Demand-controlled ventilation

Monitoring carbon-dioxide levels in a building enables ventilation to be controlled according to occupancy and to maintain indoor air quality — efficiently. Bill McConnell examines the benefits that can be obtained.As regulation concerning ventilation increases and as technology provides us with the opportunity to improve air quality in a building, it is worth considering the scope of benefits that a ventilation strategy can bring. Demand-controlled ventilation (DCV) enables the ventilation provided to be controlled by occupancy levels — fitting it to actual need and offering the prospect of saving energy compared to uncontrolled ventilation. Ventilation strategy In the light of EN13779 of the Green Building Regulations, the Energy Performance of Buildings Directive, the Part L requirements to reduce energy consumption by 22% by 2010 and local initiatives across Europe a ventilation strategy for a building should be planned with options such as DCV in mind. Such ventilation is controlled on the basis of measurements that enable estimates to be made of the number of people occupying a space. Air intake rates can be set and reset based on these estimates. Intake should be optimised at less than maximum capacity. The use of carbon-dioxide sensors in an air-conditioning system, for example, enables the ventilation quality to be refined on the basis of the needs of the occupiers for an improvement in air quality. DCV is not synonymous with measuring carbon dioxide to control overall ventilation strategy, which has been a minor trend in recent times. Carbon-dioxide data is better used as a measure of air quality rather than an overall measure of occupancy. It is an important feature of IAQ, and the detrimental effects on occupants of high levels should be taken into account. Ideally in a ventilation strategy, ventilation rates and filtration quality should both be optimised. Typical of the technology required to implement a ventilation strategy are variable airflow devices to control the input of fresh air by the air conditioning system and the newest, most-effective ‘error-free’ carbon-dioxide sensors. Many researchers and engineers have demonstrated that the use of carbon-dioxide sensors alone is not enough as a control driver for a ventilation system or as the key element in a ventilation strategy. Carbon-dioxide measurement is only an indirect indicator of occupancy, and an appropriate base ventilation rate must be continuously maintained to dilute contaminants which cannot be effectively measured or removed from within the building. Carbon-dioxide measurement can help to determine what the base ventilation rate should really be. Demand-based ventilation using carbon-dioxide offers designers and building owners an ability to monitor both occupancy air-quality requirements and ventilation rates in a space to ensure there is adequate ventilation at all times. Compared to a fixed-ventilation approach, this type of DCV offers considerable advantages. It should be noted that carbon-dioxide informed DCV does not affect the design ventilation capacity required to serve the space; it just controls the operation of the system to be more in tune with how a building actually operates. Excessive over-ventilation is avoided while air quality is still maintained. Energy savings of 5 to 80% have been recorded compared with a fixed percentage ventilation strategy. System paybacks can range from a few months to two years and are often substantial enough to help pay for other system or building upgrades. The payback from carbon-dioxide informed DCV will be greatest in higher density spaces that are subject to variable or intermittent occupancy that would have normally used a fixed ventilation strategy such as theatres, schools, retail, and conference and meeting spaces. In spaces with more static occupancies like offices, an informed DCV strategy can provide control and verification that adequate ventilation is provided to all spaces. For example, a building operator may arbitrarily and accidentally establish a fixed position for an air-intake damper that results in over or under ventilation of all or parts of a space. A control strategy based on carbon-dioxide measurements can ensure the position of the intake dampers is appropriate for the ventilation needs and occupancy of the space at all times, with carbon dioxide measurements serving as an early warning signal that adequate ventilation may not be being provided. Infiltration In some buildings, infiltration air or open windows may be a significant source of outside air. A carbon-dioxide sensor will provide measurements that will enable managers of the ventilation strategy to consider the contribution of infiltration in a space and decide that the mechanical system is only needed to make up what is necessary to meet required ventilation levels. The savings made by such a decision would be in addition to those quoted above. When ventilation sensors and controls are integrated with an appropriate building-control strategy and appropriate air-conditioning technology, ventilation can be controlled on a zone-by-zone basis according to actual occupancy. This allows for the use of transferred air from under-occupied zones to be redistributed to areas where more ventilation is required. A control strategy can be used to maintain any per-person ventilation rate. As a result this approach is highly adaptable to changing building uses and any changes that may occur in future recommended ventilation rates. This is a strategy that time-proofs a building to quite some degree and may be attractive to forward-thinking developers. Monitoring and controlling carbon dioxide is considered an important part of green-building design. Low-cost carbon-dioxide sensors for DCV first appeared on the market over 10 years ago. As with any apparently new technology these early products encountered initial market resistance. Issues with carbon-dioxide sensors included price, calibration frequency, size and appearance were very definitely issues, and their early introduction almost certainly held back the development of sophisticated ventilation strategies and the introduction of more sophisticated and more effective products. There are still products on the market that have problems but both the most accurate new sensors or manual electronic measurement tools should be considered as effective tools to inform the development of an energy and comfort conscious ventilation strategy that aims to comply with the latest regulations. Carrier has discovered that these issues can be solved with the right technology. Sensors are now available that can self calibrate and offer thermostat-like dependability at nearer to thermostat-like prices. Technology for measuring carbon dioxide is no longer a barrier to utilising this promising ventilation-control technology and saving energy and money. Bill McConnell is with Carrier UK Controls.

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