2 Penetrations and additions
In cooperation with AEE and NKUA
2.1 Building envelope penetrations
2.1.1 General problems through penetrations concerning an air tight building envelope
Several ducts for air, water and energy supply or telecommunication necessarily have to penetrate the air tight envelope of a building. Penetrations can be required in exterior walls or roofs (for example water ducts from solar panels) as well as in interior walls, floors or ceilings between conditioned and non-conditioned areas (like cellars or attics).
Basically only the essential number of penetrations should be realized within the retrofit of historic buildings. Distribution boxes with numerous junctions should be placed within the air tight building envelope. In this case only the main supply duct has to penetrate the wall construction. This also applies to ventilation ducts or hydraulic ducts. Figure 10 for instance shows the case of an exhaust air ventilation system for which the different air ducts should be brought together in one main exhaust air duct, which is let through the air tight building envelope to the exhaust air ventilator. Penetrations have to be located in areas, where sufficient working place for sealing works can be provided. Penetrations should not be placed in the corner of a room or immediately next to walls or beams.
Figure 10: Number of penetrations of supply networks with distribution ducts depending on their positions
The outside of penetration ducts, like protection tubes or air ducts, have to be connected exactly and permanently to the air tight layer of a building. Further unwanted airflows within the installation ducts, as there is the risk in cladding tubes of electrical cords, protection tubes of water or air supply ducts, have to be eliminated through air tight sealing provisions.
2.1.2 Penetrations for mechanical ventilation grids
Penetrations in the envelope for ventilation grids automatically cause thermal bridges. On the one hand warm exhaust air escapes through the ventilation duct, which means energy losses through ventilation, and on the other hand cold outside air can enter the heated area through the ventilation grid. To avoid this in case of roof penetrations the exterior ventilation grid head has to be insulated on its outside surface. The interior ventilation grid has to be insulated from the penetration point for about one meter length.
If a heat recovery unit is used for the ventilation system it should be placed as near as possible to the penetration point. The connection grid between penetration point and the heat recovery unit has to be insulated very well in order to avoid energy flows between the grid and the heated area.
Smooth, large-sized ventilation ducts generally are easy to seal by using special adapters for wall, ceiling and foil penetrations. Adapters pulled on by ventilation ducts have to be mounted before the installation of the duct network. Fittings with glued seams can be mounted after the installation of the duct network, if the penetrations are well accessible. Ducts with very big diameters and fluted surfaces, like flexible aluminium tires, should not be used in the area of penetrations, because they cannot be sealed properly to the air tight layer of the building. The correct use of an adapter for a roof penetration is shown in Figure 11 [NEI00].
Figure 11: Flexible ventilation duct with smooth part in the area of the penetration
2.1.3 Penetrations for natural ventilation
Historic buildings often have many spread chimneys in order to assure a permanent heat distribution through single ovens. Using modern energy efficient building service and heat distribution systems, most of the chimneys are closed down. Under the precondition that the chimney is dry and not mouldy, these chimneys can be used as a natural ventilation shafts. Chimneys can be used to lead exhaust air away, while fresh air is supplied as natural ventilation through windows or grids. Also under the precondition that the cellar is dry and not mouldy, the chimneys can be used for night ventilation so the warm air ascending through windows is substituted by the pre-cooled cellar air.
The ventilation openings have to be tightly closable, the chimney has to be plastered along its whole length and the connections to ceiling and roof construction have to be sealed exactly and permanently. These chimneys however form important thermal bridges. For instance when the attic is part of the heated or insulated volume, the chimney should be insulated from the outside (at least one meter of height), to avoid surface condensation on the inside or outside of the chimney and to reduce thermal conduction-losses. If a natural ventilation system is not envisaged, it’s better seal the roof perfectly.
2.1.4 Duct and cable penetrations
Power cables without protection tubes penetrating brick walls should be sealed by plastering the passage point exactly and completely. After plastering the cables must not be flexed or dragged in order to avoid the break out of the sealing material. By installing an air tight connection box this risk can be eliminated easily (Figure 12) [NEI00].
Figure 12: Air tight connection box seen from the backside [LUD03]
Ribbed protection tubes of sanitary ducts or power cables penetrating plastered brick walls can be sealed sufficiently by plastering the passage point. The sum of the free cross section areas within the protection tube causes considerable air passages. After drawing the tubes through the wall construction the interior space of the protection tube should be sealed or filled out in one point at least. For this purpose some manufacturers offer air tight connection boxes or protection tube systems.
Water ducts for cold and hot water should be sealed between the duct itself (and not its insulation sheath) and the air tight building envelope. While the sealing material of cold water ducts has to be resistant against moisture of condensate, the sealing material of hot water ducts has to be resistant to high temperatures [NEI00].
2.2 Outside devices - exterior building additions
In the following, general guidelines with respect to outside building additions are presented. These additions are necessary outside the building in order to integrate renewable energies in old buildings.
2.2.1 Outside biomass storage
Biomass that is used for fuel can be divided into the following two main categories: solid biomass and liquid biomass. This guide only covers biomass that is commonly used in buildings, such as pellets, wood chips and wood blocks as solid biomass and pure plant oil as liquid biomass. Other forms of biomass, such as animal wastes and industrial and municipal wastes are not covered in this guide because these types of feedstock require industrial conversion technologies, such as anaerobic digestion, which are generally not used in buildings.
Figure 13: Wood pellets
Points of attention
Additions in the exterior of the building for the storage of the biomass depends on the annual demand for biomass fuel. More specifically, the annual demand for biomass fuel for a particular site depends on the following parameters:
§ Scale of the installation
§ Conversion option (heat only, combined heat & power, boiler or stove)
§ Operating hours
§ Boiler efficiency
§ Moisture content of the fuel
Rules of thumb and practical examples
In order to know the additions that need to be made outside the building, knowledge of the annual fuel consumption is required. In general biomass fuels, when compared with fossil fuels, are of relatively low energy density which is the amount of energy stored per unit volume. Therefore, large volumes of biomass are needed to be stored. The primary purpose for the storage will be to retain the biomass in good condition in a convenient place for it to be transferred to the next stage of processing, combustion or energy conversion. One major concern is to keep biomass protected from moisture. If it is not absolutely dry, then loss of energy may occur or even moulds (the spores of which may be dangerous if inhaled) may be formed. In Figure 14 the variation of energy density and net calorific value with moisture content is presented. Net Calorific Value (NCV) is the total energy minus the energy to evaporate moisture to steam.
Figure 14: Variation of NCV and energy density versus moisture content
Important features to be taken into account regarding biomass storage:
§ Ventilation: Sufficient ventilation is crucial in order to prevent the build up of condensation. Insufficient ventilation could also lead to loss of energy content, and the build up of excessive temperatures with the risk of fire.
§ Turn up: In the case of large stores, it will be necessary to turn the fuel regularly. This can be accomplished by withdrawal the fuel from the bottom of the store at a higher rate than is required and return the excess to the top of the pile.
§ Drainage: Drainage is necessary, for the cleaning of the store (e.g. fungal contamination), as well as in case of inadvertent ingress of water,
Biomass can be stored either above ground, or underground. In general the biomass can be stored in a built structure that is special designed for this purpose, or in an unit which can be adapted, (e.g. shipping container) or an off the shelf, prefabricated unit designed for a specific range of fuels (e.g. pellets) [BIO08].
§ It is important to mention that in the case of integration of above ground constructions in historical buildings, special care should be taken in order not to disturb the aesthetic value of the building. Even though underground storage is likely to cost more than many above ground store designs, it may have advantages, such as easing space requirements and aesthetic benefits. On the other hand, for underground storage protecting the fuel from humidity may be more difficult, due to the fact it is harder to incorporate ventilation and drainage facilities.
Figure 15: Left: Woodchip silo and blower in Wexham nursery Right: Shredded wood supply [TAR8]
The transfer of the fuel from the storage device to the boiler is achieved with an Archimedes screw or auger, which transfers the fuel to the boiler.
Figure 16: Revolving spring arms which sweep fuel on to an Archimedes screw or auger, which transfers the fuel to the boiler
Rules of thumb and practical examples
2.2.2 Ventilation units – Heating and cooling units
The integration of new heating, ventilating and air conditioning systems in historic buildings may be required for the improvement of human comfort, control of indoor climates in museum collections or for the operation of complex computer equipment. Nevertheless, HVAC integration in historical buildings may result in visual and/or physical degradation of the historic aspects of the construction. More specific:
§ A number of potential problems may rise during HVAC integration processes concerning aesthetic aspects, due to the removal of historic materials in order to install new systems, or spaces that are altered in order to incorporate dropped ceilings, boxed chases, grilles and various equipments.
§ The structural system may be weakened by the weight and vibrations caused by large equipments.
§ Material’s degradation due to the moisture introduced into the building and migrating on historic materials may encounter. As natural vapor pressure moves moisture from a warm area to a colder, dryer area, condensation may occur on building materials in the colder area. On the other hand, too little humidity in winter can dry and finally crack historic wooden surfaces.
Figure 17: Example of Rooftop HVAC Equipment [WBD07]
In order to prevent potential drawbacks, certain precautions can be taken:
§ Exterior historic building walls should not be removed in order to add through-wall heating and air conditioning units, which are visually disfiguring and destroy historic fabric. Condensation falling off from such units can further damage historic materials. Also historic features of the building should not be masked. Furthermore, solar panels, chimney stacks, vents or other equipment should be placed on invisible portions of roofs or at insignificant locations on the site.
§ Caution should be taken in order not to put extra stress on materials through vibrations produced by the HVAC system. Overloads in the building structure should be avoided.
§ In certain cases it is more desirable to improve energy efficiency of existing buildings by installing insulation in attics and basements than to add insulation and vapor barriers to exterior walls. Only when it can be done without further damage to the resource.
§ It is preferable to use separately zoned HVAC systems to serve areas with different users, loads, orientations and hours of operation rather. Computer server rooms should have separate cooling systems when rooms require continuous cooling. Accordingly, distributed air-handling mechanical rooms should be considered in order to reduce the size and complexity of ductwork systems. Special attention should be paid to noise control.
§ Air intakes should be isolated and be placed upwind and away from building exhaust air, loading dock, or parking lot vehicular exhaust air, as well as away from adjacent spray, combustion gases, sanitary vents, trash storage and other sources of undesirable air contaminants.
§ Rooftop units should be avoided if other roof top uses are precluded and maintenance is obstructed (which may result in inefficient operation). Always install air-handling units in accessible locations where they can be maintained.
A typical outside feature of the ventilation system is the earth to air heat exchanger. This is a buried pipe, through which a fluid circulates by means of electric fans or pumps. The fluid may be air or a water solution. Earth heat exchangers can be used combined with a HVAC system for preheating or pre- cooling the working fluid. It is a simple construction with low to medium construction cost.
Figure 18: Horizontal buried pipes [TAR08]
Earth-to-air heat exchangers can be applied in either an open-loop or a closed-loop circuit. In an open loop circuit the inlet is located outside of the building, while in a closed-loop circuit, both inlet and outlet are located inside the building. Plastic, concrete or metallic pipes are used in modern applications. They are used to cool the ambient air before injecting it into the building, or the indoor air if the system is used in a closed loop. The temperature decrease of the air depends upon the inlet air temperature, the ground temperature at the depth of the exchanger, the thermal conductivity of the pipes and the thermal diffusivity of the soil, as well as the air velocity and pipe dimensions. Detailed calculations are needed to optimize such a system. As a threshold value for the application of the system, the ground temperature around the tubes should be at least 5-6 C lower than the ambient air temperature.
The closed system easiest to install is the horizontal ground heat exchanger. Due to restrictions in the area available, in Western and Central Europe the individual pipes are laid in a relatively dense pattern, connected either in series or in parallel (Figure 19).
To save surface area with ground heat collectors, some special ground heat exchangers have been developed. Exploiting a smaller area at the same volume, these collectors are best suited for heat pump systems for heating and cooling. The main thermal recharge for all horizontal systems is provided for mainly by the solar radiation to the Earth's surface. It is important not to cover the surface above the ground heat collector.
Figure 19: Horizontal ground heat exchangers
Because the temperature below a certain depth (15-20 m) remains constant over the year, and because of the need to install sufficient heat exchange capacity under a confined surface area, vertical ground heat exchangers (borehole heat exchangers) are widely favored. In a standard borehole heat exchanger, plastic pipes (polyethylene or polypropylene) are installed in boreholes, and the remaining room in the hole is filled (grouted) with a pumpable material. Several types of borehole heat exchangers have been used; the two possible basic concepts are:
• U-pipes, consisting of a pair of straight pipes, connected by a 180°-turn at the bottom. One, two or even three of such U-pipes are installed in one hole. The advantage of the U-pipe is low cost of the pipe material, resulting in double-U-pipes being the most frequently used borehole heat exchangers in Europe.
• Coaxial (concentric) pipes, either in a very simple way with two straight pipes of different diameter, or in complex configurations.
Related chapters
Part 2 Chapter 7: Integrating ventilation systems
Part 2 Chapter 8: Integrating heating and cooling emission systems
2.2.3 Cooling towers/condensors/air evaporators
As in the case of the integration of new heating, ventilating and air conditioning systems in historic buildings, integration of cooling towers, condensers and air evaporators may be required for the improvement of human thermal comfort. Integration in historical buildings may result in visual and/or physical degradation of the historic aspects of the construction.
§ The damage from installing a ducted system without adequate space can be serious for a historic building.
§ The place of the equipments needs to be foreseen from the beginning of the project. It should be carried in mind that the equipment may severely contribute to the aesthetic degradation of the building. The systems need constant balancing and can be noisy. The best place for the position would be on flat roofs, or on the ground.
§ Even if it is generally not recommended to place cooling towers and condensers in hot flat roofs due to the fact that it could severely reduce the performance of the equipments, in many cases it may be the only available solution.
§ Equipment should not be far away from the technical rooms, in order to minimize duct lengths and consequent energy losses and aesthetic degradation of the building.
2.2.4 Heat pump applications
Heat pump applications always need a heat source outside the building from which heat is extracted at a low temperature to be upgraded to a high temperature by the heat pump. Thermal energy can be extracted from a variety of renewable sources, including air, earth and water and upgrade it to a higher, more useful temperature.
Different type of heat sources exist:
Table 1: Commonly used heat sources [HPQ08]
§ Air source heat pumps extract heat energy by the surrounding air. They usually consists of a compressor and an evaporator coil and heat exchanger. The amount of energy consumed to operate the pump is much less than would be required to heat the building by conventional means.
Figure 20: Air source heat pumps [HPN08]
§ Water source heat pumps are presented in open or closed systems that are used to tap into this heat source. In open systems the ground water is pumped up, cooled and then re-injected in a separate well or returned to surface water. Closed systems can either be direct expansion systems, with the working fluid evaporating in underground heat exchanger pipes, or brine loop systems. Due to the extra internal temperature difference, heat pump brine systems generally have a lower performance, but are easier to maintain. A major disadvantage of ground water heat pumps is the cost of installing the heat source.
Figure 21: Pipe work passing through external wall to the receptor panel of a water [SBS08]
§ Ground source heat pumps move heat from underground into the house for space heating and domestic hot water. They cool the house by moving heat from the house into the ground and the hot-water tank. Underground temperatures remain almost constant year round, regardless of air temperature variations. The area of constant temperature is the heat source for heating and the heat sink for cooling.
Related chapters
2.3 References
[NEI00] Niedrig Energie Institut, Detmold, „Baupraxis für Niedrigenergie-Häuser in Nordrhein-Westfahlen“, http://www.nei-dt.de/neh-baupraxis/6__Luftdichtheit/Kap605.pdf, accessed March 26, 2008
[LUD03] „Technische Neuerungen zum Thema Luftdichtheit“, October 2003, www.luftdicht.de, accessed March 26, 2008
[BIO08] www.biomassenergycentre.org.uk. Last accessed 7/09/2008
[TAR08] The TAREB project: http://www.learn.londonmet.ac.uk/packages/tareb/en/index.html Last accessed 7/09/2008
[WBD07] http://www.wbdg.org/resources/env_hvac_integration.php. Last accessed 7/09/08.
[HPC08] www.heatpumpcentre.org
[HPN08] www.heatpumpnet.org.uk
[SBS08] www.sbsa.gov.uk/pdfs/4.3.pdf. Last accessed 7/09/08.