|What is a demand-controlled ventilation system?
Demand-controlled ventilation is a control strategy that adjusts the amount of outside air based on the number of occupants and the ventilation demands that those occupants create. This strategy is used to both control energy costs as well as assure sufficient ventilation.
How will a demand-controlled ventilation system impact the occupants' living/working environment?
The occupants should perceive little or no difference in their environment. A properly designed, installed, and maintained HVAC system will have appropriate fresh air and temperature control. The demand-controlled ventilation (DCV) system will reduce outside air intake at times when it is not needed. When people enter a room, the system will increase ventilation. This should happen without any noticeable change or buildup of contaminants.
1. Obtain a summary of the particular technology proposed by the mechanical system designer.
2. Share information with the building owner.
The building occupants may be aware of the presence of this system. Often, the system provides a sense of security for occupants that some issue is being resolved. If the system also serves the purpose of monitoring indoor air quality, a display can be provided for the customer to view. This might consist of a CO2 monitor in a common area, or a damper position feedback to the HVAC control system.
1. Review the customer's need to monitor system status.
2. Share the customer's needs with the design group.
How might the DCV system interact with architectural design?
The design of any ventilation system should be integrated with the architectural and customer needs. It is important to be aware of conditions that may disrupt proper sensor operation. For example, flooring that is commonly buffed with propane-powered floor buffers can create temporarily high levels of CO2.
1. Establish a list of the different operating conditions (seasonal business cycles, occupancy cycles, and transition periods).
2. Obtain from the customer the business-related ventilation needs (non-people-based pollutants: raw material odors, fugitive process gases, water vapor, or particulate).
3. Meet with the customer and HVAC designer to compare ventilation design to expected operating conditions.
4. Identify the responsible party for each of the system's implementation and maintenance phases.
How will the customers have to interact with the DCV system?
The customer should not have to interact with the system unless they are the owner/operator. The maintenance requirements of the HVAC system will need to cover the routine calibration checks. Technicians should be called in to make the adjustments if there are perceived problems. Larger facilities may have in-house staff that can be trained to do the routine maintenance.
1. Ensure customer is provided with a maintenance plan.
What are likely questions the customers will ask about the DCV system?
System Design Issues
What is demand-controlled ventilation? It is fresh air ventilation that is actively controlled for the number of people present in a room.
How much does the extra equipment cost? It ranges from $300 - $1000 per zone. This covers the sensors and their integration.
What do I get out of it? It saves energy and energy costs. Paybacks are often as short as two years. It also assures fresh air needs are met.
What if it malfunctions? The failure mode of the system is based on the design. It can be set to go full open, to minimum, or some other preset value that allows proper ventilation to occur. The system should also notify the user or maintenance organization that a failure has occurred.
It has been said that demand-controlled ventilation is more of a system control effort as opposed to a new technology development. This is primarily because ventilation needs are known and ventilation hardware is available. With the maturity of the CO2 sensor, there are now enough systems to provide an array of options from which the designer can choose.
What information do I need to collect to select a DCV system?
The building needs often dictate which system is most cost-effective. Key information to collect includes:
Design guides are also available from HVAC manufacturers that provide step-by-step instructions. (click here for an example).
What is the design occupancy?
What is the expected actual occupancy?
Is the occupancy predictable on an hour-to-hour or day-to-day basis?
How rapidly do the spaces fill and empty?
What activities are conducted in the space?
Are there other non-occupant contaminants present?
Are the quantities of non-occupant contaminants constant or variable?
Are the occupants sensitive to contaminants?
Can the occupant maintain the system?
What does the system need so it will be properly maintained (access to sensors, etc.)?
How will the occupant behavior affect the system (operable windows)?
Is it possible that the building use will change in the long-term?
What DCV systems are available?
Some of the options include:
· CO2-based sensors, which measure the buildup of CO2 from the occupants present
· Occupancy sensors, which use infrared light and sound to detect occupants
· Real-time data such as turnstiles, ticket sales, registrations, scheduled events, to count occupants and or know with high reliability when they are occupying a space
CO2 sensors are best suited for highly variable occupancy in larger assembly spaces. They are an accepted method of determining occupancy for the purpose of controlling ventilation. The larger space (and volume) will allow the concentration of contaminants to grow more gradually. CO2 sensors take a little time to respond to changes in the environment. As the occupancy changes, the ventilation can easily keep up.
The largest benefit of CO2-based DCV is that the ventilation system can manage a wide range of occupancy and respond accordingly. This saves energy as well as providing good ventilation during peak periods. Single rooms, with single or multiple air handling systems are the most easily controlled.
The drawback to CO2 is that the sensors are subject to local concentrations of CO2 and they may or may not represent the entire space needs. A sensor placed near an entryway or in a dead-air corner may provide false high or low readings. A single CO2sensor placed in a return duct senses an average CO2 concentration of the air returning through that duct. It does not take into account actual room conditions, short circuit airflow, and the concentrations that are leaving the room through doors and windows. In variable-air-volume (VAV) and constant-air-volume (CAV) multi-room systems, there may be the cost of additional room sensors and related maintenance.
Occupancy sensors are commonly used to control lighting. These sensors are also available with an extra set of electrical contacts. This extra set of contacts can be used to reduce ventilation when the room is not occupied. This is a low-cost option if the design already has occupancy sensors to control the lighting and computer controls to control the HVAC system. The additional costs will be for a different model occupancy sensor and to pull an extra set of control wiring. Consideration needs to be given to the occupancy patterns in the area. If occupants leave the rooms that contain the sensors yet still remain in the zone (hallways, commons area) ventilation must be maintained. This can be accommodated with additional occupancy sensors or a CO2 sensor in the return duct.
"Real time" demand-controlled ventilation uses characteristics of the building's use to help control ventilation. Examples include theater ticket sales that track occupants, or turnstiles that count occupants entering a space, initiating ventilation when a preset number of people enter the space. In using this method there may be assumptions (e.g. a ticket holder will actually go to the showing for which they hold a ticket.) The benefit to these systems is that there is little lag in ventilation. When occupancy changes rapidly, there is a rapid buildup of contaminants. With a real-time actual count of occupants, ventilation rates can be increased before contaminants build up to threshold limits. The drawback is that some energy savings may be forfeited. The ASHRAE standards allow lag time in ventilation for some cases. For short duration events, the contaminants may be allowed to build up. The minimum ventilation following the event will gradually dissipate those contaminants. If real-time demand-controlled ventilation control is used, the minimum ventilation can be adjusted accordingly less, given that the occupant-based ventilation occurred during the actual occupancy.
What systems are impacted?
HVAC systems are impacted. There are additional sensors, design calculations, programming needs, and maintenance requirements. In addition, there may be additional damper controls required. The ventilation system design must be designed for the maximum occupancy. This could increase the size of the outside air mechanical equipment.
Economizers are frequently installed to use the free cooling effects from outside air when temperatures permit. They will normally be programmed to override the ventilation requirements and provide additional outside air for cooling. In this situation, it is frequently a small additional cost to add the sensors to upgrade the system to demand-controlled ventilation.
Other equipment that affects the sensitivity or calibration of the demand sensors must also be considered. This includes CO2 scrubbers or stray infrared sources that will cause the sensors to provide incorrect signals from which to control ventilation.
What does ASHRAE say about demand-controlled ventilation?
ASHRAE standard 62-2001, Ventilation for Acceptable Indoor Air Quality, addresses the issue of intermittent or variable occupancy (section 126.96.36.199). This section allows the adjustment of ventilation for variable occupancy provided ventilation needs are met. When contaminants do not present short-term health hazards, and are dissipated during subsequent unoccupied periods, the outside air may lag occupancy. A committee of design professionals has provided interpretations to the ASHRAE standard to clarify how the standards may be used. The interpretations include many examples of how demand-controlled ventilation can be acceptably implemented and what conditions apply.
It is not appropriate to reduce the size (capacity) of the ventilation system when demand-controlled ventilation is being used. There is a common practice to size ventilation systems when the diversity of the space usage will allow the size to be reduced. ASHRAE standard advises that the reduction be no more than one-half. This concept depends on the purging effects of a system that is not demand-controlled. However, demand-controlled systems may reduce ventilation after the occupants leave the space, eliminating this purge period. Demand-controlled ventilation should be sized based on the peak occupancy. Systems with economizers may already have sufficient capacity to meet this need.
What building codes apply?
The State of Oregon building codes allow demand-controlled ventilation. A recent code change will require demand-controlled ventilation for larger spaces with high occupant densities.
The State of Washingtonbuilding codes allow demand-controlled ventilation via the use of "alternate methods" utilizing ASHRAE 62.
The State of Idaho adopted the International Building Code (IBC) effective January 1, 2003. There is a clause in the IBC that permits reduced ventilation during reduced occupancy.
The State of Montana has adopted the Model Energy Code for residences and ASHRAE 90.1 for commercial spaces. These ASHRAE standards allow for ventilation to be adjusted for variable occupancy.
Plan on commissioning
Systems must be designed, installed, and operated properly to provide the intended results. Proper use of a commissioning agent to review the design and performance of the DCV system, along with the HVAC system, can help assure good performance. If a commissioning agent is used in a project, involve the agent in the design development stage of the project. This will help the agent understand the decisions on system design and integration, and reduce review delays during startup. Guidelines for commissioning and indoor air quality are addressed by the LEED Green Building Rating system sponsored by the U.S. Green Building Council .
Construction material offgas
Ventilation is intended to displace contaminants in the air. The predominant source of contaminants is normally from human occupants. There are also contaminants from the paints, solvents, glues, cleaners, and materials used in construction and retrofit. The ventilation required following construction might be high for a period of time. The ventilation rates should be set to provide higher dilution following construction, and then periodically reviewed and reprogrammed as offgas rates diminish. This review should be included in the maintenance plan for the new space.
Ventilation for assembly spaces in large buildings may be integrated with demand-controlled ventilation for the entire facility. Design considerations can be found at by clicking here.
DCV Systems and ASHRAE