Each maintenance session, the technician provides us with the energy efficiency report and a flue gas receipt with values. What do they mean? Obviously, each value is checked by the specialized worker who knows what is up to standard and what is not. Otherwise, they report it in the control report through the Observations, Recommendations, and Prescriptions fields.

Before continuing with the article, please note that all information is provided for knowledge purposes and can only be corrected by external personnel.

Flue Gas Temperature

Flue gas temperature is at the top of the values in the flue gas test, along with the combustion air temperature, used to calculate the boiler's efficiency. It is, therefore, an important factor that affects the overall efficiency of the boiler itself.

In general, the higher the exhaust gas temperature, the lower the efficiency of the boiler. This is because, at high temperatures, a greater amount of heat is lost through the exhaust gases. To improve boiler efficiency, it is important to minimize the exhaust gas temperature.

In a condensing boiler, to lower the exhaust gas temperature, you can simply lower the temperature of the domestic hot water or the heating system.

However, in:

  • open chamber boiler, or natural draft boiler (Type A appliances)
  • balanced flue low efficiency boiler (Type C appliances)

The temperature of the flue gas analysis should be high in proportion to the length of the flue. These flue gases are full of condensate, and during their journey to the chimney, they tend to cool down, and the condensate sometimes precipitates back into the boiler. The condition of these types of boilers (B and low efficiency C) after several years of operation at low flue gas temperatures shows rich oxides and dirt due to the unexpelled condensate.

Another note on natural draft boilers is that low flue gas temperatures can reduce the draft value of the flue.

Regarding flue gas temperature, we said it affects efficiency; lowering it increases efficiency but must be managed carefully to avoid generating condensate if it is not a condensing boiler.

Combustion Air Temperature

The temperature of the combustion air is an important factor to consider in the flue gas analysis of a boiler, as it affects the formation of nitrogen oxides (NOx) and incomplete combustion.

The combustion air temperature can depend on the type of boiler and the flue piping used. For type C boilers (balanced flue or condensing), the combustion air temperature is quite low in the winter months because it is necessarily drawn from outside. However, if the boiler is connected with a coaxial pipe, higher combustion air temperatures can be achieved as it is preheated for a part of its journey.

Conversely, in open chamber boilers (Type A), as they draw combustion air from the environment, if they are located indoors, we have higher values.

Oxygen (O2)

This value detected during the boiler's flue gas analysis is proportional to the CO2 or carbon dioxide value. It is itself important for calculating efficiency and balancing the Air:Fuel ratio. In open chamber boilers, it is usually very high (14-16%), which is normal; a lower value would create condensate or increase CO.

In sealed chamber boilers, also known as balanced flue, this value is lower because the Gas:Air ratio is more controlled. Finally, we have condensing appliances with lower oxygen values (around 5-6%), which is normal because combustion occurs in a chamber designed to retain heat and use most of the fuel.

The oxygen value in a boiler's flue gas analysis is an important indicator of the combustion efficiency of the fuel. In general, the oxygen value in a boiler's exhaust gases should be as low as possible, as an excess of oxygen can indicate an incorrect fuel balance (see Excess Air).

Carbon Dioxide (CO2)

We skip this point because it is proportional to the previous value.

Carbon Monoxide (CO)

We come to a gas that deserves all attention; carbon monoxide is a combustion product, also known as partially combusted part. It consists of a gas that has not fully oxidized and still has oxidation potential. It is particularly harmful because it is absorbed by red blood cells in our lungs' alveoli, and due to its valence, it binds to hemoglobin, a quaternary protein with ferric ions essential for oxygen transport to tissues.

The legal limit for this value is 1000 parts per million, but it should ideally be closer to zero, especially in open chamber or natural draft boilers.

The causes of high monoxide values are due to unbalanced combustion in the Air:Fuel ratio; boilers that are stifled with too high fuel gas settings generate monoxide.

Another crucial factor to monitor is a possible blockage in the flue, as bird nests can increase this value and often cause boilers to lock out.

What does it mean if my boiler has a high level of carbon monoxide?

Consider that the CO (carbon monoxide) level must be strictly less than 1000 ppm (Parts per million), or the system will be shut down.

From 300 ppm to 1000 ppm is a recommendation. However, carbon monoxide indicates poor cleaning or an incorrect balance of the Gas/Air ratio.

At this point, after performing thorough cleaning, evaluate the excess air value to see if the combustion is deficient in combustion air.

Excess Air (λ)

This parameter is also used for calibrating the gas valve, and the value should be as close as possible to 2. Lower values indicate a possible obstruction in the flue or vent pipe (such as protective nets full of cobwebs or nests), and higher values can affect the boiler's efficiency. Therefore, it is important to find the correct balance with the technician.


Efficiency is the last value because it depends on all the other values mentioned in this article. It is derived from a complex calculation that current instruments perform automatically. You can intervene in efficiency indirectly to a certain extent because much depends on the boiler's quality. Another factor to consider is that too high efficiencies on low efficiency B and C type boilers inevitably lead to premature structural degradation of the appliance, so it is essential to balance safety and efficiency.

As mentioned above, the technician knows (or should know) all these things; relevant values are corrected or reported on the boiler or in the control report, always prioritizing safety. Corrections should NOT be performed by the user but always by a specialized technician. However, you can check the boiler's air intake grid to the outside; a brush or vacuum cleaner is enough to clean it.

What are the possible causes of low efficiency in the flue gas test?

Efficiency depends on several factors:

  • The temperature delta between the heating system's supply and return.
  • The calibration of the gas valve.

If the system is already at operating temperature (hot), we get lower efficiency because it is less able to absorb heat. At this point, the high modulation of the condensing appliance is crucial to achieve higher efficiency.

In the case of low-efficiency B and C type generators, it may depend on the temperature delta or an incorrect gas valve calibration.

In any case, proceed with the gas valve calibration.

Questions and Answers

What to do if the flue gas test values are out of norm?

It depends. In B type appliances, the draft value must meet 3.0 Pa. If this value is lower, it may be due to:

  • A square-shaped flue (known for not facilitating draft)
  • A flue unable to heat up adequately, such as those made of refractory cement.
  • A visible flue pipe with an inadequate slope or an overly long horizontal section

In the first two cases, if the draft is less than 1.1 Pa, the system will be shut down. Therefore, consider replacing the B type appliance with a C type condensing boiler.

The third case is manageable; consider replacing the visible piping with a compliant and sealed one (e.g., stainless steel with gaskets) and adjusting the slope to favor flue gas flow.

How often should I perform a flue gas test on my boiler?

Although it is mandatory every two years with the Energy Efficiency Report, a complete flue gas test is suggested annually but not mandatory.

However, the annual draft check is mandatory for B type appliances.

What is the difference between condensing and non-condensing boilers in terms of the flue gas test?

The differences between all types of boilers and condensing boilers are evident. Condensing boilers have much lower flue gas temperatures due to the heat recovery by the primary heat exchanger:

  • A Low Efficiency boiler has flue gas temperatures above 90 degrees up to 140 degrees at the sampling point.
  • A Condensing boiler has much lower flue gas temperatures, e.g., 40 degrees.

This affects the efficiency calculated on the temperature delta between combustion air and flue gas temperature.

Regarding Oxygen, higher values are found in low efficiency boilers and lower in condensing boilers. This is due to a more precise balance in the condensing appliance's premix.

The efficiencies are evident:

  • In low efficiency boilers, we have around 90% with a 3 percentage point margin, considering a boiler from 2000 onwards.
  • In condensing boilers, efficiencies exceed 98%.