October 2000
The following matrix is entitled Building Environmental Control Strategy . The purpose of this document is to serve in explaining, through comparative analysis, three proposed general strategies for providing environmental controls within a building located in the Northeast and more specifically, Vermont. The building typology investigated is a college residence hall.
KieranTimberlake
James Timberlake, FAIA
Kevin Rasmussen
Introduction
The following matrix is entitled Building Environmental Control Strategy . The purpose of this document is to serve in explaining, through comparative analysis, three proposed general strategies for providing environmental controls within a building located in the Northeast and more specifically, Vermont. The building typology investigated is a college residence hall. The three general strategies are as follows:
Strategy A - (Mechanically assisted) Natural Ventilation (no air conditioning)
Strategy B - (Mechanically assisted) Natural Ventilation (no AC but provide internal infrastructure for future ease of addition of AC)
Strategy C - Air Conditioning
The document is intended to present an objective matrix of facts as they describe each system, operation and value.
| KieranTimberlake | |||
| Atwater Commons Middlebury College | |||
| October 2000 | |||
| Building Environmental Control Strategies | |||
| Strategy C | Strategy B | Categories | Strategy A |
| Air Conditioning | (Mechanically Assisted) Natural Ventilation | (Mechanically Assisted) Natural Ventilation (no Air Conditioning) | |
| FF 10'-6" (9'-6" ceiling height. No fans to be provided.) | Same as Strategy A (Fans become less relevant if cooling is added) | Architectural Components Floor-to-floor height/Ceiling height | FF 11'-6" (10'-6" ceiling height) (9'-0" to bottom of fan. Fan moves air through spaces. Provide one 4'-0" OD (outside diameter) fan in each bedroom, two fans in common room. Additional height allows for clearance of ceiling fan and stratification of inside air - warm above, cool below) |
| No cross ventilation required although desirable - (therefore plan arrangement is less relevant except as attempt to reduce cooling and heating loads) | Same as Strategy A | Building (Plan) Configuration | Plan arrangement supports cross ventilation. Vertically accessed units provide natural 'stack' effect. Each four bedroom suite with common room is arranged as a 'flat' (this offers east/west exposures for each flat and cross ventilation from one side of flat to the other. Vertical entries offer 'stack' effect vertical ventilation opportunities increasing evacuation of superheated air. Thinner building results in better arrangement opportunities on site and ensures cross ventilation) |
| Similar to Strategy A - reduction in size (Optional - with addition of operable windows less control may be effected regarding opening and shutting of windows to control cooling and heating costs - students have a tendency with operable windows to open them all year round regardless of mechanical system provided. Size of windows relative to naturally ventilating building is irrelevant) | Same as Strategy A (If cooling is provided then window size is less relevant) | Windows/Exterior Configuration | Provide large operable windows with screens - type optional. Larger size benefits views provided. (Operable windows on east and west sides enable through ventilation) |
| Similar to Strategy A (shading to be provided so as to reduce cooling loads) | Same as Strategy A | Shading Devices | Shade windows - with shutters; shades; blinds; deep setting window in wall; or, 'brow' (an overhead projection) (to reduce solar heat gain). Device(s) to be determined during design cycle. |
| Same as Strategy B | Same as Strategy A (If cooling is provided then window size is less relevant) | Window /Interior configuration | Provide operable transom windows above each bedroom door (Advantage of flat configuration; encourages through ventilation) |
| Similar to Strategy A - options on wall types greater since mass is less of a consideration (Conform with campus practice) | Same as Strategy A | Exterior Materials | Consistency with the material palette of the campus is a primary consideration. Exterior would utilize stone, masonry and concrete or some combination thereof. (Primary purpose is to increase mass of exterior walls to retain cool evening temperatures during the day and help in heat exchange at night) |
| Strategy C involves use of individual fan coil units located at or near windows piped with cold water during cooling cycles. | Same as Strategy A (If cooling system is elected then same as Strategy C) | Mechanical Components Cooling System | Ceiling fans. Stack. (Mechanically assisted) naturally ventilating structure. |
| Strategy C involves use of individual fan coil units (for cooling as well - in a 2 pipe system). | Same as Strategy C | Heating System | Radiator (Runtal units or similar, or baseboard) or panels at perimeter beneath windows with individual thermostat control (2 pipe system). |
| Not applicable | Same as Strategy A | Ceiling fans | Install one 4'-0" OD fan per bedroom, two per common area in each flat. (Movement of inside air through fan circulation provides a proven comfort equivalent of -4 degrees) |
| Not applicable | Same as Strategy A | Mechanical ventilation | Promote 'stack effect' ventilation by venting each flat into a vertical chase with rooftop exhaust fan and through stairwell exhaust. (Provide 5-10 air changes per hour. This further encourage inside/outside air exchanges during cooler morning and evening hours more rapidly exchanging hot air for more temperate air. Further, the added air changes each hour promotes a healthier environment) |
| 72 degrees F to 76 degrees F with lower humidity level. | Same as Strategy A | Comfort Expectations General | Indoor accepted comfort threshold is from 84.5 degrees at 60% relative humidity to 86.5 degrees at 20% RH, with air movement. This threshold is defined for 80% of the population given generally accepted guidelines for dress and activity. (See ASHRAE 55-1992 guidelines within backup material. Above that temperature, or higher humidity levels, or lack of air movement in some combination all contribute to less comfortable conditions). |
| System would be able to easily maintain 72-76 degree range inside air temperature throughout cooling seasons and most obviously summer months. Air conditioning would control inside humidity and temperature when exterior air temperature/humidity ratio is u | See expectations for Strategy A (right) and Strategy C (at left). If air conditioning is installed (Strategy C) the addition of air conditioning would obviate Strategy A. Therefore the concept of only employing summer air conditioning to control inside air humidity and temperature when outside air temperature/humidity ratio is high (7-10 days a year in Burlington VT) is conceptually false. We believe the availability of the 'tool' would generate the constant use of the 'tool.' | Specific Comfort Conditions | This system meets or exceeds the comfort threshold criteria for most seasons and 98% of summer climate conditions. Mechanically assisted natural ventilation would maintain the exterior ambient air temperature and humidity throughout spring, summer and fall seasons within the interior. The interior comfort threshold would be exceeded when outside air temperature reaches and sustains a temperature at or above an average of 85 degrees with 20-60% relative humidity levels. This period would be approximately one week to ten days of a typical year in mid- to- late July in Burlington VT ( or roughly 231 total hours). (With this system no dehumidification will occur. The likely most uncomfortable time would generally be during a few hours a day during a period from 2-to-5 or 6PM in mid- to late July. During this period the levels of air temperature and/or humidity would exceed the indoor comfort threshold.) |
| Highest first cost. | Same or higher first cost than Strategy C. Additional volume, fans and air conditioning infrastructure - which may or may not be employed. | First Cost (construction cost) General | Generally lowest additional cost. |
| No interior operating transoms. Internal stack/chase not required for ventilation purposes. | Same as Strategy A | Architectural Components | Increase exterior and interior envelope height (over Strategy C) by approximately 1'-0" per story; total volume increase is 8%. Increase size of vertical stack/chase to allow for additional ventilation load over and above that of bathrooms and kitchenette |
| Heating employed through fan coil units - hot water circulation. Beyond that required for heating this Strategy requires full air conditioning system equipment - DDC controls, piping, chillers, pumps, etc. | Same as Strategy A and C. 2 pipe system will serve both heating and cooling. | Mechanical Components | Beyond that required for heating the system is generally low tech - ceiling fans; vertical stack (for mechanical ventilation/exhaust; and exhaust fan. Occupancy sensors may be added to control lights and fans when rooms are unoccupied to reduce power con |
| Operational cost generally the highest. Provide power and maintenance for air conditioning system. | Similar to Strategy A if no air conditioning is completed. Internal infrastructure if not utilized will at some point in the life-time of the building require maintenance. Strategy A: Provide electrical power to fans; maintain/replace fans. If Strategy | Long Term Cost (maintenance and operation cost) General | Operational cost generally lowest. Fans utilize 1/6 the power of a full air conditioning system. Students will be more likely to turn on or off a fan as opposed to an air conditioning system that provides air to the whole flat. Provide electrical power to fans; maintain/replace fans. |
| The value of air conditioning a residence hall in the Vermont climate stratum - for potentially 7-10 days of weather that tests the comfort threshold in mid- to late July (during summer school) - is of questionable value both in terms of initial cost, amo | Same or higher first cost than Strategy C. This strategy leans toward Strategy C. Why put the infrastructure in unless you elect to use it? However, up-front installation of air conditioning infrastructure eases future installation of full system if C | Additional Considerations General | Best value in terms of cost benefit in short and long term. The occupational cycle of a college residence hall is different than that of a library, classroom or administrative buildiing. Residential rooms are generally vacated by 9AM (for classes) and return to above average occupation after 5PM. Higher cooling cycle loads peak at a period of less than average occupation with this building type. Higher ceilings improve light and proportion of flats; are more in keeping with the older residence halls on campus in loft and proportion of rooms; loft storage in bedrooms is an additional benefit. The geographical location of Middlebury enables a strategy such as this and is most consistent with the stated long term environmental policies of the College. |
Background and Approach
We have been through an intensive pre-schematic, feasibility study and program stage with a client of the firm, a liberal arts College in Vermont. We have planned, through the development of options, from the early stages of design the proper siting, position, orientation and massing of any proposed structures. The options and strategies proposed have been in concert with the College's program. Internal plans, sections and massing for the proposed options have taken into account the location on the campus, nearby structures, the College's architecture, and needs for the proposed structures to meet the program. Planning decisions have been consistent with provisions for a naturally ventilated structure.
We have had discussions with members of the College's various committees. We have reviewed the environmental policies of the various College constituencies. We have worked with our engineers LKPB in St. Paul, MN to review some initial strategies for heating and cooling the proposed structures. In addition, we have consulted independently with Don Prowler, FAIA, a Philadelphia architect, internationally renowned for sustainable design consultation, who has been a principle developer of the Federal Sustainable Design criteria for federal structures. He has given us critiques on approach, general and specific guidance toward the natural ventilation strategies and the comparison with an air conditioned approach. They each in turn have provided us background with which to help us develop this matrix.
This document represents only the surface of the body of work we have completed to understand the possibilities related to natural ventilation but also the methods, means, positives and negatives of adopting this strategy. We have familiarized ourselves with ASHRAE standards and definitions particularly 90.1-1999 for general, recent energy design guidelines and 55a and 55b-1992 for Thermal Comfort criteria. We have checked our climate criteria for the region. Excerpts of the research that we have done or has been provided to us, is attached.
Proposed Strategy/Architecture and Mechanical Approach
The proposed strategy involves three, four or five bedroom suites - in 'flat' arrangements - with vertical entryways allowing for thorough and adequate cross ventilation (see the attached mechanical scheme diagrams). The buildings are proposed to be sited with their long sides to the east/west taking advantage of spring, summer and fall prevailing winds (see the attached site plan). The maximization of thermal comfort comes through 'mechanical assistance' to the natural ventilation. Vertical ventilation stacks will help to draw out heated air. Internal room ceiling fans will assist air flow. Transoms provide through air flow across suites. A slightly higher floor-to- floor dimension helps to stratify warmer from cooler air aiding comfort. Thicker walls of stone/masonry, consistent with the other buildings on campus, add gain mass and a thermal 'sink' for cooler air. Window shading will further serve to reduce heat gain. All of these are principles of good architectural practice which many of the earlier campus buildings relied upon. Here it is interpreted in a 21st century way.
Recommendation
It is our opinion and recommendation that the region setting, site and architectural strategy that has been developed is the best candidate for a mechanically assisted natural ventilation system.
Three facts help us to this conclusion - climate, geography, program/occupation of the buildings. First, the northeast regional climate, and that of Vermont particularly, is considered arid and underheated. This means that with the exception of 7- to- 10 days of the year, generally mostly situated in July, exterior temperature and humidity levels are well within the optimum thermal comfort range for human beings.
Second, the site on a ridge line set between two major mountain ranges with north/south valleys provides a channeling effect which helps to flush stagnate air and provide most days with air movement. The proposed siting of the buildings is intended to aid this effect. Each of the buildings stands alone. The buildings are located parallel to current small ridge lines which are wooded and intended to remain so. The geometry of the buildings is intended to assist in flushing the 'common' outdoor space of the residential commons. Neither new building blocks another.
Third, residence halls on campus are not occupied 100% of the time during peak cooling load periods of the day - from 9AM until 5PM. While partial occupancy is to be assured nearly year round on campus with summer/language schools, students and faculty have other campus buildings during that period of time which are air conditioned - notably the library, a main classroom building, the student center and dining halls - which serve class, eating or studying functions during that period most likely to require cooling. By the time students are returning to their rooms the building is beginning a potential flushing cycle thereby negating the need for mechanically generated cooling.
Conclusion
The proposed mechanical strategy we believe will save money in the short and long terms. It will achieve 'earth in the balance' guidelines.
Links
http://www.naturalcapitalism.org/
http://www.buildinggreen.com/
http://www.sbicouncil.org/
© 1997– KieranTimberlake
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