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March 8, 2007
Like any living organism, buildings need to breathe for their own health and longevity, as well as the health of their occupants. Using natural or mechanical means, a building’s ventilation removes contaminated air from occupied space and replaces it with outside air.
In high-rise buildings, designing ventilation systems becomes very challenging.
Let’s look at a typical 35-story residential building. Indoor air is removed by kitchen, bathroom and dryer exhaust systems. The exhaust air could be removed through a vertical shaft going to the roof. This shaft could be 3 square feet for each toilet, 5 square feet for each dryer, and 8 square feet for each kitchen. If we have 10 units per floor, these shafts will use about 160 square feet. At a cost of $800 per square foot, this represents a “loss” of $128,000 per floor, or $4.48 million for 35 floors.
This number only applies if all of our shafts are perfectly straight. It is very common for top floors to have different configurations requiring the shaft to be offset to avoid going through the middle of the living room in the unit above. Off-setting fire-rated shafts is costly, and requires significant ceiling plenum space.
Installing ventilation systems horizontally offers an alternative. This eliminates all vertical shafts, but requires installing ventilation ducts in the non fire-rated soffits or in the structural slab. Now, we need an architectural solution to penetrations of the building façade.
The stack effect
So far, our discussion concerns removing air from the building. However, without replacing it with outside air, there will be significant negative pressure inside the residential units. This causes excessive air leakage through the building’s exterior, wastes energy used to condition indoor air, and deposits outdoor moisture and dust within the building’s envelope, causing deterioration.
One of most challenging tasks for engineers is minimizing impacts of the stack effect, also known as the chimney effect.
Stack effect results from warmer indoor air rising to the top of the building where it exits through available openings. In a high-rise building, these could be elevator shafts or the walls and doors of the mechanical penthouse. The openings at the bottom that allow incoming airflow are just as important. In a high-rise these could be ground-level entrance doors, or even the ramp to the underground parking.
In any tall building the stack effect can create significant pressure differences between spaces that cause undesirable movements of air throughout the building, reducing the effectiveness of ventilation and causing odors to leak between residential units.
The latest ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publications state that in high-rise buildings, mechanical ventilation is best achieved by keeping a slightly positive indoor air pressure. This could be accomplished with either a horizontal or vertical system. The horizontal approach requires more soffits; the vertical one requires shafts and fire-smoke dampers. The vertical shafts for outside air will increase the cost of the building (compared to the horizontal or Z-duct approach) by $400,000-$600,000. As an added benefit, the outside air supply will be filtered.
CB Engineers’ projects have not demonstrated that a vertical approach to designing ventilation systems is any better than the horizontal approach. Each building should be considered separately, evaluating various architectural features (floor layouts, exterior skin, floor to ceiling height, etc.) and cost considerations (vertical rated shafts, horizontal non-rated soffits).
Watch out for wind
Wind presents another challenge. It changes the balance of air pressure inside the building by causing infiltration on the windward side of a building and corresponding exfiltration on the leeward side and sides parallel to the wind direction.
In Seattle’s mild climate, operable windows reduce energy consumed by fans and mechanical cooling, and in most cases give occupants control over their living space. Further benefits include eliminating fan noise and sometimes, the mechanical cooling system. However, the desirability of this approach could be limited by concerns about urban noise.
Acoustical requirements can be met with passive air systems (Z-ducts, operable mullions, for example) for exterior ventilation. These may solve the acoustical issues, but this system could contribute to problems with the building’s stack effects, because Z-ducts are basically acoustically treated holes on the building exterior, and allow uncontrolled infiltrations of outside air.
Different strategies can improve the effectiveness of building ventilation systems. To start with, improvements to the building’s sealing will prevent stack effect: starting with the top and bottom of the building, then the shafts, and last, the outer shell. Zoning the building into compartmentalized areas can help equalize pressure differences, and finally, decoupling floors will prevent vertical air leakage.
Designing residential kitchen ventilation systems deserves careful analysis to identify the best approach.
Of the three ventilation systems in a typical dwelling (kitchen, bathroom and dryer), kitchen ventilation systems usually have the largest impact. This is because kitchen exhaust usually requires 400-600 CFM of exhaust, and with a gourmet kitchen it could reach 9001,000 CFM per hood.
To deal with kitchen ventilation systems, engineers use three basic approaches:
1. A ducted hood to the outside with a typical exhaust quantity around 400 CFM. In this case, we will need 400 CFM of make up air to keep the room in balance.
2. A recirculating hood not ducted to the outside. This type of hood does not contribute to negative pressure in the kitchen; however, they do not remove moisture generated during cooking and require frequent changes of charcoal filters to control odors.
3. Combining recirculating hoods with general exhaust from the kitchen will cut exhaust air quantities approximately in half, thus reducing requirements for make up air by half or even more in the case of large hoods, compared to conventional ducted hoods.
While there have been many advances in building technologies, ventilation strategies have not changed significantly over the past three decades. We would like to stress that the mechanical ventilation system should not be used to try to overcome problems caused by poor envelope design and construction. Pressurizing buildings in an attempt to counteract stack effect wastes energy. Only very close cooperation between architects, engineers and contractors will result in well-designed and well-functioning ventilation systems that maintain healthy buildings.
Igor Tartakovsky, PE, is president of CB Engineers, a mechanical and electrical engineering firm with offices nationwide, including Seattle. The company’s portfolio encompasses residential high-rises, commercial/office, higher education, biomedical/laboratory, hospitality, fitness, entertainment and correctional facilities.