Demand Control Ventilation

Demand controlled ventilation (DCV) where the ventilation rate is controlled to maintain a certain air quality is today common in offices, schools and other building where the number of people using the building varies. Moreover, a DCV system based on room temperature control also eliminates the need of additional heating in rooms when the cooling capacity of the supply air exceeds the cooling capacity needed, e.g. when the room is unoccupied or when the solar heat load is low. Figure 1 illustrates this situation. This advantage in terms of energy savings is often overlooked. The fundamental requirement on a DCV system is to assure good indoor climate with reference to indoor air quality, thermal comfort and acoustic environment. In addition, this goal should be achieved cost-eff ectively and with a minimum of purchased energy.

Principle and concept of Demand Control Ventilation

The concept of DCV has been known for over 20 years. The sensors of the first generation did not provide the required reliability, and the cost of the sensors was high. In recent years, the advances in sensor technologies have made demand controlled ventilation both reliable and cost effective. The ASHRAE Standard 62.1-2004 indicate that the demand controlled ventilation is acceptable when correctly designed and installed.

CO₂-based demand controlled ventilation is a combination of two technologies

CO₂ sensors continually monitor air in a conditioned space. Since people exhale carbon dioxide, the difference between the CO₂ concentration in the interior of the building and the level in the exterior of the building indicates the occupancy and activity level in a space and, thus, its ventilation requirements. The sensors send carbon dioxide data to the ventilation controllers, which automatically increase ventilation when carbon dioxide concentrations exceed a certain level in a space. Ventilation rates can be measured and controlled based on real occupancy. This contradicts the conventional method of ventilating at a fixed rate independent of occupancy. This results in much larger air fl ow rates coming into buildings than necessary. That quantity of air must be taken into account, because it increases energy consumption and costs. In humid climates, the excess ventilation also can result in uncomfortable humidity and mould growth, making the indoor air quality quite inappropriate. Furthermore, the lack of fresh air can make building occupants drowsy. To avoid the problems of excessive and insufficient fresh air, people can apply demand controlled ventilation.

Demand-controlled ventilation is created by adding an IAQ control loop to an existing thermal comfort control system (Fig. 2). An IAQ sensor continuously assesses the air renewal requirement and converts this into an outside air demand signal. The IAQ sensor assesses the quality of the indoor air as it would be perceived by a person on fi rst entering the space. Today’s sensors are CO2 sensors and/or VOC sensors (VOC: Volatile Organic Compounds). For definitions, specifications and test data refer to VDMA Standard 24 772: “Sensors for the measurement of indoor air quality”. However, simply adding an IAQ control loop does not make demand-controlled ventilation. As shown in the figure 3, Another highly significant feature is that control via a time schedule is replaced by a number of demand switches responsible for enabling the system.

Control strategy

When implementing demand-controlled ventilation, there are three different categories to consider, based on:

Type of fan control

•- On/Off
– Step control (e.g. 0 / 1 / 2)
-• Variable speed control.

Method of heat recovery

• – Plate heat exchanger
– Mixing dampers for re-circulated airs
– Thermal wheel.

The principle for the control of a demand-controlled partial air conditioning system for heating/cooling and heat recovery with a plate heat exchanger is illustrated below figure 4: IAQ is controlled by adjusting the fan speed (on/off , multistage or variable speed (control). Control of IAQ operates in a similar fashion in systems where a thermal wheel is used for heat recovery.

Benefits of Demand Control Ventilation

Following are the benefits of Demand control ventilation: •

– Automatic provision of optimum ventilation
– An increased sense of well-being and higher productivity
– Energy cost savings of 20 to 70% and, hence, less damage to the environment
– Good IAQ, supported by documentary evidence
– Demand controlled ventilation saves energy by avoiding the heating, cooling, and dehumidification of more ventilation air than it is needed. According to the observations, the savings range from 5 to 80 percent in contrast to the conventional ventilation system.

The payback can vary from several months to two years and can often be significant enough to facilitate to pay for other building systems.

The payback from CO₂ -based DCV will be greatest in higher density spaces, where occupancy constantly changes (e.g. schools, theatres, retail establishments, and meeting and conference areas). In spaces with more static occupancies (e.g.offices) DCV can provide control and verification that adequate ventilation provided to all spaces. For example, a building operator may arbitrarily and accidentally establish a fixed air intake damper position that results in over- or under ventilation of all or some parts of space. A CO₂ control strategy can ensure the position of the intake air dampers is appropriate for the ventilation needs and occupancy of the space at all times. Active control of ventilation system can provide the opportunity to control indoor air quality. Demand controlled ventilation creates improved IAQ by increasing ventilation if CO₂  level rise to an unacceptable level.

Design considerations for DCV

CO₂ -sensing is a rather uncomplicated technology, and installation of CO₂ sensors is a trouble-free procedure. Sensor voltage, power and control of output requirements are similar to those ones commonly used in thermostats. There are two types of sensors: wired and wireless. Data from wireless sensors is delivered with the use of signal communications. Wireless sensors have self contained power supply. Such sensors are used on-board power controlling to alert a building operator when battery charge is low and needs be changed.

All suppliers of HVAC systems frequently offer systems for located demand controlled ventilation and reading data from sensors. Therefore, putting into operation of CO₂ -based DCV is not a complicated process. However, upgrading previous systems with pneumatic controls for operation with DCV may be more challenging. Sensors are typically mounted on walls similar to thermostats. Some manufacturers off er standard sets, which include a thermostat and sensor. The standard sets which can monitor temperature, CO2 and humidity are also available. They are used in systems that include a drier to control humidity in ventilation air.

Data from CO₂ -sensors delivered to HVAC control system in a building or to an actuator that controls the amount of ventilation air. For reconstruction of HVAC system it may become necessary to repair or upgrade dampers. Good operating of dampers that can be automatically controlled is of great importance. Pneumatic controls will need to be replaced with electronic control or Direct Digital Control (DDC). Actuators which do not have input points for the sensors will need to include these points. But it is not simple to upgrade and calculate HVAC systems for more complex systems, such as variable-air-volume systems, as it may seem. One needs a more complex algorithm. CO2-sensors can be mounted in the interior of the building or by integration into an air handling system. The data from sensors to regulate the amount of supply outdoor ventilation air are applied in them. The illustration of this is provided in Figure 6.

Energy saving potential

If demand controlled ventilation lowers excessive supply outdoor air in a building during heating and cooling seasons, then annual energy expenses for heating and cooling the outdoor air reduce correspondingly. In addition, lower outdoor air requirements decrease the fan energy expenses to supply or extract air from a building. Actual occupancy levels in buildings are generally significantly lower than the design occupancy levels. The experience indicates that actual occupancy levels may be 25-30% and 60-75% lower in some buildings than the design levels. The first and last, saving energy potential using demand controlled ventilation may vary depending on climate, type of a building, Type of HVAC system and occupancy in the space in which DCV is implemented and other operating conditions. The capability of authorized staff to maintain and operate equipment properly may also positively affect savings. Available data suggest that demand controlled ventilation reduces ventilation, heating and cooling loads by 10% to 30%. Buildings with large fl uctuation of occupancy, such as office buildings, shopping malls, cinemas, auditoriums, schools, nightclubs etc., realize the largest saving energy. Demand controlled ventilation reduces electricity requirements when actual occupancy level is below than design occupancy level during the demand periods. Lower amount of supply of outdoor air reduces cooling and ventilation loads and thus, air-conditioning power reduces. Generally speaking, energy saving potential varies from building to building. It depends on its occupancy. Figure 7 shows an example of graphical representation of energy saving potential.

Market factors

The quick pay-off period of CO₂ sensors can be expected in spaces, in which occupancy is variable and unpredictable (auditoriums, some school buildings, shops etc.), as well as in the areas with high heating and/or cooling demand and high utility rates. On the average demand controlled ventilation has a payback period of two to three years that can be cost-attractive for many customers. But many buildings do not use DCV that is due to some disadvantages, namely that CO₂ -sensors of DCV system do not respond to other indoor pollutants and expensiveness of operational personnel. DCV is a new concept for standards and local building codes, which one should not hurry to apply. Contractors and designers have questions and doubts about liability of systems, if they can meet indoor air quality standards. Because of that it may be due to incorrect installation of CO₂ -sensors and presence of large amount of non-human pollutants exceeding the acceptable level. On the other hand, DCV requires installation and operational personnel, which are more expensive and difficult to find. But the energy saving can compensate these disadvantages.

Discussion and Conclusions

DCV system controls the amount of outdoor fresh air supply, depending on the number of people in a building and their activity. DCV makes it possible to maintain the needed ventilation and improve indoor air quality while saving energy. Such systems benefit both building operators and building occupants. DCV reduces electricity requirements when an actual occupancy level is below than the design occupancy level during the demanded periods. The lower amount of supplied outdoor air reduces cooling and ventilation loads and, therefore, air conditioning power reduces. Maximum saving energy using DCV is provided in buildings, where the number of people continuously changed, is unpredictable and attains a high level, for example, office buildings, shopping malls, cinemas, auditoriums, schools, nightclubs etc. In buildings with a more stable occupancy level, DCV provides enough amount of fresh air supply per person all the time. But it could be uneconomical, because DCV reduces energy costs less in the areas with a high utility level. Saving energy potential can change as well, depending on climate, the type of a building, the type of a HVAC system with which DCV is implemented and other operating conditions. Demand controlled ventilation creates improved indoor air quality by increasing ventilation, when CO₂ level rises to an unacceptable level. One of the most important aspects of designing DCV is correct control strategy selection.

In such a manner the set-point control strategy can be designed for spaces with high occupant densities, which reach full or nearly full occupancy rapidly once occupancy commences. While the proportional control strategy is applicable to a wide range of occupant densities and patterns. A proportional control approach starts to open a damper or increase the introduction of outdoor air when indoor CO₂ levels are a certain amount above outdoor levels. This lower control set point in the control range is 100 to 200 ppm above outdoor levels. As CO₂ levels in the occupied zone rise, the damper opens wider. Two important criteria for any CO₂ control strategy are that the target per-person ventilation rate is met at all times and that during periods of changing occupancy the lag times as prescribed in ASHRAE standard 62.1-2004 are met. It is possible to determine the number of sensors & to select types of sensors, when a control strategy is chosen correctly.