The Role of Controls for Indoor Air Quality Kent W. Peterson, PE, Fellow ASHRAE P2S Engineering, Inc. Mid Columbia ASHRAE Chapter

Introduction 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 Control fundamentals Relative humidity control Building pressurization control Ventilation control  ASHRAE Standard  Ventilation Rate Procedure  Demand Controlled Ventilation

ASHRAE Standard u 5.4 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 Mechanical Ventilation u The 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 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 Fundamentals

Control Reliability Accuracy and inaccuracy Measure controlled parameter Capable controls Commissioning Control Fundamentals

Control Accuracy 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 [ (error 1 ) 2 + (error 2 ) 2 + … + (error n ) 2 ] Accuracy Control Fundamentals

Ventilation Control Strategies for VAV Common OSA Control Methods  Fan tracking  Use of commissioning data  Plenum pressure differential  Flow monitoring Control Fundamentals

Outside Air Control A fixed minimum outside air damper position with no control is not a valid control strategy for a VAV system under any condition Control Fundamentals Not Recommended

Fan Tracking OSA Control Methods n Assumption: The difference between the measured supply airflow and return airflow is the amount of outside air. 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 Not Recommended

Use Commissioning Data u Assumption: u 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 Not Recommended

Plenum Pressure Control  Maintain a fixed MA plenum pressure differential by modulating the RA damper OSA Control Methods Control Fundamentals

Measure the amount of ventilation air to control Flow measurement  Velocity must be high enough  Typically need minimum 2D before & 1D after the flow element (will increase if turning vanes are not used) Flow sensing built into dampers Flow Monitoring OSA Control Methods Control Fundamentals

Measuring Airflow Different sensing means advantages can depend on the application  <1000 fpmThermal anemometry  >1000 fpm Pitot tube Understand how accuracy can impact the control method selected Caution when controlling differential Accuracy Control Fundamentals

 Combination damper-flow station Flow Monitoring (continued) P V = P T - P S OSA Control Methods Control Fundamentals

Relative Humidity Control Humidity control is required for good IAQ ASHRAE Standard 62.1 Section 5 recommends that RH be maintained below 65% Controls for Good IAQ

Building Pressurization Control Negative building pressure causes infiltration Infiltration in hot and humid climates can cause IAQ problems Controls for Good IAQ

Standard Ventilation Control Classifications  Ventilation Rate Procedure  Indoor Air Quality Procedure Variable Supply Air Operations and Maintenance Demand Controlled Ventilation Ventilation Control

Standard Ventilation Rate Procedure 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 Ventilation Control Standard 62.1

Standard Ventilation Rate Procedure People Component Building Component Breathing Zone Outdoor Airflow CFM Minimum CFM/Person Zone Population Minimum CFM/ft 2 Zone Floor Area V bz = R p P z + R a A z Ventilation Control Standard 62.1

Excerpt from Table 6.1 of Standard Minimum Ventilation Rate In Breathing Zone cfm/personcfm/ft 2 Offices Office space50.06 Reception areas50.06 Conference rooms50.06 Public Spaces Corridors and utilities0.06 Public Restroomsnote Application

Standard Ventilation Rate Procedure n Terms u Vbz Breathing Zone Outdoor Air u Ez Zone Air Distribution Effectiveness u Voz Zone Outdoor Airflow u Vpz Zone Primary Airflow u Zp Primary Outdoor Air Fraction u Vot Uncorrected Outdoor Air Intake u Ev System Ventilation Efficiency u Vot Outdoor Air Intake Ventilation Control Standard 62.1

Standard Reset  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

Demand Controlled Ventilation (DCV) 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 CO 2 sensing is typically used as an occupancy indicator Ventilation Control

Understanding CO 2 CO 2 is not an indicator of IAQ If ventilation rate is 15 cfm per person (1.2 met units) the resulting steady-state CO 2 concentration relative to outdoor is 700 ppm differential Standard 62.1 Appendix C Ventilation Control DCV Using CO2

Demand Controlled Ventilation Avoid CO 2 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 CO 2 setpoint of ppm Ventilation Control DCV Using CO2

Demand Controlled Ventilation Good candidates  Unpredictable variations in occupancy  Climate where cooling and heating is required most of year  Low pollutant emissions from non- occupant sources DCV guidance is available Centralized CO 2 monitoring Ventilation Control DCV Using CO2

IAQ Control System … 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 Controls for IAQ

For Further Information ASHRAE Publications  Standard  ASHRAE Guideline  Research Project RP-980  Techniques for Measuring and Controlling Outside Air Intake Rates in Variable Air Volume Systems  ASHRAE Journal Articles  Transactions  Humidity Control Design Guide Trade journals Controls for IAQ