Transcript The Role of Controls for IAQ - Mid Columbia ASHRAE Chapter

The Role of Controls for
Indoor Air Quality
Kent W. Peterson, PE, Fellow ASHRAE
P2S Engineering, Inc.
[email protected]
Mid Columbia ASHRAE Chapter
Introduction
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Proper operating controls are
fundamental to providing acceptable
indoor air quality
Many control factors must be
considered to be effective in
maintaining air quality
Topics of Discussion
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Control fundamentals
Relative humidity control
Building pressurization control
Ventilation control
ASHRAE Standard 62.1-2004
 Ventilation Rate Procedure
 Demand Controlled Ventilation
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ASHRAE
Standard 62.1-2004
u5.4
Ventilation System Controls.
Mechanical ventilation systems shall
include controls, manual or automatic,
that enable the fan system to operate
whenever the spaces served are
occupied. The system shall be
designed to maintain the minimum
outdoor airflow as required by Section
6 under any load condition.
IMC Section 403.3
Mechanical Ventilation
uThe
ventilation system shall be
designed to supply the required rate
of ventilation air continuously during
the period the space is occupied,
except as otherwise stated in other
provisions of the code.
Control System Goals
Control Fundamentals
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Provide required amount of
ventilation air throughout the
complete range of operating
conditions without wasting energy
Must function reliably over a long
time
Must be usable by the building
operating staff
Control Reliability
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Control Fundamentals
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Accuracy and inaccuracy
Measure controlled parameter
Capable controls
Commissioning
Control Accuracy
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Accuracy
Control Fundamentals
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Describes the total of all deviations
between the true value and the
measured value
Typically expressed as the percent
of full-scale range
Inaccuracy refers to the departure
from the actual value to which all
causes of error contribute
Total Error = SQRT [ (error1)2 +
(error2)2 + … + (errorn)2 ]
Ventilation Control
Strategies for VAV
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Common OSA Control Methods
Fan tracking
 Use of commissioning data
 Plenum pressure differential
 Flow monitoring
Control Fundamentals
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Outside Air Control
Control Fundamentals
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A fixed minimum outside air damper
position with no control is not a valid
control strategy for a VAV system
under any condition
OSA Control Methods
Control Fundamentals
100,000 cfm SA
85,000 cfm RA
15,000 cfm OSA
3% sensing
accuracy
3,000 cfm SA error
2,550 cfm RA error
37% Error on OSA
Fan Tracking
n
Assumption: The difference between the
measured supply airflow and return airflow
is the amount of outside air.
Use Commissioning
Data
u
OSA Control Methods
Control Fundamentals
u
Assumption:
That for any given airflow in a
system, there is some OSA
damper position that provides the
required OSA quantity
OSA Control Methods
Control Fundamentals
Plenum Pressure
Control
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Maintain a fixed MA plenum pressure
differential by modulating the RA damper
Flow Monitoring
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Velocity must be high enough
 Typically need minimum 2D before
& 1D after the flow element (will
increase if turning vanes are not
used)
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OSA Control Methods
Control Fundamentals
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Measure the amount of ventilation
air to control
Flow measurement
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Flow sensing built into dampers
Measuring Airflow
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Different sensing means
advantages can depend on the
application
<1000 fpm Thermal anemometry
 >1000 fpm Pitot tube
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Accuracy
Control Fundamentals
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Understand how accuracy can
impact the control method selected
Caution when controlling differential
OSA Control Methods
Control Fundamentals
Flow Monitoring
(continued)
PV = PT - PS
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Combination damper-flow station
Relative Humidity
Control
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Controls for Good IAQ
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Humidity control is required for good
IAQ
ASHRAE Standard 62.1 Section 5
recommends that RH be maintained
below 65%
Building Pressurization
Control
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Controls for Good IAQ
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Negative building pressure causes
infiltration
Infiltration in hot and humid climates
can cause IAQ problems
Standard 62.1-2004
Ventilation Control
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Classifications
Ventilation Rate Procedure
 Indoor Air Quality Procedure
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Ventilation Control
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Variable Supply Air
Operations and Maintenance
Demand Controlled Ventilation
Standard 62.1-2004
Ventilation Rate
Procedure
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Ventilation Control
Standard 62.1
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New procedure for calculating
outside air flow rates
Default values for ventilation
system efficiency and multiple
spaces effects for simplicity
Reduced rates in most
occupancies
Standard 62.1-2004
Ventilation Rate Procedure
People
Component
Building
Component
Ventilation Control
Standard 62.1
Breathing
Zone Outdoor
Airflow CFM
Vbz =
Minimum
CFM/Person
Zone
Population
R p Pz + R a A z
Zone Floor
Area
Minimum
CFM/ft2
Excerpt from Table 6.1 of
Standard 62.1-2004
Application
Offices
Office space
Reception areas
Conference rooms
Public Spaces
Corridors and utilities
Public Restrooms
Minimum Ventilation Rate
In Breathing Zone
cfm/person
cfm/ft2
5
5
5
0.06
0.06
0.06
note
0.06
note
Standard 62.1-2004
Ventilation Rate
Procedure
n
Terms
u
u
Ventilation Control
Standard 62.1
u
u
u
u
u
u
Vbz
Breathing Zone Outdoor Air
Ez
Zone Air Distribution
Effectiveness
Voz
Zone Outdoor Airflow
Vpz
Zone Primary Airflow
Zp
Primary Outdoor Air Fraction
Vot
Uncorrected Outdoor Air Intake
Ev
System Ventilation Efficiency
Vot
Outdoor Air Intake
Standard 62.1-2004
Reset
6.2.7
Dynamic Reset. The system
may be designed to reset the design
outdoor air intake flow (Vot) and/or
space or zone airflow as operating
conditions change.
Variations on occupancy
Variations in the efficiency with which
the outside air is distributed
Higher fraction of OSA due to free
cooling or exhaust air makeup
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Demand Controlled
Ventilation (DCV)
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Ventilation Control
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It is “any” method used to control
ventilation that modifies intake rates
based on changing “demand”
“Demand” is usually measured as a
change in the occupancy
CO2 sensing is typically used as an
occupancy indicator
Understanding CO2
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DCV Using CO2
Ventilation Control
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CO2 is not an indicator of IAQ
If ventilation rate is 15 cfm per
person (1.2 met units) the resulting
steady-state CO2 concentration
relative to outdoor is 700 ppm
differential
Standard 62.1 Appendix C
Demand Controlled
Ventilation
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Avoid CO2 sensors in RA ducts
Zone sensors in breathing zone
Outdoor ventilation control
Minimum 0.15 cfm/ft2 OSA
 Modulate airflow rate to space first
 Modulate OSA rate to maintain the
CO2 setpoint of 1000-1100 ppm
DCV Using CO2
Ventilation Control
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Demand Controlled
Ventilation
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Good candidates
Unpredictable variations in
occupancy
 Climate where cooling and heating
is required most of year
 Low pollutant emissions from nonoccupant sources
DCV Using CO2
Ventilation Control
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DCV guidance is available
Centralized CO2 monitoring
IAQ Control System …
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Controls for IAQ
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Must understand system
relationships and accuracy
Requires careful selection
Should be commissioned
Must be usable by the building
operating staff
Must function reliably over a long
time
For Further Information
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ASHRAE Publications
Standard 62.1-2004
 ASHRAE Guideline 16-2003
 Research Project RP-980
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for Measuring and Controlling Outside Air Intake
Rates in Variable Air Volume Systems
Controls for IAQ
Techniques
ASHRAE Journal Articles
 Transactions
 Humidity Control Design Guide
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Trade journals