HVAC system designers and design/build contractors are familiar with the many "pros" of demand control ventilation systems. Such systems:
- Save energy
- Reduce overall heating/cooling load
- Reduce outdoor air volume
- Can provide operational flexibility
- Can reduce re-balancing when used with a dedicated outdoor air system
- Decrease outdoor air handling unit filter maintenance
- Can document proper indoor air quality performance. However, as always there are "cons" to go along with the pros. For demand control ventilation systems, these include:
- Increase in initial cost
- Increase in system complexity
- Increase in installation time, control mapping, and commissioning
- Increase in maintenance cost with sensor monitoring and adjustment
- Can reduce indoor air quality if installed improperly.
American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 62.1, Ventilation for Acceptable Indoor Air Quality, is the guide for most demand control ventilation systems. Standard 62.1 is typically the foundation for local building codes, and addresses outdoor air placement into the occupied breathing zone. It also defines standard office, conference room, and classroom ventilation rates, which are predominately driven by occupancy.
Most base building systems do not apply demand-control ventilation. The design engineer will calculate code-compliant outdoor air volumes based on the maximum projected building occupancy. Although this practice will net a system that is compliant with code and comfortable, it typically results in excess ventilation since buildings are rarely filled to capacity. Building owners are often unaware of how such systems affect energy consumption and air-handler maintenance, both of which directly affect their budget.
Therefore, improving energy and operating efficiency should start with the design process. Design teams applying a demand control ventilation strategy must be aware that such systems will typically have a short payback time in humid regions where economizer hours are limited.
Here is an example of the impact humidity has on a design day. Washington DC and Los Angeles’ LAX airport have been chosen to represent peak summer for a humid climate and for a dry climate respectively. Note the coil loading:
Washington DC ≈ 71 Btuh/cfm summer coil load
(93°F EDB/75°F EWB)
LAX Airport ≈ 44 Btuh/cfm summer coil load
(83°F EDB/68°F EWB)
A Washington DC facility will require approximately 63% more energy to condition outdoor air during summer design conditions, due to moisture alone. This does not include fan energy to address the larger latent coil pressure drops, or winter design loads.
Demand control ventilation can also be used to increase the performance of existing systems. For example, small rooftop equipment or heat pump units dehumidify when the compressors are on. When the compressors are off, the unconditioned humid outdoor air passes directly into the space. The units’ performance may be improved by limiting unnecessary outdoor air. This also applies to an existing unit that has insufficient capacity for a renovated space or future addition. By incorporating demand control ventilation, the unit’s diversified loads may allow continued operation without replacement.
Pick the Low-hanging Fruit
The most basic form of demand control ventilation involves documenting where the occupants will be in the building. For instance, are conference room users the same employees walking from zone to zone, or new visitors arriving for meetings? Do occupants leave for lunch or telecommute on a regular basis? Since this data is difficult to collect for a new facility, sensors and other methods can be employed to identify when occupants are in the building. There are several methods to do this, and as usual it’s best to start with the low-hanging fruit.
For small offices, schools or conference facilities where occupied hours can be tracked with confidence, using a simple timer is possible. Overrides are provided for the user in case after-hour facility use is required. Conference facilities, gyms, and auditoriums typically have the most diversity, largest outdoor air loads, and are typically the first to be reviewed for demand control ventilation controls. The impacts to adjacent space should also be reviewed as part of this process, as older schools and office buildings may rely on conference room or auditorium systems to support kitchen hood exhaust air requirements.
For portions of the country using ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, as the energy code, lighting systems require an automatic shut-off capability. This can necessitate occupancy sensors for each individual office. If this is the case, it’s important to coordinate with the electrical contractor to acquire a second set of contacts or output from each sensor. Doing so will provide a control signal that identifies when the occupant is not in the room and outdoor air is no longer required for that space.
Typically, a timer will be used to delay the reduction in outdoor air after occupancy or initiate a room purge prior to occupancy. In this scenario, loss of space cooling may result with the reduction of dedicated outdoor air quantities, and may require an override by the room thermostat. This is especially true for perimeter rooms with high solar or plug heat loads.
Page 2 of 2
Open office spaces and conference rooms can benefit from occupancy sensors, but CO2 sensors are typically used to calculate occupancy level. This is simplified to a delta between indoor and outdoor CO2 readings. This creates an active and modulated outdoor air control sequence, which results in greater complexity for the control system. Additional coordination with the general contractor is usually required to support sensor placement and installation. In addition, outdoor airflow measurement is usually necessary, and outdoor air sensor placement is important.
One Piece of the Puzzle
Applications for demand control ventilation are limited. Design engineers must verify that ventilation values are based on the occupancy count, and not equipment, chemical emissions (copiers, uninterruptible power supply battery rooms), odor control and exhaust requirements (bathrooms, kitchens), moisture removal devices, or cooling control (chilled beams, radiant cooling).
The designer must also verify the impact of other energy-saving features. Daylighting controls may fool the system into thinking the perimeter zones are unoccupied. A reduction in outdoor air may reduce heat transfer in exhaust-heat recovery devices, or allow increased infiltration that affects room heating and cooling loads. Even worse, the increased infiltration may result in moisture driven through the building shell, resulting in damage of the façade.
As CO2 sensor counts increase, a central sampling unit may be considered. The benefit is a higher quality sensor and a single location for maintenance. Additional sampling tubing to each zone and controls are an initial cost increase, and tubing placement requires coordination to avoid damage by subcontractors working above the ceiling. The payback can be analyzed based on future sensor maintenance along with space arrangement and flexibility.
Simple and Complex
Demand control ventilation can be beneficial to the mechanical equipment or for future energy savings. Integration with existing lighting or building automation system controls is typically straightforward. However, the control sequence required to implement their operation can be complex and should be implemented based on local design conditions and codes.
Keep in mind that a demand control ventilation system’s design and installation can be thought of as being similar to an automobile. Low gas mileage can be achieved using a relatively simple design. This design requires less maintenance but results in higher fuel costs. Highly efficiency automobiles such as hybrids have greater complexity and lower fuel costs, but may require additional maintenance.
Like any quality life-cycle review, the most important questions regarding demand control ventilation are how long the building be used, and what the expected payback time frame is. The answers to these questions will determine how complex the system will be.
Scott Winkler, P.E., is managing principal engineer, Southland Industries, Dulles, VA. He can be reached at 703/834-5570 or by e-mail at [email protected].