Combustion heat in the Passive House

The heating demand of Passive Houses, which is reduced by a factor of four in comparison with conventional new buildings, constitutes a fundamental prerequisite for meeting the demand sustainably, by using renewable energy resources. Regenerative fuels based on biomass play an important role here, as they must be generated regionally due to ecological and ethical reasons (competition for food). The energy balance is unnecessarily affected by long transport routes, and materials cycles are interrupted. The regional supply is thus limited and can only be technically improved to a small extent.

Since biomass represents the main renewable carbon carriers, it will increasingly serve as an organic raw material for the chemicals industry in the future too. This will also restrict the quantities available for simple heating applications (and increase prices in the process). This distinguishes biomass from other forms of energy that are produced in a purely technical way (for example electricity). Therefore the level of efficiency in the Passive House is an essential prerequisite for a completely regenerative supply of energy for heat generation. The examination of the potentials available shows that mainly solid fuels are available for the heating of buildings. With a population of 81 million, the ascertained potential capacity of about 550 PJ/a connotes a per capita supply of 1880 kWh/(pP*a), which is equivalent to a continuous output of 215 watts. This is sufficient for the heat supply in the Passive House; there is even a surplus of energy .

The generation of heat by means of combustion processes has been practised since prehistoric times. Fire is one of the oldest and fundamental discoveries of man and is often equated with heat. The fire in the hearth of a home has always had a great symbolic value and was closely associated with religion and mythology. This non-rational aspect is still important today and is still in demand. Wood fires in the form of stoves in living rooms are particularly desirable, especially with visible flames. A “side-effect” is the increased heating up of the room in which they are located.

At the moment there are still some technical hurdles, not least due to a lack of heat generators with a small output of less than 5 kW suitable for the demand in the Passive House.

An important aspect, especially in the Passive House, is the operation of the heat generator inside the heated volume of the building. A mechanical convenience ventilation system should also be operated at the same time. The provision of combustion air for the fireplace and in particular the safety mechanisms that may be necessary for any malfunctioning of the ventilation system must be ensured satisfactorily. The airtightness of the construction must also be assured for a high quality Passive House building envelope.

Firing systems
Room-sealed operation

• Safety considerations

Non-room-sealed operation
Air conveyance
Airtightness
Chimneys

The preceding overview of the conditions for the application of biomass combustion in Passive Houses shows that in compliance with safety requirements (monitoring of the pressure difference), particularly pellet systems can be used successfully.

The provisions that are currently being developed for the parallel use of room-sealed stoves and ventilation systems may be too inadequate for Passive Houses, so that additional safety installations (monitoring of the pressure difference) are strongly recommended for buildings with a high level of airtightness. As devices which have not been approved as “room-sealed” (as long as they are room-sealed in the technical sense of the word) may also be used under this condition, there is a wide range of suitable stoves available for this purpose. The possibility of an improved class of airtightness should be considered.

The effort in relation to safety is very great for firing systems for solid fuels, which makes such systems comparatively expensive, particularly as the level of integration of currently available devices is small and a pellet-based system solution is not yet available for Passive Houses.

More detailed examinations would be necessary to clarify how much differently those requirements can be assessed that necessitate official approval, such as the soot-fire resistant chimney on the roof, airtightly implemented room-sealed Passive House systems with lower outputs (≤ 3kW) or condensing boilers. If it is possible to use an air/exhaust pipe, significant savings can be made in the costs of setting up and maintaining the system. Currently, this is not permitted in Germany [M-FeuVO 2005], but in other countries (e.g. Austria) such devices are available on the market.

For gas and oil-powered condensing boilers, tightly enclosed designs with air/exhaust pipes belonging to the firing system are now standard. A similarly extensive level of integration is not yet discernable for systems using solid fuels; these are still based on the conventional concept of the separate stove. Even improvements like the possibility of the room-sealed operation could not call this concept into question. Just like the technical concept, the relevant standards are also lagging behind the technical possibilities at present.

[FeuVO 1977] Land Hessen: Verordnung über Feuerungsanlagen und Brennstofflagerung 1977; Wiesbaden 1977 (The State of Hesse: Regulations for firing systems, and fuel storage 1977, Wiesbaden 1977)
[M-FeuVO 2005] Bauministerkonferenz: Muster-Feuerungsverordnung; Berlin 2005 (Conference of construction ministers: Sample firing regulation, Berlin 2005)
[AkkP 20] Passivhaus-Versorgungstechnik; Protokollband Nr. 20 des Arbeitskreises kostengünstige Passivhäuser Phase II; Passivhaus Institut; Darmstadt 2000 (Passive House Supply Technology; Protocol Volume No. 20 of the Research Group for Cost-effective Passive Houses Phase II; Passive House Institute, Darmstadt 2000)
[DIN 18894] Deutsches Institut für Normung: DIN 18894-2005-02 Feuerstätten für feste Brennstoffe - Pelletöfen - Anforderungen, Prüfung und Kennzeichnung, Beuth Verlag, Berlin 2005 (German Standards Institute: DIN 18894-2005-02 Fireplaces for solid fuels – Pellet-stoves - Requirements, testing and labelling, Beuth Verlag, Berlin 2005))
[DIN 1946] Deutsches Institut für Normung: DIN 1946-6:1998-10 Raumlufttechnik, Lüftung von Wohnungen, Beuth Verlag, Berlin 1998 (German Standards Institute: DIN 1946-6:1998-10 Indoor air technology, ventilation in homes, Beuth Verlag, Berlin 2005)
[FNR 2007] Fachagentur Nachwachsende Rohstoffe e. V. (Hrg.): Handbuch Bioenergie-Kleinanlagen, Gültzow 2007 (Agency for Renewable raw materials e.V, Handbook of small-scale bioenergy facilities, Gültzow 2007)
[Strauß 2006] Strauß, R.-P.: Innovative Lüftungsgeräte mit „AirSwitcher“ […], Tagungsband 10. Internationale Passivhaustagung Hannover 2006, Dr. W. Feist (Hrsg.), Darmstadt 2006 (Strauß, R.-P.: Innovative ventilation units with “AirSwitcher” […], Conference Proceedings of the 10th International Passive House Conference in Hannover 2006, Dr. W. Feist, Darmstadt 2006)
[Peper 2000] Peper, S.: Luftdichtheit bei Passivhäusern […] Drucktest ohne Blower Door, Tagungsband 4. Internationale Passivhaustagung Kassel 2000, Dr. W. Feist (Hrsg.), Darmstadt 2000 (Peper, S.: Airtightness in Passive Houses […] Pressure test without the blower door, Conference Proceedings of the 4th International Passive House Conference in Kassel 2000, Dr. W. Feist, Darmstadt 2000)
[HKI 2006] HKI Industrieverband Haus-, Heiz- und Küchentechnik e.V.: HKI Technik Info Raumluftunabhängigkeit / externe Verbrennungsluftversorgung, HuK-Te-INFO 014, Frankfurt/M 2006 (Industrial association of building, heating and kitchen services e.V.: HKI Technical Information Room-sealed operation/external combustion air supply, HuK-Te-INFO 014, Frankfurt/M 2006)