How can we determine the soot measurement practically?

The exhaust gas measurement in a heating system is used to determine the pollutants released into the earth's atmosphere with the exhaust gas (e.g. carbon monoxide CO) and the heating energy lost with the warm exhaust gas.

It is a legally prescribed measure that aims to ensure that the atmosphere is not polluted indefinitely by pollutants and that energy is not wasted. Prescribed maximum amounts of pollutants and maximum energy losses (exhaust gas losses, not higher than about 10%) must not be exceeded. Legal provision and implementation in Germany.

The flue gas measurement (not to be confused with the flue gas test) is a measurement to be carried out by the chimney sweep that affects gas and oil boilers in the house heating system (except for room heaters up to 4 kW output and in the drinking water system drinking water heaters up to 28 kW output and condensing boilers). It is an important study that is prescribed by the Federal Immission Control Act and the Small Fires Ordinance (1st BImschV) in Germany and the Air Pollution Control Act in Austria. The measurement is carried out with a lambda probe in the core flow of the connecting pipe between the boiler and the chimney / flue gas pipe, the measured values ​​are recorded by a small computer and can be printed out. The measurement is carried out once for boilers with a nominal output of 4 kW to 11 kW four weeks after commissioning, and it is a recurring measurement for boilers from 11 kW and drinking water heaters from 28 kW. For drinking water heaters up to 28 kW and for combination heaters at maximum output (hot water preparation), only one CO measurement of the flue gas is required. If it is a pellet heating system, the dust and SO2 emissions are measured instead of the CO2 and CO content.

Concentrations of the exhaust gas components

Carbon dioxide

So that the exhaust gas loss qA is low, the CO2 content in the exhaust gas must be as high as possible. If the fuel is ideally mixed with the supply air, the CO2 content in the exhaust gas is a maximum of 10.8% for natural gas LL, and a maximum of 15.4% for EL heating oil. Since this mixture preparation is practically impossible to achieve, an excess of air is required, which, however, increases the exhaust gas loss (heated air).

Carbon monoxide

Carbon monoxide CO is highly toxic even in small quantities and occurs when there is a lack of oxygen (lack of air), which on the one hand results from too little excess air, from contaminated burners or from insufficient fresh air supply at the boiler's installation site. The CO content in the undiluted exhaust gas (calculated with lambda) should be below 80 ppm (0.008%), at 500 ppm (0.05%) an inspection is urgently recommended. Exceeding the limit of 1000 ppm (0.1%) carbon monoxide initially results in a complaint by the district chimney sweep and after a period of 6 weeks a re-measurement must be carried out. If the CO content is high and the exhaust gas is backing up in the installation room, the fireplace is shut down with the cooperation of the municipal utility (gas supplier) (to avert danger).


The oxygen content O2 goes hand in hand with the excess air ratio lambda and should be as low as possible, depending on the boiler and operating mode, it assumes the values ​​below.

Soot number for oil heaters

Furthermore, the possible carbon content C in the exhaust gas in the case of oil heating is determined according to Bacharach and specified as the soot number, which may be max. 1, for old systems before 1988 2. Soot is unburned carbon that is created when the mixture is insufficiently mixed. Reasons can be a lack of air, an incorrectly selected oil nozzle (spray angle) or an atomization pressure that is too low. Oil derivatives (unburned oil), which show up as yellow deposits on a filter paper sample, must not be present in the exhaust gas.

Nitrogen oxides

Not part of the exhaust gas measurement in Germany, but the device manufacturer has to provide evidence of nitrogen oxides (NOx), which are formed from the nitrogen in the supply air at too hot combustion temperatures. In general, the combustion temperature and thus the amount of nitrogen oxides can be reduced by cooling the burner or, more simply, by partially returning the hot exhaust gases from the tip to the root of the flame (internal recirculation, blue burner [2]).

The flue gas temperature is around 60–140 ° C for gas boilers, 120–180 ° C for oil heating and around 40–50 ° C for calorific value. Lower flue gas temperatures worsen the chimney draft and, if they are below the dew point temperature, can lead to condensation of the water vapor in the flue gas, which can lead to chimney sooting or corrosion of the flue system. This can be prevented by renovating the chimney (e.g. pulling in a stainless steel exhaust pipe). High exhaust gas temperatures indicate poor heat transfer at the heat exchangers, which can be a result of soot deposits and can be remedied by cleaning the boiler.

If the exhaust gas temperatures are still too high even with cleaned heat exchangers, this indicates that the heat exchanger surfaces are overloaded. Since the set combustion output is usually considerably higher than the actual heat requirement of the house, this can very often be corrected by reducing the output of the burner. A reduction in output is only possible, however, if the new, reduced burner output is still within the output range of the burner used. With gas burners, the gas throughput is reduced, with oil burners either the atomization pressure is reduced or the size of the atomization nozzle is reduced. In any case, however, the manufacturer's instructions for the burner and heat generator must be observed. After each reduction in output, the burners must be readjusted using a flue gas measuring device, just like when starting up again.

Supply air temperature

The supply air temperature corresponds to the room temperature of the boiler installation room, provided that it is fuel oil or gas devices that draw the air from the installation room. Gas devices and heating oil devices that work independently of the ambient air and draw in fresh air from the outside are equipped with an air-exhaust system (LAS), which diverts exhaust gas in a pipe-in-pipe system and draws in fresh air in countercurrent. This preheated fresh air is included in the exhaust gas loss calculation as the combustion air temperature.

Dew point temperature

The dew point temperature of the water vapor in the exhaust gas can be used to estimate whether water is condensing in the chimney or exhaust pipe. It is assumed that the flue gases cool down by 5 ° C per meter of chimney height. If you subtract the cooling related to the chimney height from the flue gas temperature, you get a flue gas temperature that can be present at the chimney outlet. If this temperature is below the dew point temperature, water vapor condensation is possible; if it is higher, water formation is unlikely.

Excess air ratio

The excess air ratio lambda indicates the ratio of the actual to the theoretically required amount of air, which is equivalent to the ratio of CO2max. to CO2. Atmospheric burners work with an excess air ratio of around 1.8; Fixed fan burners with around 1.2 to 1.4. Regulated fan burners (measurement with an integrated lambda probe) achieve λ = 1.03. The excess air is 0.3% to 80% depending on the type of burner. A small excess of air reduces the exhaust gas loss and improves the heat transfer.

Chimney draft

The chimney draft has the task of removing the hot flue gas produced during combustion and at the same time drawing in the cold combustion air required. Leakages on the flue gas side have a positive or negative influence on the combustion, as the chimney draft is reduced by secondary air. Exhaust gas temperatures that are too high also measured when the chimney draft is too high, but the excess air during combustion is normal. The installation of a secondary air flap can help here.

Do you have interest? Then get in touch with us and we will be happy to get back to you.

Your ERFA team