The heat of combustion is the total energy released during fuel combustion, including the heat released during the condensation of water vapor from the exhaust gases. The calorific value is the same energy minus the heat of steam condensation—assuming the steam leaves the boiler uncondensed. Therefore, the heat of combustion is always higher than the heating value. In older sources, these were referred to as the upper and lower heating values, respectively.
To sum it up
- Difference consists of the heat of condensation of water vapor formed during the combustion process (and evaporating from the moisture in the fuel).
- Heat of combustion (upper heating value, GCV, HHV) > net calorific value (lower heating value, NCV, LHV).
- Scale: For solid and liquid fuels the difference is usually about 5–6%, for natural gas – about 10%.
- In practice: A condensing boiler recovers part of the condensation heat, so its efficiency, calculated based on its calorific value, can exceed 100%.
Heat of combustion – definition
Heat of combustion — is the amount of heat released during the complete combustion of a unit mass (or volume) of fuel, when the combustion products cool to their initial temperature and the resulting water vapor condenses. It is usually expressed in MJ/kg (solid and liquid fuels), MJ/m³, or kW h/m³ (gases).
The heat of combustion is determined in the laboratory calorimetric bomb . For solid fuels, the procedure is described in the PN-ISO 1928 standard: the measurement result is the heat of combustion at constant volume, when all the water in the combustion products is in a liquid state. Only then is the calorific value calculated.
Determination of calorific value
Calorific value — is the amount of heat released during complete combustion of fuel in the unit, provided that there is water vapor in the exhaust gases does not condense . This corresponds to the operating conditions of traditional furnaces and boilers: exhaust gases exit the chimney hot enough that water exits the unit as steam, carrying some of the energy with it.
Water vapor in exhaust gases is formed from two sources: hydrogen, contained in the fuel, which burns, turning into water, and moisture in the fuel itself. The more hydrogen and water in the fuel, the greater the difference between the heat of combustion and the calorific value.
Calorific value and heat of combustion – comparison.
| Characteristic | Heat of combustion | Calorific value |
|---|---|---|
| Other names | Upper heating value, GCV, HHV (Hs) | Lowest calorific value, LCV, HCV (high) |
| Water vapor from exhaust gases | Compressed - including heat of condensation | Non-condensed – heat of condensation is not taken into account. |
| Size | Always higher | Always below |
| How is this determined? | Measurement in a bomb calorimeter | Calculations based on heat of combustion, hydrogen content, moisture content. |
| Where can I meet her? | Gas cost calculation (conversion from m³ to kW h), laboratory analysis. | Fuel consumption tables, comparison of fuel types, boiler efficiency |
| Typical difference | Approximately 5–6% for solid and liquid fuels, approximately 10% for natural gas. | |
Why does a condensing boiler have an efficiency of more than 100%?
The efficiency of a boiler is traditionally expressed in calorific value . This is due to practical reasons: in the past, measures were taken to prevent steam condensation in exhaust gases, as condensate caused corrosion of the boiler and chimney. Therefore, the heat of condensation was considered wasteful.
In a condensing boiler The exhaust gases are intentionally cooled below the dew point, and some of the condensation heat is recovered. This allows for more energy to be extracted than predicted by the calorific value—hence the efficiency "above 100%." This is a result of the chosen reference point, not a violation of the laws of physics: relative to the calorific value, the efficiency always remains below 100%.
Where in practice will you encounter both concepts?
Gas bill
The gas meter measures the volume, and the supplier issues an electricity bill in kWh. The conversion is done using gross calorific value (GCC), determined based on gas measurements in the network—in simple terms, energy is volume multiplied by gross calorific value. This is indicated on the bill as a conversion factor, which varies depending on the quality of the supplied gas.
Choice of fuel: coal, wood, pellets.
Fuel offers and comparison charts usually state calorific value (HV) ), as it better reflects the amount of energy available in a standard furnace. When comparing fuels, ensure that both figures refer to the same parameter and the same moisture content—"dry" and "working" calorific values are two different numbers.
This is especially noticeable in the example wood . The water in the logs must evaporate, and the energy expended in this process is lost up the chimney. Therefore, damp wood has a significantly lower calorific value than dry wood and burns less efficiently, contributing to the accumulation of soot and resin in the chimney.
How to read fuel parameters – practical tips
- Please note the designations: Hi, Qi, NCV, LHV or "calorific value" is the lower parameter; Hs, Qs, GCV, HHV or "heat of combustion" is the upper one.
- Compare equal values. Comparing the calorific value of one fuel with the calorific value of another will lead to an exaggeration of the advantages of the first.
- Pay attention to the moisture content. The calorific value of wood or coal means little without information about its moisture content.
- Consider boiler efficiency in context. Efficiency values above 100% in boiler data refer to the heating value - this is typical for condensing boilers.
- For detailed fuel and equipment specifications, see. in the product instructions, fuel quality certificate and boiler manufacturer's manual.
Frequently Asked Questions (FAQ)
Which is greater - the heat of combustion or the calorific value?
Heat of combustion. This also includes the heat released during condensation of water vapor from the exhaust gases, which is not included in the calorific value.
Is the heating value the same as the heat of combustion?
Yes, these are two names for the same thing. Similarly, net calorific value is simply calorific value.
Why is calorific value more important for a conventional stove?
In a traditional furnace or boiler, water vapor exits the chimney uncondensed, so the heat from its condensation does not enter the house. The heating value better reflects the amount of energy that can actually be used.
What is the difference between the calorific value and the heating value of natural gas?
For natural gas, the heating value is about 10% higher than the heating value because methane contains a lot of hydrogen, so a lot of water vapor is produced during combustion.
How does humidity affect the calorific value of wood?
The wetter the wood, the lower its calorific value, since some of the energy is lost to evaporation of water. Therefore, well-seasoned wood is best for stoves.
See also
- What is the difference between peas and eco peas?
- What is the difference between boiling and evaporation?
- What is the difference between liquefied gas and propane-butane?
- What is the difference between E5 and E10 gasoline?