- Alex Mathers
Demand Control Ventilation – Multizone Issues and Sequence Gaps
You know the old saying “Build tight, ventilate right…and always hire a commissioning agent by DD at the latest!” Proper ventilation (in conjunction with proper exhaust) is critical to creating a safe and comfortable indoor environment. While in theory, demand control ventilation (DCV) provides an economical means of providing outdoor air to occupied spaces at the rates required by local building codes, in practice, there are difficulties which prevent DCV implementation as intended.
Studies have documented the adverse health effects associated with poor indoor air quality, ranging from minor discomfort, decreased worker productivity, to respiratory illness. Ensuring indoor air quality requires source contaminant control, commissioned and clean HVAC systems, proper filtration systems and increased, or appropriate ventilation rates.
The widely recognized commercial ventilation standard, ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality (IAQ) defines the minimum ventilation requirements to provide IAQ that is acceptable to human occupants to minimize adverse health effects. Standard 62.1 contains provisions that allow building ventilation systems to vary the amount of outdoor air (OA) ventilation delivered to occupied zones based on feedback from CO2 sensors that monitor indoor and outdoor air CO2 concentrations.
Although not a contaminant of concern, CO2 levels are used as an indicator of space occupancy and proportionate IAQ.
Modulating the outdoor ventilation air while maintaining proper indoor air quality has the potential for large energy savings compared to constant rate ventilation systems that are typically designed to provide outdoor ventilation air based on maximum anticipated occupancy. While the primary purpose of DCV is to save energy, improving indoor air quality in targeted locations is a close secondary objective. ASHRAE Standard 90.1-2019, focusing on energy, requires demand control ventilation (DCV) of high occupant density spaces.
ASHRAE 90.1 and 62.1 are commonly used as a basis for most state building mechanical and energy codes. The 2021 International Code (IMC) used for Texas and Washington Chapter 4 Ventilation references 62.1 as well as in California, Title 24, Part 6 (California Energy Code) aligns (mostly) with ASHRAE 62.1 and 90.1.
Per 62.1 Demand-control ventilation (DCV) provides automatic reduction of OA intake below design rates when the actual occupancy of spaces served by the system is less than design occupancy. Most control approaches use CO2-based DCV.
With a single-zone system the sequence is simple. The measured CO2 concentration can be used to directly control the AHU or RTU outdoor air (OA) damper.
With a multiple-zone variable air volume (VAV) system, each zone in the system requires a different fraction of OA, but the AHU or RTU delivers the same fraction of OA to all zones, resulting in some zones being over-ventilated and some other zones being under-ventilated. This presents some problems for designers as well as installers and integrators.
Ventilation Design at the Zone Level
The Breathing Zone Outdoor Airflow per ASHRAE Standard 62.1 is calculated by outdoor airflow rate required per person times the zone population plus the outdoor airflow rate required per unit area times the floor area. This is then divided by the Zone Air Distribution Effectiveness to obtain the Zone Outdoor Airflow that must be supplied by the ventilation system.
For example, a 400 sf, 8-person office building conference room would require a minimum ventilation rate of 64 CFM for overhead cooling and 80 CFM for overhead heating.
To be clear, that is the minimum outdoor air required, not the total airflow. Outdoor air drawn into the air handler is fully mixed with recirculated air, so it is impossible to deliver discrete quantities (cfm) of outdoor air to the individual breathing zones. Instead, the supply air is delivered as a homogeneous mixture of outdoor air and recirculated air. So you may wonder, how does the VAV know how much OA is being delivered to the space?
In a recirculated mutli-zone system the percentage of outdoor air to the supply airstream (outdoor-air fraction (OAF)) maybe 20% or lower. To get the required ventilation at a 20% OAF, the VAV would have to deliver ~400 CFM just to ensure 80 CFM of outdoor air is going to the space…or does it?
Ventilation Design at the System Level
ASHRAE 62.1 standard suggests two methods to calculate outdoor air intake requirements. In both methods system ventilation efficiency (Ev) and outdoor air fraction should be calculated. These depend on the critical zone which requires the highest percentage of outdoor air. This critical, or primary zone determines the system level outdoor air percentage.
For example, if 50% of the system primary air must be outdoor air to properly ventilate the critical zone all the zones will receive 50% outdoor air and any zone with a lower primary outdoor air fraction that the critical zone will be over ventilated. This over ventilation results in "unused" outdoor air that is recirculated in the return air coming from these zones and can be used to offset the ventilation requirements of the system. The final outdoor air fraction may be closer to 30%. So, going back the zone level example, while the percent OA may be low, the OAF assuming unused OA in the return maybe 30%.
In simpler terms, once the OA is in the system, engineers do not care where the OA goes as long as the design ventilation rate (DVR) is introduced into the system at all fan speeds. The DVR for a VAV system is the summation of ventilation requirements of all the zones served. There will be times when one zone is fully occupied and therefore calling for high ventilation rates while other zones may be unoccupied calling for minimum ventilation rate. Any unused OA will return to the AHU/RTU, mixing with more fresh OA and returned to the supply air with a higher OAF.
DCV Sequences at the Zone Level for Multizone Systems
Integral to any mechanical design is the all-important Sequence of Operation. This part of the HVAC controls specification is extremely significant, and an essential part of any successful project. A typical DCV sequence for VAVs is below:
When the measured zone CO2 is below the sensor setpoint (400 PPM), the VAV will modulate its damper to maintain the desired zone temperature set points.
If the measured zone CO2 at the zone exceeds the sensor setpoint (400 PPM) the VAV will modulate its damper open.
This seems simple enough, except when it conflicts with the zone airflow control based on temperature. If the space is in heating mode - at its maximum heating CFM - and requires additional ventilation, what does the VAV do? Some energy codes do not allow for additional reheating of conditioned air.
Another temperature control conflict is when the space temperature is satisfied, but the space CO2 is above setpoint. Going by the sequence above the VAV will modulate its damper open, but this additional conditioned air will drive the space temperature down, below its setpoint. Unless additional programming is implemented to open the reheat valve the space may become uncomfortably cold for the occupants.
DCV Sequences at the System Level for Multizone Systems
ASHRAE 62.1 and energy codes stipulate zone level controls for DCV but notably absent is any requirement at the system or AHU/RTU level. The control of the AHU/RTU is left up to the engineer to specify, and if they do not, a zone with measured CO2 exceeding the sensor setpoint may not receive the OAF required to reduce the CO2 concentration.
For projects where engineers do address the system level, a typical DCV sequence for AHUs is below:
If the maximum VAV damper position is reached and the zone CO2 setpoint is still not satisfied the AHU will modulate its OA damper open to increase the OA concentration in the SA and reduce the zone CO2 level below setpoint.
Again, this is straightforward at first glance, but when the OA temperature is at or outside of the design day conditions, supply air temperature control may be an issue. AHU coils are typically sized at 1% or 0.4% ASHRAE Climatic Design Conditions with the OA assumed to be at minimum. If OA is increased during these times, the coils may not have the cooling, heating or dehumidification capacity. This will affect the temperature control at the zone level resulting in dissatisfied occupants.
DCV Implementation on Tenant Improvement Projects
While it is usually assumed the zone level and system level will be designed and installed as part of one project, this again is not always the case. For renovations, retrofits and tenant improvements in warm shell buildings where there are existing AHUs/RTUs, the scope of the project may end at the VAV. Any sequence controlling the CO2 in a zone will have to rely on an existing sequence at the system level, present or not.
Here again, the VAV at the zone level has no knowledge of the OAF in the supply air and has no control over the AHU to increase OA to its zone based on CO2 levels or decrease OA to save energy if CO2 levels are below setpoint.
The performance of DCV systems is highly dependent on accurate balanced and measured OA supply and exhaust airflows. Differences between sampled, measured and design airflows can be greater than 10%. Therefore, extensive TAB and commissioning for new buildings and retro-commissioning for renovated buildings should be deliberately performed.
Sensor accuracy and calibration also affect the ability of a DCV system to operate as intended. Inaccuracies result from the use of sensors to measure CO2. This error may double if two sensors are used (one indoor and one outdoor). A typical accuracy specification of a common CO2 sensor used for DCV is +/- 75 PPM. With each sensor error range of +/- 75 PPM, the accuracy of the differential measurement is double that of an individual sensor or 150 PPM.
If an outdoor sensor is not installed and the assumed outdoor level is 400PPM, any concentration exceeding the assumed level will decrease the efficiency of the system. Outdoor background levels can vary by more than 100 PPM. As a result, if instead the outdoor level is 500 PPM, and the assumed background level is 400 PPM this will create an error of 100 PPM or 25% error. With 500 PPM CO2 OA delivered to the zones they may exceed their zone setpoint and incorrectly assume full occupancy.
Energy is required to add or remove heat to fresh air introduced into a building. Over-ventilation is one of the largest indirect contributors to a building’s energy use. Buildings that deliver fresh air to the building’s occupants at a fixed or constant volume represents the core of the financial and environmental case for DCV. Compared to a fixed ventilation approach, DCV offers considerable advantages if implemented and commissioned correctly.
Applying Demand Controlled Ventilation, BY XINGBIN LIN; JOSEPHINE LAU, PH.D., MEMBER ASHRAE