Using Demand Controlled Ventilation in a Multi Zone Office Building
Demand controlled ventilation can be used in both single-zone and multi-zone systems. In both cases, DCV saves energy by adjusting airflow according to the number of occupants. In addition, if the system is able to measure the concentration of certain air pollutants, it can respond to pollutant sources that are independent from the number of people in a zone.
In a single-zone ventilation system, the outdoor air supply is calculated for the needs of only one area. This allows a simpler and more affordable control system. On the other hand, a multi-zone system must balance the needs of many building areas. The control requirements are more complex, since the system must perform the following functions:
- Delivering enough outdoor air to meet the needs of all zones.
- Ensuring each zone gets the correct airflow.
Ventilation control is achieved with zonal air dampers, which adjust their position according to the airflow required. As the ventilation needs of a zone increase, the corresponding damper moves to a more open position, and vice versa. A thorough calibration of the damper system is required for them to respond correctly to control signals.
Improving the Efficiency of Multi-Zone Ventilation Systems
Multi-zone ventilation systems in buildings waste plenty of fan power if they lack proper controls. For example, if all zones have reduced air needs and all dampers are partially closed, fan power is being wasted by the airflow restriction. However, the following control sequence can increase efficiency in these cases:
- The central air handler reduces its fan speed, lowering the total cfm provided to the multi-zone system.
- Air dampers open slightly to compensate for the reduction in fan power, reducing the static pressure drop caused by them.
- Since the system now has less pressure, the cfm increase again. The same airflow is possible with less fan power, since the pressure drop across zone dampers has been reduced.
- This process can continue until one damper has reached the fully open position.
- Beyond this point, any reductions in fan speed also reduce airflow, which is detrimental for air quality.
Since this process requires a fan speed adjustment, it works best in air handlers that have a variable frequency drive. Air handlers that restrict airflow with a main damper cannot achieve the same efficiency, since the partially closed damper causes a significant waste of fan power.
A high ventilation efficiency is possible when airflow is modulated in coordination with zone dampers, but there is one important requirement. The airflow needs of each zone must be determined by an occupancy sensor, air pollution or both. Equipping each zone with sensors increases the project cost, but the reward is the highest possible efficiency. This configuration also optimizes air conditioning and space heating costs, without compromising the comfort temperature setpoint, since there is less outdoor air to heat or cool.
Simpler control sequences are possible in multi-zone DCV systems. For example, if the system monitors the total occupancy, instead of zonal one, the total outdoor air supply will be correct. However, some zones may be over-ventilated or under-ventilated when individual controls are lacking.
Finding the Optimal Design for a Multi-Zone Ventilation System
A modulated air supply not only improves energy efficiency. If the ventilation system can deliver the outdoor air required by each zone accurately, the building can improve air quality at the lowest possible cost. Ideally, a multi-zone DCV system should operate as follows:
- All zones have CO2 detectors to account for the number of people present, as well as air pollution ones. This allows a precise calculation of outdoor air needs.
- The control system optimizes the air handler fan speed and all zone damper positions to deliver outdoor air.
- HVAC equipment does not waste energy by heating or cooling more air than necessary.
DCV is not the only measure improving efficiency in a multi-zone ventilation system. A heat recovery system can exchange heat between the outdoor air supply and the exhaust air, precooling or preheating it depending on the season. DCV and HRV complement each other.
An additional measure that could improve the efficiency of the system consist in the installation of economizers. The decision to install economizers heavily depends on the kind of opeation being carried out in the facilities and the dominant climate, since DCV and an economizer normally oppose each other. For example, large data centers may require cooling all year around, and teconomizers can provide substantial savings during the winter months.
Single-Path and Dual-Path Ventilation Systems
Ventilation systems can use air transfer, which consists on circulating air that has already traveled through another building zone. However, ASHRAE standards and local building codes place limits for this. For example, air that circulates through kitchens and restrooms must be removed from the building, since these areas have many sources of air pollutants and odors.
It is important to understand the difference between single-path and dual-path ventilation systems, since both can be used in a multi-zone installation.
- A single-path system delivers air independently to each zone, which is then collected with a return plenum. A portion of the return air may be recirculated and mixed with outdoor air, but this occurs at the central air handler.
- A dual-path system has two ventilation paths, as implied by its name. The first path provides a mixture of outdoor and recirculated air from the main air handler, while the second path only provides recirculated air from inside the building.
Assuming the same ventilation needs, a dual-path system will generally have a higher efficiency than a single-path system. However, a dual-path system must prevent recirculation from zones with odors or major pollution sources. If this requirement is overlooked, air quality will suffer.
Since multi-zone ventilation systems have a more complex layout and more components than single-zone installations, their design requirements are also more complex. However, energy efficiency measures can be deployed nevertheless. A good strategy, aimed at maximizing efficiency, consists of the combination of DCV and HRV, especially if used with a dual-path ventilation design.