American Aldes Controlling Stack Effect User Manual

							INTRODUCTION
Central ventilation and toilet exhaust risers are designed for the 
purpose of providing mechanically controlled ventilation and 
protecting against poor indoor air quality. In high-rise buildings 
(buildings over four stories), exhaust ventilation risers and 
subsequent fans are often dramatically affected by environmental 
factors such as stack pressure. When buildings are built tightly to 
conserve energy, stack pressure has a greater effect on a system’s 
ability to regulate indoor air quality, ultimately detracting from 
a building’s energy efficiency. 
The central duct riser used for air exhaust and/or ventilation air 
distribution in tall buildings is the main focus for building designers 
and engineers looking to improve energy efficiency and indoor 
air quality. Maintaining proper airflow rates in duct risers is the 
key for both indoor air quality and energy efficiency assurance; 
Difficulty balancing the system, poor maintenance practices, and 
fluctuations in system pressure due to stack effect make it very 
problematic to maintain proper flow rates, let alone minimize 
energy consumption. 
One challenge designers face is how to minimize the effect stack 
pressure has on a particular system, while minimizing fan motor 
power consumption. To combat seasonal fluctuations in system 
pressure, designers can either increase fan-induced duct pressure 
or find a means to modulate the opening at each intake point. In 
the absence of either solution, these seasonal pressure variations 
will result in over- or under-ventilation, increased thermal load on 
the building, and fluctuations in sound levels at the intake points. 
This application guide discusses how stack pressure is determined, 
its effect on vertical riser system performance, and what can be 
done to overcome this ever-present condition.
Controlling Stack Effect in Ventilation Duct 
Risers Promotes Energy Efficiency and IAQ
APPLICATION 
GUIDE
ENERGY EFFICIENCY AND IAQ VS. STACK EFFECT
The difficulty of maintaining proper ventilation system riser airflow 
balance in areas with large shifts in climatic conditions is mainly 
due to stack effect. Stack, or hydrostatic pressure, is created when 
differences exist among air temperature, altitude, and vertical 
distribution of air from indoor and outdoor conditions. As discussed 
in ASHRAE Fundamentals Chapter 26, “stack pressure differences 
are positive when the building is pressurized relative to outdoors, 
which causes flow out of the building.  Therefore, in the absence of 
other driving forces, when the indoor air is warmer than outdoors, 
the base of the building is depressurized and the top is pressurized 
relative to outdoors; when the indoor air is cooler than outdoors, 
the reverse is true.”
Stack effect is unavoidable, and increases with building height 
and as temperature differences between inside and outside air 
increase.  The level of stack pressure within vertical chases in a 
compartmentalized building (multiple floors) is also affected by 
the degree of air tightness between floors and with the exterior 
walls.  Tall buildings, larger differences in indoor and outdoor 
temperatures, and tight construction all contribute to greater 
pressure differences within elevator shafts, stairwells, and exhaust 
risers. 
DETERMINING STACK PRESSURE
Since vertical duct risers penetrate the floors of compartmentalized 
buildings and provide an open vertical chase throughout the 
length of the duct itself – usually the height of the building – 
stack pressure within these ducts can be calculated using the 
following formula:
ΔPs = C1 · g · p · (T1 - T0) /T1) · H
Where : 
Ps=stack pressure, in. of water
C1=unit conversion factor = 0.00598 
(in. of water) x ft x s2/lbm
g=gravitational constant, 32.2 ft/s2
p=indoor or outdoor air density
T0=outdoor air temperature (R)
T1=indoor air temperature (R)
H=Height (ft.)
A simple rule of thumb can be derived from the same formula 
as follows:
ΔPs = .0000274 in. w.g. per ft x (TF1 - TF0)                                   
						

							Example:  A 200 ft. building in Chicago, 0°F winter design condition, 
70°F indoor temp:
ΔPs = .0000274 x 200 (70° F - 0°F)
ΔPs = .00548 (70)
ΔPs = 0.38 in. w.g.
If the duct riser extends only partially throughout the building 
height, apply the same formula for only the length of the duct 
riser.  To determine the stack pressure at each intake point, apply 
the same formula for the length of each point to the fan.  This 
assumes that the fan at the top of the riser is capable of handling 
the increase in pressure and resulting increase in flow, and the 
neutral pressure point is at or above the fan itself.
If the increase in stack pressure and flow results in conditions 
beyond a fan’s performance capability, the neutral pressure 
point would be lower than the fan at a point within the duct 
riser itself.  This would actually result in positive pressure, or 
outflow of air above the neutral pressure point at the top of the 
riser, and completely eliminate the ventilation performance in 
those areas.  This scenario is often identified as a cause of poor 
indoor air quality.
CHANGES IN AIRFLOW CAUSED BY STACK PRESSURE
The increase in pressure from stack effect results in increased 
flow through the duct riser.  The flow at each floor’s intake points 
varies as a square root of the difference in pressure through 
the opening.  Assuming that the fan can effectively remove this 
increase in flow, the percentage of change in flow at each intake 
point is taken as follows:
Where : 
Q1 = Q0
ΔP0 + ΔP1
ΔP0
Q0=Flow at design ΔP0 in the absence of stack 
pressure.
Q1=New Flow under stack pressure conditions
ΔP0=Pressure in the absence of stack
ΔP1=Pressure including stack
Example:  Flow at an exhaust grille located on the first floor is 100 
CFM, ΔP0 at .10 in. w.g., the increase in flow as a result of increase 
in stack pressure:
Q1 = 100.10 + .38
.10
Based on our original example of a 200 ft. building, and assuming 
design of 100 CFM per floor with a total of 14 floors, the total system 
airflow would increase from 1400 to 2286 cfm.  This represents an 
increase of 63.2% in total flow, or 886 CFM of unwanted ventilation 
and additional load on the building! 
BALANCING AIRFLOWS IN THE PRESENCE OF STACK 
PRESSURE 
Ultimately, the effect stack pressure has within a building relates 
to the amount of unwanted infiltration of unconditioned air and/
or exfiltration of conditioned air.  This unwanted movement of air 
relates to increased thermal load on the building and uncontrolled 
energy consumption.  Since the mechanical ventilation riser 
is a contributor to overall building pressure buoyancy, not to 
mention proper regulation of ventilation for IAQ, it is important to 
recognize that proper balancing and regulation of these systems 
has a significant effect on energy consumption. 
One technique to minimize the effect stack pressure has on exhaust 
ventilation system prescribed airflows is by increasing the internal 
duct pressure created by the fan.  The greater the internal duct 
pressure, the less effect stack pressure can have on the system; 
however, increased pressure also relates to increased fan motor 
BHP and relative energy consumption in watts.  To determine the 
increase in pressure necessary to overcome stack pressure within 
a tolerance of +/-10% in a balanced static system, the following 
formula can be applied:
Q1 / Q0 = 1.1 =ΔP0 + ΔP1
ΔP0
Squaring both sides to solve
for ΔP0 : 1.21 =ΔP0 + ΔP1
ΔP0
ΔP0  =wΔP1= 4.760. 21
(times the increase in stack effect pressure)
Where Tolerance factor
Q1 = Q0 +/- 10 %
Simply stated, the pressure drop at each grille for static balancing 
must be 4.76 times the anticipated stack effect at each respective 
intake point to maintain the airflow within 10% of design values.  
This is true for all the grilles regardless of elevation within the 
building.  In the absence of stack effect, the formula does not 
apply.  When applying the increase in pressure factor of 4.76 to 
our example, and given the original stack pressure of 0.38 at the 
first floor grille, the fan must now operate at a level to ensure 1.81 
Ps in. w.g. at this same grille.  The increase in necessary pressure 
will not only result in excessive energy consumption, but excessive 
noise generated at each grille as well.
Controlling Stack Effect
2
= 100√4.8 = 100x2.2 = 220cfm 
						

							and modulate the opening to regulate flow in response to these 
changes.  This will allow the use of lower-pressure fans for energy 
savings and prevent stack pressure from effecting flow rates and 
resulting in over- and under-ventilation.  Unfortunately, most 
modulating dampers on the market today are designed using 
pitot tube pressure-sensing devices and electric drive motors 
and controllers to actuate a damper for flow control.  Using one 
of these devices on every intake point in a riser is often more 
costly than years of energy penalties on systems without them.
0 cfm
0 cfm
18 cfm
28 cfm
52 cfm
81 cfm
83 cfm
94 cfm
78 cfm
117 cfm
132 cfm
150 cfm
Unregulated Unbalanced Airflows
Over-ventilation causes energy waste,
Under -ventilation causes poor indoor air quality (IAQ)
MOTOR/FAN PERFORMANCE AND ENERGY PENALTY TO 
OVERCOME STACK PRESSURE
After solving for the increase in pressure necessary to maintain 
balanced flows, simple fan laws can be applied to determine 
the required increase in fan RPM.  Fan laws show that pressure is 
proportional to the square of the RPM.  
SP1 / SP2 = (RPM1 / RPM2)2
Therefore, using the previous example, the increase in RPM can 
be determined as follows.  Assuming the original fan RPM is 1000 
and the pressure to achieve design airflows in the absence of 
stack pressure is 0.22, derived from 0.10 at each grille and 0.12 to 
account for duct loss:
RPM2 = RPM1 
SP2= 10001.91= 2 9 5 0  rpmSP10.22
The result is an increase of almost three times the original RPM 
design in order to prevent changes in airflow due to stack pressure 
effect.  When applying this increase to energy consumption of fan 
motors, the increase varies with the cube of the RPM.
HP1 / HP2 = (RPM1 / RPM2)3
Following the previous example:
HP2 / HP1 = (2950 / 1000)3
HP2 / HP1 = 25.7
Therefore, the final result is a more than 25-times increase in power 
consumption to operate a fan at the higher pressure required to 
ensure proper system balance in the presence of stack effect.
ANALYSIS OF BALANCING AND CONTROL OF STATIC RISER 
SYSTEMS
Through analysis of traditional central duct riser system 
designs, and factors that effect overall airflow performance, it 
is determined that excessive energy consumption will increase 
as stack pressure increases.  Since statically controlled systems 
have no means of adjusting to fluctuations in stack pressure, 
the amount of excessive energy consumed will either come in 
the form of additional thermal load on the building, which will 
result in increased heating costs, or from increased fan power 
to control the flows at higher pressure.
The other negative factors associated with statically controlled 
risers are excessive noise and duct leakage created by high fan 
pressures, or the potential for under ventilating portions of the 
building.  Either scenario can result in an unsuitable environment 
for the building’s occupants.  
The only solution to dealing with stack pressure effect on vertical 
risers is to monitor the pressure at each intake point into the riser 
Controlling Stack Effect
3 
						

							TYPICAL SPECIFICATIONModel CAR-II Constant Airflow Regulators by American ALDES Ventilation Corporation, Bradenton, Florida, shall solely operate on duct pressure and require no external power supply.  Each regulator shall be pre-set and factory calibrated requiring no filed adjustment to the airflows as indicated on the schedule, and shall be rated for use in air temperatures ranging from -25°F to 140°F (-32°C to 60°C).  
Constant Airflow Regulators shall be capable of maintaining constant airflow within +/-10% of scheduled flow rates (15% for units 50 CFM or less), within the operating range of 0.2 to 0.8 in. w.g. differential pressure, or 0.6 to 2.4 in w.g. on high-pressure models (CAR-II-HP), or 0.1 to 0.42 in. w.g. on low-pressure models (CAR-II-LP).  Sound power levels shall not exceed those for each size and CFM rating as scheduled.  Regulators shall be provided as an assembly consisting of a 94V-0 UL ABS plastic body housed within a round sleeve for mounting in round duct.  Each round sleeve must be fitted with a lip gasket to assure perimeter air tightness with the interior surface of the duct.   All regulators must be classified per UL 2043 and carry the UL mark indicating compliance. All Constant Airflow Regulators will require no maintenance and must be warranted for a period of no less than five years.  Constant Airflow Regulators shall be installed in tight ducting systems in accordance with all applicable codes and manufacturer’s instructions.
HOW THE CAR-II WORKS
Constant airflow is achieved by controlling the free area 
through the device.  At minimum static pressure, the aero-wing 
is parallel to the air stream.  As the static pressure increases, 
the aero-wing lifts, thereby reducing the amount of free area 
through the regulator.  At the same time, the higher static 
pressure increases the air velocity resulting in CONSTANT 
AIRFLOW.  This occurs regardless of pressure differences in 
the range of 0.2 to 0.8 in. w.g. (50 to 200 Pa).  The air velocity 
in the duct is in the range of 60 to 700 ft/min. (0.3 to 3.5 m/s).
Controlling Stack Effect
4
American ALDES Ventilation Corporation  •  4521 19th Street Court East, Suite 104  •  Bradenton, FL 34203 – USA
941.351.3441  •  800.255.7749  •  941.351.3442 (fax)  •  [email protected]  •  www.aldes.us
© 2013 American ALDES Ventilation Corporation.  Reproduction or distribution, in whole or in part, of this document, in any form or by any means, without the express written consent of American ALDES Ventilation Corporation, is strictly prohibited. The information contained within this document is subject to change without prior written notice.
controlling stack effect_application guide_1113   
THE PHYSICAL CHALLENGE OF TEST AND BALANCE
Balancing and commissioning of a ventilation riser is usually 
considered difficult and tedious.  Low airflows through small, 
often inaccessible, sidewall-mounted registers located on multiple 
floors, is challenging to any test-and-balance contractor.  It requires 
special instrumentation and many man hours for typical riser 
systems.  Even with modulating duct openings, the balancer’s 
job is compounded by fine-tuning controllers to specified set 
points before and after airflow measurements. 
In addition to the physical constraints of balancing vertical risers, 
the time of year and stage of construction dramatically affect the 
measurement readings the contractor will record.  This goes back 
to stack pressure effects on the system.  
THE AMERICAN ALDES CONSTANT AIR REGULATOR 
SOLUTION
The ultimate solution to ensure proper system balancing and 
airflow regulation is the American Aldes CAR-II Constant Airflow 
Regulator.  The CAR-II is a factory-calibrated passive airflow 
regulator that eliminates the need for balancing airflows at the 
grilles.  It does not require any external power since if automatically 
adjusts to the proper airflows in response to duct inlet pressure.
The active control element of the CAR-II is a unique aerofoil. Using 
Bernoulli’s Principle, the aero-wing damper lifts in response to 
increasing static pressure. This operation regulates the free-area 
opening through the control, resulting in maintenance of velocity 
and specific airflow setpoints.  
Because the CAR-II will maintain the prescribed airflows as it 
adjusts to changes in pressure caused by stack effect, it eliminates 
over-ventilation caused by the exhaust riser, which saves energy.  
The use of CAR-IIs also allows for fan operation at the lowest 
pressure level possible without sacrificing airflow performance, 
which saves fan energy consumption.  Finally, CAR-IIs eliminate 
under-ventilation caused by imbalances of the exhaust system, 
which protects against poor indoor air quality.
CAR-IIs are employed in thousands of buildings in the United 
States and around the world.  This well-proven technology was 
developed to minimize fan energy use in the late 1970s.  Today, 
CAR-IIs serve as a simple solution to indoor air quality ventilation 
regulation and energy savings.  The CAR-II by American Aldes 
continues to lead the industry in economical passive airflow control 
regulation.  Consult the factory, or an American Aldes certified 
representative to discuss how CAR-IIs can save money, conserve 
energy, and protect any building against poor ventilation control.