technicalmonograph Natural ventilation strategies for refurbishment projects Can we avoid mechanical ventilation?

Transcription

1 tecnicalmonograp 3 Natural ventilation strategies for refurbisment projects Tis tecnical monograp is one of a set produced as part of te REVIVAL project an EU Energie Programme supported demonstration project of energy efficient and sustainable refurbisment of non-domestic buildings in Europe. Te monograps explore some of te main energy and comfort issues wic arose during te Design Forums eld wit eac of te six sites. Te four monograps are entitled: Termal mass and pase cange materials in buildings Adaptive termal comfort standards and controls Natural ventilation strategies for refurbisment projects Hig performance dayligting Refurbisment projects often involve canges to te ventilation strategy. Commonly tis will include te reduction of uncontrolled infiltration, usually as part of a general upgrading of te envelope. Te reduced infiltration will reduce eating loads, but may result in indoor air quality (IAQ) dropping below acceptable standards. Tis will often prompt te installation of mecanical systems as part of te refurbisment package. Furtermore, mecanical systems may often be installed to combat overeating, te result of poor design features suc as large areas of unsaded glazing. An integrated approac to te refurbisment specification may allow natural ventilation, wit te following advantages: Save electrical fan-power Reduce plant costs and maintenance Save space Improve ealt and well-being of occupants Can we avoid mecanical ventilation? Natural ventilation is becoming an attractive alternative to mecanical ventilation or full airconditioning in new and re-furbised buildings. It is of course, a re-discovered tecnology, since before te 20 t century all buildings were naturally ventilated. Furtermore, almost all residential buildings, and many non-residential buildings still rely on natural ventilation. Wy te big issue ten? Figure 1 Te Albatross building in Den Helder as an advanced passive ventilation system incorporated in a secondary glazed skin. Tere are two important factors tat ave canged te situation. Firstly buildings ave got muc bigger and more complex, and secondly, te influence of energy conservation as required tat wasteful over-ventilation is avoided. Traditionally buildings ad leaky envelopes and tis often provided a failsafe for air quality. But wit te growing trend to make air-tigt envelopes to save energy, ventilation as to be designed more precisely. Tree different purposes for ventilation can be identified and eac needs to be considered separately altoug teir provision may be by common elements:

2 2 Natural ventilation strategies for refurbisment projects Ventilation Purpose Ventilation rate Minimal ventilation To maintain air quality Typical winter case ac/ Space cooling To vent unwanted eat Typical summer case 2 12 ac/ Pysiological cooling To provide direct air movement to occupants Typical summer case m/s Tese purposes make very different demands on te building. In winter, te problem is to excange just enoug air to maintain sufficient air quality. Build Tigt Ventilate Rigt implies tat te ideal is to ave an airtigt envelope wit purpose made controllable ventilation openings, positioned to give te best mixing and minimise discomfort from cold draugts. Openings may be windows, or closable grilles or trickle vents. If tey are windows only, tey must be able to be set at a very small opening area. Te driving forces of natural ventilation Wind generates pressure differences across te building wic cause air to flow troug openings in te building envelope, figure 2. Temperature difference between inside and outside causes a vertical pressure gradient wic causes air to flow vertically (upwards if building is warmer tan outside). Tis is known as buoyancy flow or more commonly as te stack effect, figure 3. Te control of ventilation rate For muc of te time bot of tese forces are present simultaneously. Te problem is tat bot are igly variable. Overall air cange rate on a cold windy day will be many times tat on a warm calm day, unless te openings respond to cange te flow resistance, figure 4. Rules for winter ventilation Figure 2 Wind pressure distribution. Airflow takes place between openings at different pressures. Figure 3 Temperature difference between inside and outside creates a pressure difference across te envelope driving airflow in troug openings at te base and out te upper part of te building. Openings sould be small and controllable to account for different wind strengts and temperature differences. Openings sould be ig up in te external wall of te room to encourage mixing and minimise draugts. If only fixed trickle vents are provided, tey ave to be large enoug to cope wit te worst conditions i.e. no wind and small temperature differences. Tis will lead to over-ventilation in conditions of significant wind and large temperature differences, and ence wasted energy, for muc of te time. Automatic vents are now becoming available, wic close up as te pressure differences get greater, tereby stabilising te air-flow rate. Or active ventilators under BMS control can be modulated in response to temperature and wind speed, or indoor air quality (IAQ). Rules for summer ventilation Openings sould be large and easily controllable (good access to andles, stays, locks etc) Openings sould be well distributed

3 tecnicalmonograp 3 3 vent. rate 1.0 average 0.4 rate exceeded for 80% of time days Figure 4 Due to te variability of te driving forces, fixed openings sized to provide sufficient minimum ventilation will result in over ventilation (and energy waste) wen driving forces are large. vertically and orizontally to encourage flow between parts of te façade at different pressures (see figures 6 and 7). Consideration must be given to ow te incoming air will affect te occupants. Sading devices must not block summer ventilation openings (see Tec. Mon. 4) Openings may require special design features to reduce transmitted noise. In bot cases, consideration must be given to te distribution of fres air witin te space. Different distributions of openings allow different depts of floor plan to be naturally ventilated. Te following table (figure 5) gives a rule of tumb. Figure 5 Dept of effective natural ventilation in rooms from side openings Note tat te dept of effective ventilation is dependent on floor to ceiling eigt. Te removal of a suspended ceiling may represent a refurbisment opportunity for improving natural ventilation (and dayligting), provided oter problems suc as servicing and acoustics can be solved. Single-sided Double sided (cross ventilation) Figure 6 Wind generates complex pressure distributions on buildings, particularly in urban environments. Tis assists ventilation, provided tat openings are well distributed and flow pats witin te building are available. Figure 7 For a given total area, ventilation is improved wen openings are well distributed vertically and orizontally. Tis is because air flows at different pressure between openings. It also leads to a better distribution witin te room. Single opening Multiple openings well distributed vertically and orizontally Dept of floor in units of floor to ceiling eigt 2 3 6

4 4 Natural ventilation strategies for refurbisment projects poor good Figure 8 Stacks can ventilate a deep plan building; wind and buoyancy forces bot create negative pressure causing fres air to flow in from te perimeter Figure 10 Nigt ventilation can reduce daytime temperatures by as muc as 4 K. However it only works were tere is termal mass available internally, and ig rates of nigtime ventilation. 23 o C 17 o C Advanced ventilation tecniques Ducts and cimneys So far tis guidance as considered ventilation provided by openings in te facades of te building, and usually doubling up as windows. Tis imposes some limitations on plan dept and layout. To get sufficient quantities of air in and out of an existing deep-plan space, ducts and/or cimneys may be provided, figure 8. Large vertical ducts can generate larger airflows, by te so-called stack effect, tan can be obtained in a single room by side openings, because of teir greater eigt. Furtermore, wen te wind does blow, tere is always a negative pressure across te top of a building; tus te wind driven and stack driven flows complement eac oter. In order to keep flow resistance low, te cross-section must be quite large, typically between 2% and 5% of te floor area tey are serving, assuming a similar area of inlet is available at te perimeter. Tis may be difficult to provide in an existing o C 27 C o Figure 9 Solar cimneys use solar gains to eat te air column. However, te driving force is dependent on te eigt of te warm air column and te average temperature difference. It follows tat it is no good eating up te air at te top only. building, altoug use of existing ligt wells or redundant lift safts migt be a solution. Te performance of te stack can be enanced by eating te air in te stack by solar energy, but te air must be eated from te bottom, not just as it leaves te stack, since it is te eigt of te warm column of air tat drives te flow, figure 9. Tese elements are often referred to as solar cimneys; tey may ave problems in cold and sunless weater wen te poorly insulated glass, cools te air and generates reverse flow. Because tey need to be eated for te eigt of te column, tey cannot be located in te centre of a deep plan building, unless te solar part protrudes a long way above te roof level. Nigt ventilation Te termal mass of buildings can be used to soak up eat gains in te day, tus reducing peak temperatures, figure 10. But te eat as to be taken out of te building at some time. Tis is done most efficiently by maximising te ventilation rate at nigt. Tis tecnique can be so successful tat te daytime temperatures in te building can be considerably lower tan te peak outdoor temperature (up to 3 or 4 o C). In tis case it pays also to reduce te ventilation rate in te day wen (and if) te outdoor temperature is above te indoor temperature. Maximum ventilation G Minimum ventilation acoustic vents In nigt ventilation strategies te following sould be considered: Provide openings wic can be left open at nigt, but maintain security. Consider ow large volumes of air will flow troug te building, from room to room and floor to floor.

5 tecnicalmonograp 3 5 Termal mass is only effective if it is coupled to te occupied space, and to te nigt ventilating air. Ligtweigt internal finises to eavyweigt buildings makes tem beave as ligtweigt buildings te benefit is lost. Consider exposing massive elements if covered e.g. suspended ceilings. In ligtweigt buildings, consider adding termal mass (e.g. new partitioning) or even pase cange materials (see Tec Mon 1). Note tat refurbisment may involve te application of insulation internally to solid masonry walls. Tis will significantly reduce accessible termal mass. If tis as to take place, ensure tat internal partitions, and if possible floor slabs, are eavyweigt and not isolated. Hybrid systems Natural and mecanical ventilation need not be mutually exclusive. Obviously, certain spaces in a naturally ventilated building can be mecanically ventilated if tey are internal or ave ig ventilation demands, suc as toilets or kitcens. Muc fan power is wasted ventilating unoccupied or ligtly occupied spaces. Demand control is were fans are only run wen te air quality (as detected by CO 2 levels) is deemed unsatisfactory. CO 2 detecting controls are now ceap and reliable. Fans can be used to supplement natural air-flow in ducts and cimneys wen te wind and buoyancy forces are too weak. Tis function also needs to be activated by a control system wic detects a reduction of air-flow or a fall in air quality. Te arguments for and against ybrid systems are: For Passive systems would ave to be oversized in order to cope wit worst case scenarios by accepting mecanical intervention, wit appropriate controls, an optimum balance between energy efficiency and comfort can be struck. Summary conclusions Natural ventilation can acieve ig level of comfort, However, it requires considerable co-operation from te occupants and management, so it is important tat bot understand te principles and are aware of te problems. Natural ventilation cannot provide suc consistent and uniform conditions as mecanical systems. However, te positive side of tis, is tat tere is growing evidence tat variation in indoor conditions is tolerated and even enjoyed, provided te occupants are in control (see monograp Adaptive termal comfort standards and controls ). Oter factors wic ave to be considered include te outdoor conditions, for example natural ventilation may be impractical in very noisy or polluted environments. However, it is observed tat even in noisy urban environments, people will open windows, trading termal comfort for traffic noise. It is important to consider te effect of te wole package of refurbisment measures since tey interact wit on and oter. For example, a building were air-conditioning ad been identified as te only solution to comfort problems, migt provide a satisfactory environment wit natural ventilation, provided measures suc as sading and fabric improvement were also carried out. Hybrid systems sould be considered were mecanical systems act as failsafe, activated only wen air quality or termal comfort falls below an acceptable level. Prepared by Nick Baker te Martin Centre University of Cambridge Against Necessitates te capital expense and maintenance of two systems One tecnique were a small amount of mecanical power can be used wit great effect is te use of ceiling or desk fans. Tese devices provide air movement but not fres air. Te air-movement can make a reduction in te effective temperature, i.e. as perceived by te occupants, of as muc as 3 o C. Altoug tey improve termal comfort, tey do not improve air quality.