Managing Stack Effect in Tall Buildings
MANAGING STACK EFFECT IN TALL BUILDINGS
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American Institute of Architects
This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.
Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
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Course Information
This course explores stack effect impacts in tall buildings. Stack effect represents an uncontrolled energy loss and can cause irritating noise, infiltration of humidity or odor, drafts and thermal discomfort, etc. Viewers will have an increased awareness of how to control and manage stack effect through passive strategies.
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Learning Objectives
1. Describe physics of stack effect and what it means for sustainable, efficient building design and occupant comfort
2. Discuss how to control location of neutral plane, and its importance for stack effect and natural ventilation
3. Articulate role of ventilation system in managing stack effect impacts
4. Illustrate methods of controlling stack effect to meet sustainable building and energy reduction goals through use of case study examples Copyright© 2019 by RWDI. All rights reserved.
Presented By
Duncan Philips, Ph.D., P.Eng Principal
Duncan is RWDI’s Global Practice Leader for Building Performance, heading up a talented pool of building performance engineers and scientists as they develop climate-responsive design strategies for individual buildings and masterplans. Clients benefit from Duncan’s ability to solve tough building physics problems by analyzing air flow and heat transfer phenomena.
Duncan has been involved in the design of tall and supertall buildings across the planet, in both hot and cold climates. He has assisted in stack effect mitigation for existing buildings in cities ranging from Dubai to Chicago and presented at multiple conferences on stack effect.
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Agenda
1. Introduction to Stack Effect
2. Criteria for Assessment of “Problematic” Conditions
3. Locating the Neutral Plane
4. Effects of Building Pressurization
5. Managing Stack Effect in Buildings
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Introduction to Stack Effect
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What is Stack Effect?
It does not happen because “hot air rises” • Can exist in all buildings • Induced by buoyancy force originating from indoor-outdoor temperature differences
Winter Stack-Effect Driven Airflow
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What is Stack Effect?
Benefits • Natural ventilation • True in all climate types +15 °C (60 °F) +25 °C (77 °F)
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What is Stack Effect?
Stack Effect
Stack Effect Stack Effect Driving Force Issues/Impacts (Pressures) Cannot get rid of Can reduce issues and driving force impacts through design
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What is Stack Effect?
Frequently asked question…
“How can I get rid of stack effect in my building?”
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What is Stack Effect?
The right question…
“How can I reduce the frequency of stack effect issues/impacts in my building?”
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Case Study
1999 N Broadway Denver, Colorado • 44 stories • Height: 548 ft (166 m) • Outdoor temperature: 10 °F (-12 °C) • Indoor temperature: 70 °F (21°C) • Moderately tall • Cold winter day (not extreme)
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Visualizing Stack Effect in a Tall Building
Video copyright: Dave McGrail, “Stack Effect Demonstration.”
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Visualizing Stack Effect in a Tall Building
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Potential Stack Effect Impacts
Whistling airflow through doors & cracks Complex issue driven by: • Shape and configuration of crack / opening • Flow rate through the crack
Sound can range from whistle to hum
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Potential Stack Effect Impacts
Difficulty opening & closing doors A nuisance… and a safety issue
Choice of door openers and closers is important
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Potential Stack Effect Impacts
Difficulty controlling temperatures on floors with excessive infiltration A space can be too cold – such as residential lobby in cold climate
Upper part of a building can be too hot in warm climates
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Potential Stack Effect Impacts
Elevator door operability issues & whistling Difficult to define, tends to be intermittent
Different elevator door manufacturers have different mitigation methods
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Potential Stack Effect Impacts
Increased building energy costs Due to uncontrolled airflow from outdoors
Unclear how much energy is lost via stack effect
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Potential Stack Effect Impacts
Ingress of odors from outdoors & migration of odors indoors Controlling odor migration in buildings is critical
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How Does Stack Effect Get Setup
Tube Air of Tube Tube Water of Tube
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How Does Stack Effect Get Setup
Tube Air of Tube Tube of Water of Tube
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How Does Stack Effect Get Setup
Tube Air of Tube Tube Water of Tube
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How Does Stack Effect Get Setup
Air instantly
turned into water
Tube Air of Tube Tube Water of Tube
Water instantly turned into air
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Stack Effect Driving Force: Winter
Indoors Outdoors 350 Pa H 400 m H 400 m T +21 °C T -17 °C p 1.20 kg/m3 p 1.38 kg/m3 (0.075 lb/ft3) (0.086 lb/ft3)
For a 400 m column of air… 400 m +21 °C -17 °C 480 kg/m2 550 kg/m2 (1300 ft) 70 °F 0 °F 4700 Pa 5400 Pa (98.2 psf) (112.8 psf)
Total Pressure Difference = 700 Pa (14.6 psf)
350 Pa
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Stack Effect Driving Force: Summer
Indoors Outdoors 100 Pa H 400 m H 400 m T +21 °C T +35 °C p 1.20 kg/m3 p 1.14 kg/m3 (0.075 lb/ft3) (0.071 lb/ft3)
For a 400 m column of air… 400 m +21 °C +35 °C 480 kg/m2 460 kg/m2 (1300 ft) 70 °F 95 °F 4700 Pa 4600 Pa (98.2 psf) (94.0 psf)
Total Pressure Difference = 200 Pa (-4.2psf)
100 Pa Copyright© 2019 by RWDI. All rights reserved.
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Stack Effect Pressures ΔP
350 Pa +350 Pa (+7.3 psf)
400 m +21 °C -17 °C (1300 ft) Driving Force 70 °F 0 °F ΔP = 700 Pa
(ΔP = 14.6 psf)
(-7.3 psf) -350 Pa 350 Pa
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Stack Effect Driving Force
Pressure Difference (Interior to Exterior)
250 – 750 Pa 1 – 3 inches water 5.2 – 15.7 psf
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Stack Effect Pressure Issues
Motorized swing door Revolving door 1 set of frameless glass doors - 350 Pa on doors with airlock (two sets = approximately 80 lb force to open each door of doors)
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Criteria for Assessment of “Problematic” Conditions
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Determining Acceptability
Difficulty opening and closing of swing doors Operability threshold force to set in motion • = 30 lbf • ~ 130 Pa (2.8 psf)
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Determining Acceptability
Elevator door operability issues Operability threshold = 25 Pa (0.5 psf) • Difficult to define
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Determining Acceptability
Whistling through doors Define a threshold at 100 L/s = 200 cfm
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Determining Acceptability
Other considerations: Difficulty balancing HVAC
Inability to deliver adequate quantities of fresh air
Struggle to maintain acceptable thermal conditions
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Criteria
Ground floor pressures
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Elevator Operability Issues
Elevator door malfunctioning Difficulty closing & opening around 25 Pa (0.5 psf) pressure difference
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Winter vs. Summer Airflow
Winter (normal) – cold outside Summer (reverse) – hot outside
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Locating the Neutral Plane
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Winter: Large Opening @ Top
Large Neutral Plane Opening ΔP = Indoor - Outdoor
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Winter: Large Opening @ Bottom
ΔP = Indoor - Outdoor
Neutral Plane Large Copyright© 2019 by RWDI. All rights reserved. Opening
Winter: Distributed Openings
Real buildings have distributed openings over height
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Summer: Distributed Openings
Hot outside
ΔP = Indoor - Outdoor
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Summer: Distributed Openings
Hot outside
ΔP = Indoor - Outdoor Neutral Plane
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Natural Ventilation
Neutral Plane needs to be above inlet of highest room to be ventilated by stack effect
Neutral Plane
Etheridge, Natural Ventilation of Buildings – Theory, Measurement and Design, John Wiley & Sons, 2012 Copyright© 2019 by RWDI. All rights reserved.
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Effects of Building Pressurization
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Building Pressurization
Fresh Air (from outdoors) Return Exhaust (to outdoors)
Net Airflow Rate = Fresh Air – Exhaust
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Winter: HVAC Pressurization
Neutral Plane
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Winter: HVAC Pressurization
Neutral Plane
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Winter: HVAC Pressurization
Neutral Plane
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Summer: HVAC Pressurization
Neutral Plane
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Summer: HVAC Pressurization
Neutral Plane
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Managing Stack Effect in Buildings
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Managing Stack Effect
Design goal
Reduce the frequency and severity of undesirable stack effect impacts and issues within the building
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Managing Stack Effect
Two main options for mitigation: Passive Mitigation • Tighten building envelope, specific doors, vents • Add vestibule doors or walls • Use revolving doors
Active Mitigation • HVAC pressurization (including seasonal control)
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Mitigation Examples
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Mitigating Stack Effect
• 80 Story Building • ~1000 ft / 300m • Winter ground floor lobby • Similar to an outdoor deck at the top of a building in summer
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Mitigation Example
No mitigation
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Mitigation Example
37 Pa across elevator doors No mitigation > 25 Pa (0.8 psf) Exceeds threshold 130 Pa (6.5 psf)
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Mitigation Example
20 Pa across elevator doors Improve façade < 25 Pa (0.4 psf) air leakage Exceeds threshold 130 Pa specification
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Mitigation Example
20 Pa across elevator doors Adding < 25 Pa (0.4 psf) vestibules Exceeds threshold 130 Pa (3.4 psf)
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Mitigation Example
201 Pa across elevator doors Positively >> 25 Pa (4.2 psf) pressurizing Ok < 130 Pa (1.5 psf) ground floor only
+++ Fresh Air
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Mitigation Example
15 Pa across elevator doors Positively < 25 Pa (0.3 psf) pressurizing Ok < 130 Pa (1.5 psf) all floors
+ Fresh Air
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Challenges with Mitigation
Like trying to plug a leaky dam…. Solving one stack effect issue can cause another
Pressures and air leakage through whole building, not individual floors
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Challenges with Mitigation
Additional challenges associated with building type
Building Type Issue
Office buildings High occupant traffic during rush hour
Residential buildings Multiple penetrations through exterior façade
Mixed use Multiple building owners/operators
Podiums Connections to adjacent buildings or underground subway/shopping with large open areas and/ or different pressurization
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Sample Mitigation Attempts
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Airlock Doors Required
Shanghai World Financial Center
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Revolving Door – Attention to Details
Drip tray under revolving door not partitioned
Air flows freely underneath door
Doors and enclosures have cracks / gaps
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Basement Entrances are Important!
Very large gap under doors between underground parking and service elevator lobby
Tend to assume basement garage is outdoor space
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Case Study: Shanghai Building
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Shanghai Case Study
Building features • 3 Lift banks: Low rise, mid-rise, high-rise, & service • 40 story building • Lifts to highest floors do not work on “cold” days
Solution: Building operations put 2 lifts out of 5 on hold-open in order to get others to work
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Shanghai Case Study
Pressure across the lift doors
Low Rise Mid-Rise High Rise
L40 L40 L40 L38 L38 L38 L36 L36 L36 L34 L34 L34 L32 L32 L32 L30 L30 L30 L27 L27 L27 L25 L25 L25
L22 L22 L22
Level Level Level L15 L15 L15 L11 L11 L11 L07 L07 L07 L04 L04 L04 L02 L02 L02
-50 -40 -30 -20 -10 0 10 20 30 40 -50 -40 -30 -20 -10 0 10 20 30 40 -50 -40 -30 -20 -10 0 10 20 30 40 Pressure Difference LR Lifts Pressure Difference MR Lifts Pressure Difference HR Lifts
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Shanghai Case Study
Component issues Doors did not seal well • Façade took up ~25 Pa • Lift doors took up ~40 – 70 Pa
L1 – Exterior Door Leak: L1 – Gap in L1 – Exterior Door Leak
Partial Seal Rotating Door Copyright© 2019 by RWDI. All rights reserved.
Shanghai Case Study
Construction issues
L1 – Thermal Image of Façade Above Rotating Door Mullion conduction (expected)
Mullion conduction (expected)
Cold plume at façade (not expected)
Cold plume at façade (not expected) Copyright© 2019 by RWDI. All rights reserved.
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Shanghai Case Study
Solutions Seal the façade • A clear problem, but not a definitive solution
Create a vestibule for the HR Lifts • Solves the problem for the HR lifts • Creates a new one for the MR lifts
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Shanghai Case Study
Pressure across the lift doors
Low Rise Mid-Rise High Rise
L40 L40 L40 L38 L38 L38 L36 L36 L36 L34 L34 L34 L32 L32 L32 L30 L30 L30 L27 L27 L27 L25 L25 L25
L22 L22 L22
Level Level Level L15 L15 L15 L11 L11 L11 L07 L07 L07 L04 L04 L04 L02 L02 L02
-50 -40 -30 -20 -10 0 10 20 30 40 -50 -40 -30 -20 -10 0 10 20 30 40 -50 -40 -30 -20 -10 0 10 20 30 40 Pressure Difference LR Lifts Pressure Difference MR Lifts Pressure Difference HR Lifts
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Shanghai Case Study
Solutions Seal the façade • A clear problem, but not a definitive solution
Create a vestibule for the HR Lifts • Solves the problem for the HR lifts • Creates a new one for the MR lifts
Make a hole in the HR lift shaft wall – bypass vent • A non-standard approach, but resolves the operational problem • Likely would have some code issues to resolve in North America
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Shanghai Case Study
The current practice is equivalent to adding that bypass vent
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Case Study: North American Building
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North American Case Study
• Two lift cores (L, H) & service lift • 28 story office building • Sealed façade • Bridge to adjacent buildings on L2 • Access to exterior on L4 and L27 • Unique quirk - interior of top mechanical room was open to outdoors • Exceptionally cold in lobby area • Temperature drops down to -35°C (~ - 32°F) ASHRAE 99.6
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North American Case Study
Measurement of ∆P Across Building
Plot shows conditions at 140 moderate temperatures 120 100 Various factors can pull the 80 pressure plot 60 40 • High internal HVAC
Height Height AboveGrade [m] 20 pressurization 0 • A “very” leaky lower level -60 -40 -20 0 20 40 60 Pressure [Pa]
Expected Measured - Stair Shaft Measured - SE - Elevator Shaft
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North American Case Study
Recommendations • Improve all exterior doors - lobby level, mechanical rooms, terraces • Rotating doors on most frequently used doors • Adjust operational protocols to close vestibule doors on cold days • Better seals on doors above neutral plane • Check building mechanical pressurization • Closing off boiler combustion air intake vent or seal mechanical room
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Conclusions
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Stack effect can exist in all buildings
The driving force (strength of stack effect) depends on: • Building height • Temperature difference
We can’t totally get rid of the driving force but we can reduce negative impacts through design
Mitigating stack effect needs a holistic building approach rather than treating isolated symptoms
THANK YOU
Duncan Phillips, Ph.D., P.Eng Building Performance Group [email protected]
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