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HVAC system performance in large office buildings : ventilation, source control and filtration support air quality
Shaw, C. Y.
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HVAC system performance in large office buildings: ventilation, source control and filtration support air quality
Shaw, C.Y.
NRCC-46762
A version of this document is published in / Une version de ce document se trouve dans: Canadian Property Management, v. 18, no. 6, Oct. 2003, pp. 12-14
HVAC System Performance in Large Office Buildings : Ventilation, Source Control and Filtration Support Air Quality
By C.Y. Shaw
The author discusses some key factors that influence the performance of ventilation systems in large office buildings and provides information for the efficient operation of these systems.
Good indoor air quality in commercial office buildings is very important for the health, safety and satisfaction of the people working in them, and is of prime concern for building managers. Achieving good air quality is a challenge for designers, builders and building managers, especially with the new materials, furnishings, products and processes used in construction, some of which could be potential sources of air contamination.
Factors Affecting Air Quality
The three major factors that can be controlled to achieve good air quality are ventilation, source control and cleaning/filtration. Ventilation supplies fresh outdoor air to an enclosed space and removes the contaminated/stale air from this space through the exhaust. Source control refers to the use of environmentally friendly building materials and furnishings, floor coverings, surface finishes, adhesives and cleaning products to minimize the potential for indoor air contamination. For buildings located in urban centres or near industrial plants where the quality of the outdoor air may be worse than that of the indoor air, filtration techniques can be used to remove contaminants from the intake air. Cleaning/filtration techniques can also be used to remove contaminants from the indoor air.
Ventilation
Ventilation is the most frequently used and in most cases the only available strategy to building operators. The main factors to be considered for controlling the cost and efficiency of a ventilation system are:
• Air distribution
• Air leakage
• Local exhaust
Air Distribution
Ideally, all zones of a building should receive adequate ventilation air. The heating, ventilating, and air-conditioning system (HVAC), with supply and return ducts for each zone, can usually satisfy this requirement. However, in some buildings under certain conditions, the performance of the HVAC system can be adversely affected by the operation of exhaust fans, and wind and temperature conditions. A power exhaust fan
can lower the pressure in a zone and force more supply air into that zone. Wind pressure and temperature also affect the ventilation system. Wind blowing around and over a building causes variations in pressure around it. Temperature differences between the inside and the outside during the winter time lower the pressures in a building on lower floors and increase the pressures at upper floors as a consequence of differences in air density. This is known as the "stack effect." Such changes in pressure within an office building due to exhaust fans, wind or temperature, individually or in combination, can significantly affect the movement of ventilation air.
A study conducted by IRC revealed that wind can significantly pressurize the internal area of a building on the windward side and de-pressurize that on the opposite side. These can result in one part of the building receiving more ventilation air than that called for in the design.
Air Leakage
Air leakage is the infiltration of air through the building envelope, which depends on the airtightness of the envelope and the pressure difference across it caused mainly by wind and temperature differences between the inside and outside. Leaky buildings are more costly to heat and more difficult to ventilate properly than relatively airtight buildings. Airtightness measurements made by IRC in various types of buildings, including eight office complexes, showed that renovations to improve airtightness can lead to significant energy savings by reducing over-ventilation. The IRC study showed that designers and owners can profit by knowing how airtight a building is. Sealing large cracks and openings in the exterior wall and around windows is one good way to improve airtightness.
Contamination Source/Local Ventilation
Increasing a building’s ventilation rate to speed up the removal of localized air-borne contaminants, even when energy use is not a concern, is not a solution to such a problem, as indicated by another IRC study. The most efficient strategy to improve air quality is to remove the contaminants at the source by local exhaust and then to rely on ventilation for the rest of the building. In the case of an identifiable contaminant source such as office equipment, the exhaust from the source should be connected directly to the outside.
Energy-Efficient Control of Ventilation
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends ventilation rates for buildings based on the number of
occupants. For office buildings, which are typically occupied only during the day, it may be possible to control ventilation rates on the basis of the number of occupants at a given time. A demand-controlled ventilation system, using the concentration of
occupant-generated CO2, can be used to control the ventilation rates. For this, however, the
- The CO2 concentration must be proportional to the actual number of occupants at a
given time,
- The CO2 concentration should be the same or very close to it on all floors,
- There should be suitable locations for placing the CO2 sensors,
- The CO2 concentration must be proportional to the building’s air change rate
(ventilation rate).
An IRC study in a 22-storey office tower with an interior volume of approximately
113,700 m3 confirmed that the CO2 concentration varied according to the number of
occupants. The concentration is low during the night, begins to rise as the employees arrive in the morning and peaks around noon. It drops somewhat during the lunch hour, then picks up as the employees return to work and peaks again around 4 p.m. Once the occupants begin to leave, the concentration drops continuously until it reaches the low
level at night. The CO2 concentrations measured at various locations on a particular floor
agreed closely with the concentration measured at the return-air shaft of that floor which also agreed closely with that measured at the top of the two main return shafts to which the return ducts of each floor were connected. This finding suggests that the
measurements taken at the tops of the two main return-air shafts give representative
values of the CO2 concentrations throughout the building. Hence the tops of return-air
shafts are suitable locations for CO2 sensors. The study also established the relationship
between the measured CO2 concentration and the air change rate, which shows a good
correlation between the measured CO2 concentrations and the air change rates. Thus, this
relationship can be used as the basis for controlling the building’s ventilation rate.
Summary
• Improving the airtightness of buildings will reduce air leakage and cold drafts, and
reduce energy use by improving the performance of ventilation systems.
• Sources of contamination should be minimized as much as possible either by using
environmentally friendly furnishings, materials and products, or by exhausting the contaminants at the source. General ventilation should then be used.
• For office buildings where the number of occupants varies significantly with time, it
may be possible to further improve energy efficiency by controlling their ventilation rates based on the actual number of occupants at a given time. Before installing this type of control system, tests should be conducted to assess the feasibility of using such a system in the building.
Further Reading
Shaw, C.Y. Maintaining acceptable air quality in buildings through ventilation, Construction Technology Update No. 3, Institute for Research in Construction, National Research Council of Canada, Ottawa, January 1997.
Shaw, C. Y. Influence of air diffuser layout on the ventilation of workstations, Construction Technology Update No. 37, Institute for Research in Construction, National Research Council of Canada, Ottawa, June 2000.
http://irc.nrc-cnrc.gc.ca/ctus/ctu37e.pdf
Dr. C.Y. Shaw is a senior research officer in the Indoor Environment Program of the National Research Council’s Institute for Research in Construction. He can be reached at 1-613-993-9702 or john.shaw@nrc-cnrc.gc.ca
