Indoor air quality and HVAC expenses are determined in great part by ventilation. A higher airflow requires more fan power, and depending on the season this can also increase air conditioning costs or space heating costs. However, there is a limit to how much the ventilation airflow can be reduced, since fresh air requirements increase along with occupancy.
Demand controlled ventilation, or DCV, consists on adjusting airflow according to occupancy. The maximum airflow is only supplied at full occupancy, and the cfm provided by fans is ramped down under partial occupancy. A more innovative approach to control ventilation is monitoring not only occupancy, but also the presence of air pollutants.
The ASHRAE 62.1 and 62.2 standards are recognized design references for ventilation systems around the world. The 62.1 standard is called Ventilation for Acceptable Indoor Air Quality, while the 62.2 standard is called Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.
Main Factors That Influence Ventilation Needs
High occupancy correlates with a higher concentration of air pollutants, but there are also cases where air pollution can increase even when occupancy is low or zero. For example, large amounts of volatile organic compounds are released by new furniture, or when some cleaning products are applied. A conventional DCV system based only on occupancy cannot guarantee IAQ under these conditions, since it responds to the low number of occupants instead of the high density of air pollutants.
Conventional DCV designs also overlook the fact that different activities have a varying effect on air quality. For instance, 50 students watching a video tutorial require less ventilation than 50 students working with paint and glue in an art class. A DCV system than only monitors occupancy responds the same in both cases, while a smarter system with air quality response can determine that the 50 students watching a video require less ventilation.
Ventilation Options: Constant Air Volume and Variable Air Volume
The two main variables that can be adjusted in an HVAC system are airflow and air temperature. A constant air volume (CAV) system modulates temperature while keeping airflow constant, while a variable air volume (VAV) system modulates airflow while conserving temperature. Although CAV systems are simpler and generally less expensive, they can be very inefficient in medium or large applications.
Since demand controlled ventilation requires an adjustable airflow by definition, it can only be deployed with a VAV configuration. However, the opposite does not apply: a VAV system is only considered DCV if its controls respond to occupancy.
VAV systems often serve multiple zones that may have different cooling or heating needs. Such a variable load can be served with VAV boxes, which use dampers to modulate airflow into each zone. To meet the HVAC needs of different building areas, the position of each damper is adjusted based on a temperature sensor in each zone.
Of course, the increased complexity of a VAV installation makes it more expensive, but the additional cost is paid off by fan savings. Also consider that the load on space heating and air conditioning equipment is reduced when less air moves through. As a result, achieving a given temperature with reduced airflow consumes less energy.
The traditional way to control airflow from a fan is with a damper, but this is not the most efficient solution, since the pressure drop across the damper represents a waste of fan power. Ideally, the fan speed should be controlled with a variable frequency drive (VFD), which reduces the input voltage and its frequency to lower the fan motor rpm. Since airflow depends on fan rpm, a VFD allows modulation without a damper.
Control Options for Demand Controlled Ventilation
A demand controlled ventilation system must know the number of people in an indoor space to adjust airflow accordingly. This can be accomplished with various methods, ranging from simple people counters at building entrances, to more complex installations with occupancy sensors for each zone.
Depending on the type of building, occupancy patterns can be programmed into the DCV control system, with no direct sensing needed. Educational institutions are a good example of where to use this approach, since classrooms have a known number of students with a predetermined schedule.
However, as mentioned above, a DCV system that only responds to occupancy may miss events that produce a high air pollution independently from the number of persons present. In these cases, the ventilation system can only respond if air quality is monitored directly.
The main challenge when measuring air quality is that a large number of substances is present in the air, and the ventilation system cannot respond to a single variable like occupancy. DCV based on air quality is only possible if the control system can determine the presence and density of key pollutants.
Enhancing DCV Efficiency with an Economizer
Under some weather conditions, outside air may have a more comfortable temperature and humidity level than the recirculated air from indoor spaces. In these cases, an HVAC economizer can used outdoor air directly for space cooling needs, saving the energy that would be otherwise needed to run an air conditioning unit.
A DCV strategy that includes an economizer can achieve a very high efficiency. While the ventilation system minimizes cfm based on occupancy and air pollutants, the economizer minimizes the air conditioning cost associated with that airflow. An air-side economizer uses outdoor air directly, while a water-side economizer uses a cooling tower that would normally interact with a chiller.
Regardless of the efficiency measures deployed in an HVAC system, having equipment of adequate capacity is strongly advised. When air conditioning and space heating units are selected empirically, they often end up oversized, driving up the price and maintenance expenses.
In modern engineering practice, HVAC design and energy modeling are carried out with advanced software, performing complex calculations in just a fraction of the time that would be required through manual methods. The interaction between HVAC components and weather conditions cannot be modeled with a single equation, and this is precisely what makes design software so valuable.