Boiler size calculation of wood log boilers

Please note that with solid fuel boilers the specified output is guaranteed under full load only. You heat up the boiler and it will take about 30 minutes until the boiler will operate with its maximum output. The stated output will last for approximately 2,5 hrs. Now the burn-off period begins for about an hour with decreased performance. The firebed remaining in the combustion chamber will postheat for about an hour. Afterwards the combustible is completely used up. For an easier calculation we presume a burning period of approximately 4 hrs (at nominal output).

Attention: This is imperative for the rating of the boiler to avoid that the boiler output is rated too weak!

Boiler size calculation of wood log boilers


Assumed heat load: 75 W/m²
Heatable living area: 200 m²
[75 W/m² x 200 m² x 24 hrs = 360 kWh daily]

boiler size: 40 kW
boiler fillings with firewood: 2-3
(on the coldest day at -16 °C)

Dimensioning diagram for calculating the boiler size with the following operating conditions:

outside temperature: approx. -16 °C (coldest day)
room temperature: approx. 20 °C
persons: approx. 4
type of building: SFD
heat coverage by wood log boiler: 100%
daily fillings with wood (at -16 °C): approx. 3 times

Differing operating conditions may increase or decrease the heat requirements.
Estimate in particular cases and keep reserves.

*daily heat requirement [kWh/d]: The daily heat requirement of the building in kWh (at an outside temperature of -16°C), when known. This value can be inquired with the architect of the building. If you don’t know the exact value it might roughly be estimated. Thus, you take the heat load of the corresponding building class in the adjacent chart and then the equivalent line in the diagram in order to calculate the heat load for your living area.

**permanently needed heating output [kW]: The permanently required heating output is for example the nominal heat output of an ordinary oil/gas boiler that produces thermal heat for 24 hrs.

Building class Heat requirement at coldest day (-16 °C)
Existing building until 1977 120 to > 160 W/m²
Thermal Insulation Ordinance since 1977 80 to 120 W/m²
Thermal Insulation Ordinance since 1984 60 to 80 W/m²
Thermal Insulation Ordinance since 1994 40 to 60 W/m²
EnEV 2002 20 to 45 W/m²
EnEV 2009 15 to 30 W/m²

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