Summary and Conclusions

Dimensioneringsværktøj for brændeovne - Web baseret simuleringsværktøj for bestemmelse af den korrekte brændeovnsstørrelse til den specifikke bolig.

In Denmark, approximately 700,000 solid fuel stoves are installed. Examinations of recent date have shown that in the heating season, these stoves contribute with a not negligible share of air pollution in the cities. The reason is often inexpedient firing and an inappropriate performance of the stove. In many cases the thermal output of the stove exceeds the heating demand of the residence; and the user typically reduces the stove’s combustion air supply with the purpose of lowering the temperature of the accommodation space. The result is a sooting combustion followed by undesired and environmentally damaging emissions. To avoid this problem, a dimensioning tool is developed for solid fuel stoves, making the user able to find the optimal thermal output of the stove for the actual residence.

It is desirable to obtain a convenient and comfortable temperature in the accommodation space of a residence. Residences are constantly influenced by heat sources such as radiators, incident solar radiation through windows, human presence and domestic activities (cooking, dishwashing, the light and the TV are turned on and perhaps the residents have fire in the stove). At the same time an accumulation of the added heat takes place in building materials and in the interior furnishings. Furthermore, a transfer of heat takes place from the residence to the surroundings, which contributes to a persistent cooling. The heat is conducted away to the open because it is cold outside or because it is windy, but the heat can also move upwards to an upper floor or it can be moved to adjoining rooms that are not heated. The whole system undergoes a continuous change; the system is – so to speak – dynamic.

The simulator program is created in such a way that all these influences are taken into account. With the purpose of avoiding unnecessary complications with complicated mathematical formula, attempts have been made to simplify the calculations through use of experience values from directions coming from the Danish Building Research Institute and acknowledged technical literature. First of all we look at the residence; typically there is a connection between the heat consumption and the year of construction. This connection is used as a first indication, but also reinsulation of the residence is taken into account. The heat consumption is corrected according to possible reinsulation and modernization of windows, etc. Then the report concerns the room in which the solid fuel stove is installed. The decisive parameter for achieving a comfortable temperature is the size of the room. But also here, special conditions are taken into account, e.g. whether the room is high-ceilinged or if there are open adjoining rooms – as well as the condition of walls, floor and ceiling.

Another important element regarding the heat consumption of a building, is the ventilation, both natural ventilation and mechanical ventilation. The last-mentioned is defined with a certain precision, whereas the natural ventilation depends on the temperature difference between outside and inside, and the wind action of the residence. In these cases, experience values have been used from the technical literature, and extra heat loss is calculated, when the residence is placed in a position where it is particularly exposed to wind. Alternative internal heat contributions from residents, lighting, appliances, etc. are estimated to an average of 500 kW.

We have also examined how the solid fuel stove liberates the heat to the room. Here we have anticipated a constant relation between heat radiation and convection. Heat radiation primarily influences the surrounding areas, i.e. the radiation heats the walls, ceiling and furnishings, whereas the convection primarily heats the air of the room. The ratio between radiation and convection is notoriously set to 60, respectively 40%. Furthermore, a universal specific performance profile is calculated for solid fuel stoves taking into account the heat accumulation in the stove components, chimney loss, etc.

The report also concerns the application of the program. The user is here guided through the program’s different functions (buttons) which are arranged in such a way that all users – including those who are not quite familiar with the specific technical field – can use the program without any problems. When the necessary data entry has been made, the user can call a report containing all essential information, and the main result is the optimal performance of the solid fuel stove (nominal output) for the actual residence. The original program is on one of Danish Environmental Protection Agency’s servers and it will be accessible on the Agency’s website.

Finally, the function of the program is tested through measurements in seven residences with different heating needs. Master data for residences and solid fuel stoves appear from appendix A. Combustion processes and calculations of the output of the solid fuel stove appear from appendix B. Temperature measurements in residences appear from appendix C.