A condensing boiler is a boiler. It extracts heat from fuels such as gas and oil by not only using the heat energy of the fuel but also using the water vapor contained in the exhaust gas. This means that in a condensing boiler, the steam does not disappear unused from the chimney-like other boilers.
However, to be able to use the condensing boiler, some conditions must be present or created. Which states are this, how a condensing boiler works, which types exist, and which advantages and disadvantages it offers, you will find out here.
What is the calorific value?
The calorific value of a substance indicates the amount of heat that is released during the combustion of combustible material. The cooling of the combustion gases is also taken into account.
Why is that so important? A combustible substance reacts with oxygen during combustion to form a flame. Since hydrocarbons (petroleum, coal, natural gas) are used as fuels in most heat generation plants, the burning of the hydrocarbons produces exhaust gases which, in addition to carbon dioxide and nitrogen, also contain water vapor.
The water vapor in the exhaust gases binds heat and releases them on the cooling of the water vapor by condensation again. According to the law of conservation of energy, we are condensing the water vapor releases as much energy as would have been required for the evaporation process. This latent (hidden) power is also called condensation heat.
Modern condensing technology makes this heat of condensation usable and thus increases the efficiency of the fuel. This process is called a condensing effect.
The higher the hydrogen content in the fuel, the higher the efficiency achieved by the condensing effect. The additional benefit from the condensing fact, which corresponds to the energy-saving, is the following:
- Hydrogen: approx. 18%
- Natural gas: approx. 11%
- Heating oil (light): approx. 6%
- Coal (theoretical): 2 – 3%
Below I have put together a small list of different fuels with the respective calorific values:
- Natural gas: 10.83 kWh / kg
- Fuel oil, heavy: 11.86 kWh / kg
- Heating oil, light: 9.5 kWh / l
- Hard coal: 6,9 – 7,0 kWh / kg
- Lignite: 5.1 – 5.5 kWh / kg
The function of a condensing boiler
A physical law states that heat is created when water vapor condenses. This makes use of a condensing boiler, in addition to the heat generated by the burning of gas, which uses the heat energy hidden in the water vapor. For this reason, a condensing boiler works more effectively than a conventional boiler, in which, as mentioned above, water vapor escapes unused.
In a condensing boiler condenses the water vapor by the reliable cooling of the exhaust gases to liquid water. The energy of the water vapor can be partially used for space heating to increase the efficiency of the heating system.
Which types of condensing boilers are there?
Depending on the type of exhaust system and the supply of combustion air differ:
- Room air dependent devices
- Room air independent devices
Room air-dependent condensing boiler
These devices remove the combustion air directly from the installation room. There must be sufficient combustion air in the place at all times. For this reason, fresh air must pass through windows or door slots in the installation room.
Of importance here is that with an existing extractor hood with a deduction to the outside, there is a risk that exhaust gases from the heating enter the apartment. In this case, the cooker hood may only run when the heat is switched off.
Room air independent condensing boiler
These are devices that do not extract the combustion air from the installation room but the outdoors. This is done using a double-walled pipe, which leads, best through the roof, outside. The inner tube has the task of allowing the exhaust gases to flow out; the outer leads combustion air to the boiler.
For an existing chimney, it will be enough if the double-walled pipe reaches the chimney. Through this leads to a single-walled tube, which directs the exhaust gases outside. The free space between the pipe and the chimney usually provides sufficient space to supply combustion air to the boiler.
The advantages of an independent room condensing boiler are:
- No conflict with the cooker hood
- Increased efficiency through heat recovery
- Overflow openings between different rooms are a thing of the past.
- The boiler flows clean combustion air from the outside.
Requirements for the optimal use of a condensing boiler
To benefit from the advantages of a condensing boiler, some conditions must be met:
- Wastewater connection
- Correct setting of the entire heating system for boilers without overflow
- What does the chimney have to endure?
A condensing boiler requires a wastewater connection to discharge the accumulating condensate (several liters per day) with a high amount of condensate into the sewage system. This can usually be easily connected to the existing sewage system. With larger boilers, an additional neutralization device is often necessary because of the high acid content of the condensate.
Condensing boiler with or without overflow – the adjustment of the entire heating system
Condensing appliances often work less well in practice than the technical possibilities allow. The optimal function depends on the return air temperatures of the heating system. The colder the water that flows back from the radiators into the boiler, the better the exhaust gases cool off, which benefits the condensation effect.
At a high return air temperature, the condensing effect is often not or little to bear. At a return temperature of 50 degrees, the heat of condensation is reduced and, as a result, the condensing effect is almost zero. This means that in addition to the technology, the optimal adjustment of the heating system by hydraulic balancing is essential. A specialist best does this.
The above does not apply to condense boilers with overflow valves, which are mainly located in spas with low water contents. Here a hydraulic balancing can hurt the efficiency of the boiler. The reason is that in individual operating states, short-circuit flow and return to avoid a minimum flow rate and to overheat of the spa. This increases the return temperature and consequently, the condensing effect.
In hydraulically balanced systems, this often occurs because here, every radiator gets the amount of heating water required. If less water flows back into the boiler, the overflow device activates.
Whether an existing condensing boiler has an overflow device can be found out by an installer. When purchasing a new condensing boiler, it is advisable to make sure that it does not require an overflow valve. Furthermore, a buffer often increases the return air temperature of the heating system and reduces the condensing effect.
The right chimney for condensing boilers
The chimney must meet the high requirements for soot fire resistance and moisture resistance. Furthermore, it is the best pressure-resistant and corrosion-resistant.
Due to the low exhaust gas temperatures of condensing boilers, a lot of condensation forms. For this reason, it is essential to use resistant materials such as stainless steel so as not to damage the chimney from the inside by moisture. Due to the low exhaust gas temperatures, the otherwise usual chimney effect in the stack tends to wake up. This is also the reason why a fan is often used for the safe removal of exhaust gases.
Efficiency and effectiveness of condensing boilers
Efficiency and degree of utilization are two not synonymous usable terms. The former describes the ratio of supplied energy to useful energy during an optimal operating point. The degree of utilization describes this ratio during a specific period. For this reason, the annual utilization rate indicates how much of the energy stored in the boiler can be used in an entire heating period.
Also, there is the standard efficiency, the calculation of which is determined by DIN 4702 and which the manufacturers determine on test benches. The values ascertained appear in the manufacturer’s documentation of the respective devices and are used by specialist companies for heating systems as well as consumers for information.
By indicating the standard efficiency, consumers can recognize the ability of the condensing boiler. This means that the standard efficiency is a realistic measure of the effectiveness of a boiler. The higher it is, the better the heating system can use the energy contained in the fuel.
Advantages and disadvantages of a condensing boiler compared to a low-temperature boiler.
- Condensing boilers work more effectively than low-temperature boilers.
- Condensing boilers can be coupled well with other alternative methods of energy production.
- Condensing boilers are more environmentally friendly in the long run.
- Condensing boilers can be used solely with liquid fuels.
- For the removal of the exhaust gases in a condensing boiler, power is needed.
- The purchase price of a condensing boiler is usually higher.
Low-temperature boilers are standard because they are reliable and inexpensive systems. The efficiency of these systems (based on the calorific value of the fuel) is close to 90 percent for suitable devices. This value is not comparable with the utilization levels of condensing boilers (related to calorific value), which are much higher.
The calorific value of a condensing boiler contains condensation heat in addition to the calorific value. This means the energy that he uses by condensing the water vapor produced. For condensing boilers, we take the calorific value as a reference. In many cases, this equates to a degree of utilization (based on calorific value) of 100 or more percent. For this reason, with an optimally set system with a condensing boiler compared to a low-temperature furnace, it is possible to save up to ten percent energy per year — a point for the condensing system.
A disadvantage of a condensing boiler compared to a low-temperature furnace is that it can be operated solely with liquid fuels to make optimal use of the condensation energy.
For the removal of the exhaust gases after the energy generation, a condensing boiler needs electricity to use the water vapor. This is considered a disadvantage of a condensing boiler. Furthermore, a condensing furnace is more expensive to purchase than a low-temperature boiler. The reason is that during the installation often changes to the chimney are needed.
The clear advantages of condensing heating are: It can be easily combined with other alternative methods of energy production, such as solar thermal energy. Furthermore, it is more environmentally friendly because of the better effectiveness and protects the purse. Another advantage of condensing heating is a flexible attachment. Such a heater can be installed in attics, cellars, or living rooms.