilers, the exhaust gas is cooled down so far that the water vapor components of the exhaust gas condense. By using the heat of condensation occurs a significant improvement in combustion efficiency.
An additional gain in terms of efficiency lies in the fact that the exhaust gas temperatures and thus the exhaust gas losses are significantly lower than in conventional firing, in which, to prevent condensate formation in the exhaust, the exhaust gas temperature should not fall below about 120 ° C. In comparison, the calorific value can be operated at 60 ° C. The higher the hydrogen content of a fuel, the higher the amount of water vapor contained in the exhaust gas after combustion of the fuel. Especially for fuels with a high hydrogen content, it is therefore essential that the heat of condensation contained in the exhaust gas is used.
Condensing boilers can use a greater or lesser share of the heat of condensation, depending on the energy quality and the operating conditions. In non-condensing boilers, the temperature of condensation cannot be used, resulting in a so-called possible exhaust gas loss of about 6% for fuel oil EL and about 11% for natural gas.
The efficiency of the condensing boiler
Condensing boilers are available for oil, gas, or pellet-firing. Since the beginning of the 1990s, condensing technology has been stating of the art, initially for gas and later also for oil firing. In the condensing boiler, in contrast to other boilers, the heat of condensation of the water vapor in the exhaust gas can be used. The return temperature, i.e., the boiler inlet temperature of the heating water in the boiler, is lowered to below the dew point of the exhaust gas. For this purpose, heat exchangers are integrated into the boiler or sometimes also downstream.
Due to the utilization of this condensing boiler, condensing boilers have a higher efficiency of about 6 percent (for heating oil) and 11 percent (for natural gas).
Since this so-called potential exhaust gas loss has not yet been included in the efficiency, thus partially efficiencies of over 100 percent arise (although this is, of course, not possible).
By the way, in the chimney sweep protocol, this waste gas loss is also not shown!
Condensing boilers with efficiencies over 100 percent – how does that work?
The fact that the efficiency (also called the degree of utilization) can exceed 100% is based on the reference value of the calculation. The definition of energy goes back to Julius Weisbach (1851), who has defined the following:
Ability is the ratio of delivered to delivered energy.
Here, however, only the calorific value of the fuel was significant. The calorific value did not matter at that time. The calorific value of fuel can now be used, but the calculation approach has not (yet) changed.
The technique used is thus slightly ahead of the physically correct calculation.
Comparison of utilization rates and annual losses in low-temperature and condensing technology
Calorific value Hi: (previously lower calorific value Hu) is the amount of heat of a fuel that is released in perfect combustion. The water vapor produced during combustion in the exhaust gas is present in gaseous form.
Calorific value Hs: (formerly upper calorific value Ho) is the amount of heat of a fuel that is released in perfect combustion. The water vapor formed during combustion condenses, i.e., the temperature of vaporization contained in the exhaust gases, is also provided.
To be able to use the condensing effect, the condensing boiler (with a more extensive water content of approx. 0.5 to 1.5 liters per kW) requires shallow return temperatures of below 50 ° C (for natural gas) or 45 ° C (for heating oil), Condensing boilers (with a shallow water content of approx. 0.1 liters per kW) require low mean boiler water temperatures.
Efficiency and partial load utilization of condensing boilers
The exhaust gas losses fall disproportionately. A unique feature is that the degree of use is higher in the partial load case than at full load.
They usually have a slightly less favorable efficiency than condensing boilers but are significantly better than low-temperature boilers. With decreasing utilization, the degree of utilization decreases.
To use this condensing effect (the control and hydraulic integration into the heating system are of particular importance):
- Hydraulic balancing of the entire systems (including consideration of the overflow valve)
- Setting the low temperatures (as high as necessary, as low as possible)
- Heating system: radiator or surface heating
- Heating requirements of the building
- set heating curve
Dependence of the calorific effect on the temperature
Since the condensing result is load-dependent and returns temperature-dependent, one should try to keep the return temperatures as low as possible. This is possible, for example, by underfloor heating or surface heating. In this type of capillary heating, the set flow temperatures are shallow. Therefore the return temperatures below 30 ° C. Since after energetic renovation measures (insulation of the outer walls, window replacement, etc.), the total heat load decreases, the (still) existing radiators (convection heaters) are usually large enough to achieve correspondingly low return temperatures.
Furthermore, a hydraulic balance of the entire heating system leads to better use of the condensing effect. If the required heating load for each room is known, then the flow rate of the heating water can be limited to the appropriate amount on the radiator. This means that all heaters can be equally well supplied, and the temperature of the heating water can be optimally utilized. Ultimately, all areas of the entire heating system must be well coordinated.
In the worst case, for example, a single radiator that is installed close to the boiler could otherwise destroy the entire condensing effect.
Condensing boilers can be significantly more effective than low-temperature or standard boilers if the condensing effect is exploited.