The 2019 Guide to Demand Control Ventilation
A well-designed ventilation system is fundamental for energy efficiency and indoor air quality in buildings. Air pollutants accumulate when ventilation is too low, but an excessive ventilation airflow represents a waste of energy. Ideally, a ventilation system should keep pollutants at safe levels, while achieving the lowest operating cost possible. This can be achieved by optimizing the ventilation airflow according to the needs of the building.
Overventilation also increases the operating cost of space heating and air conditioning systems. While fans waste energy supplying more air than needed, heating and cooling units waste energy conditioning the extra air volume.
Ventilation design is a complex process that requires advanced engineering knowledge. However, in simple terms, the goal is to define three aspects: the specific ventilation equipment that will be used, its layout in a building, and how it will be controlled.
The Ventilation Design Process
Modern building designs give a high importance to airtightness, since it improves energy efficiency, and indoor temperature control. However, an airtight building depends on mechanical ventilation to keep a constant supply of fresh air.
Ventilation requirements are determined in great part by the indoor area served and the building population. The local climate also plays a role, but this is a factor that cannot be changed. Instead, building systems must adapt to the existing weather conditions.
To visualize how area and occupancy determine ventilation needs, consider a simplified example:
- Assume two classrooms have the same number of students, but one is twice as large as the other. The larger classroom will need more fan power, since twice as much fresh air is needed to fully replenish indoor air.
- A similar logic applies for equally-sized spaces with different occupant loads. For example, a restaurant with 100 customers requires more fresh air than a restaurant with only 60 customers in the same floor area.
ASHRAE standards provide equation sets to calculate the outdoor air supply for indoor spaces. In most cases, the design airflow is the sum of two smaller airflows: one determined by area and another determined by occupant load. Ventilation airflow rates are normally specified in cubic feet per minute (CFM) or air changes per hour (ACH)
Ventilation airflow values can be converted between CFM and ACH, but there is no fixed conversion factor. Instead, the relationship between CFM and ACH is determined by indoor space dimensions. For example, a value of 2 ACH has a different meaning for a small bedroom and for a large conference room. Consider the following example, where the value of 1 ACH is determined in CFM for two indoor spaces with different dimensions:
|Indoor Space||Bedroom||Conference Room|
|Volume (cubic feet)||720||9600|
|1 ACH in cfm||720 ft3 / 60 min = 12 cfm||9600 ft3 / 60 min = 160 cfm|
In the example above, the following observations can be made:
- 2 ACH means 24 cfm for the bedroom, but 320 cfm for the conference room.
- You can compare cfm values with each other, but not ACH values unless they are for rooms of equal volume.
- For the same reason, you will find that fan specifications use cfm but not ACH. A fan manufacturer does not know the volume of the room where the unit will be installed, which would be necessary for an ACH value.
Another important difference between ACH and CFM is timeframe. When ventilation airflow is specified in air changes per hour, it could refer to either constant or intermittent ventilation. However, when ventilation airflow is specified in cubic feet per minute, it deals with a shorter time frame. In other words, values in ACH are result-oriented, while values in CFM are mean-oriented.
When ventilation requirements are calculated, they are based on the maximum occupancy of the indoor space in question. However, many buildings are only at full occupancy during some times of the day, and the ventilation system supplies more air than necessary the rest of the time. To address this issue, building owners can deploy demand controlled ventilation, where ventilation airflow is adjusted according to occupancy.
Basic Principle of Demand Controlled Ventilation
When DCV is deployed, the outdoor airflow is adjusted according to the occupant load. The air-handling system only supplies the maximum outdoor airflow at 100% occupancy, and this value is ramped down when indoor spaces are not full. For example, a restaurant can be expected to reach full occupancy during breakfast, lunch and dinner, but is only sparsely occupied at other times of the day. This is an example of where DCV can be used to achieve energy savings.
DCV is also promising in office buildings. For example, a company can have 200 employees, but providing permanent ventilation for a population of 200 will be a waste of energy. Consider that employees may be away attending meetings or visiting clients, and some of them may be on vacation or sick leave. In this case, a DCV system can optimize ventilation airflow based on the building needs, and not the full occupancy of 200.
A conventional DCV system provides just the right amount of air considering the area and occupancy, adjusting airflow in real time to improve energy savings. For comparison, a ventilation system without DCV always operates at design airflow, as if the area served was always full.
Many building codes establish a minimum outdoor airflow (OA) in their ventilation requirements, to prevent the accumulation of pollutants. ASHRAE provides two options, which are the Ventilation Rate Procedure (VRP) and Indoor Air Quality Procedure (IAQP). The VRP can be considered the conventional approach, based on predetermined airflow values by square foot and by occupant. On the other hand, the IAQP focuses on measuring and controlling air pollutants directly, as opposed to the indirect approach of the VRP (area and occupancy).
The IAQP has the potential to achieve a higher energy savings than the VRP, while conserving indoor air quality:
- The IAQP does not establish a minimum outdoor airflow.
- Instead, the system monitors air pollutants directly, and the OA is adjusted based on the measured concentrations.
- This prevents overventilation when pollutants have already been controlled, which represents a waste of energy.
Foobot can be used to control DCV systems that follow the Indoor Air Quality Procedure (IAQP). It can also enhance systems that use the Ventilation Rate Procedure (VRP), by adding the capacity to monitor air pollutants.
DCV is an ongoing process, where occupancy and/or pollutants are monitored to optimize airflow. In addition, most systems use a temperature sensor to make sure conditioned air is at the right temperature. Let’s examine the many viable methods to keep track of occupancy and pollutants below.
1) Time Tables
This is not a measurement method strictly speaking, but it can be deployed in buildings with a known population and a predetermined schedule. Two examples are lecture rooms and office cubicle areas, where occupants must be present at certain hours. It is only a matter of programming the respective schedule in the ventilation controls.
This control method is affordable because no sensors are needed, but it can only be deployed when occupancy patterns are known in advance, which is never perfectly accurate. For example, employees may stay late at the office to meet an important daylight, or students may stay at school during the afternoon for tutoring.
You cannot use time-based controls for DCV if occupancy is unpredictable. Also consider that the time table method has no response to air pollutants. If a certain activity produces above-average levels of air pollutants, the ventilation system will not increase the OA in response.
2) Occupant Counters
Some DCV systems use sensors or cameras to count the number of people entering and exiting a building.
- Assume an indoor event venue opens its doors, and the sensors count 500 people arriving and 75 people leaving.
- It is safe to assume there are 425 people inside, and the ventilation output can be adjusted accordingly.
This method is useful for large indoor spaces where most people gather in a single zone, such as auditoriums and theaters. It is less reliable for buildings that are split into many zones with separate ventilation requirements, since there is no way to tell how people are distributed.
There are many possible ways to count the number of persons in an indoor space. For example, if 5,000 tickets were sold for an event, and the staff is composed of 100, the ventilation system can be set for an occupant load of 5,100. People counting systems can also be more sophisticated; for example, there are software solutions that can count people based on the input of video cameras.
Occupant counters have the same limitation as time tables, having no response for increased air pollution levels. Both methods assume that air pollution follows the occupancy pattern, which is not always the case. A ventilation system can only respond to air pollutant levels if these are monitored directly.
3) Occupancy Sensors
Occupancy sensors are normally based on infrared radiation or ultrasound, and there are also dual-technology occupancy sensors that feature both inputs. These sensors can determine if a room is occupied, but cannot determine the number of people.
- Restrooms are an excellent example of where these sensors can be used effectively.
- They normally have a fan that operates at design airflow, but only when the sensors detect someone in the restroom.
Occupancy sensors are useful as ON/OFF controls for applications where occupancy is random and brief. When used alone, they are not an efficient option for areas with extended occupancy, since they always use maximum airflow. However, occupancy data can be used to optimize ventilation when combined with other measurements, such as air pollution levels.
4) DCV Based on Carbon Dioxide Monitoring
There are compounds whose concentration is directly related to the number of occupants. Carbon dioxide (CO2) is an excellent indicator of occupancy, since all human beings produce similar amounts per hour. In addition, a DCV system with CO2 concentration sensors can tell not only the total occupant load, but also how they are distributed among indoor spaces. This way, the ventilation system can control the total airflow provided, and also how it is split among various zones.
As you might expect, a DCV system with CO2 concentration sensors is more expensive than the options described above. However, these systems also tend to achieve higher energy savings as well.
The CO2 setpoint for these systems is normally based on the outdoor concentration plus a margin. For example, if the outdoor CO2 concentration is 400 ppm (approximately the global average) and the accepted difference is 700 ppm, the threshold will be 1100 ppm.
5) DCV Based on Air Pollution Monitoring
An alternative concept is controlling ventilation systems based on pollutants like particulate matter (PM) and volatile organic compounds (VOC). In this case, the DCV system is configured to keep pollutant levels low. Ideally, pollutants should be under the thresholds established by the World Health Organization (WHO) or local authorities.
Another reason to control ventilation based on pollutants is that some activities release them in large amounts. For example, crowded meeting rooms quickly see VOC and CO2 concentrations skyrocket, many cleaning products release plenty of VOCs, and new furniture tends to carry harmful substances from the manufacturing process. There are cases when ample ventilation is needed even with low occupancy, and CO2 concentration monitoring misses these events.
DCV systems that operate based on pollutant concentrations can also be configured to control humidity. Although air humidity is not a pollutant by itself, there are many harmful organisms that thrive in humid environments – these include mold, dust mites and bacteria.
Key Elements of a DCV System
By definition, a DCV system must be capable of adjusting airflow. This is generally accomplished through the following methods, together or in combination:
- Fan speed control
- Air damper adjustment
Speed control is the most effective method to adjust airflow. To control fan speed, you must adjust the revolutions per minute (rpm) of the motor driving it. Fans above 1 hp normally use motors with variable-frequency drives, while fractional horsepower fans use electronically commutated motors (ECM) with built-in speed control.
Air dampers can be opened or closed to control airflow, but this method is less efficient because part of the fan power is lost as a pressure drop across the damper.
- Fan speed control is preferred for single-zone ventilation systems.
- On the other hand, when multiple zones are served by the same fan, the damper can be fully opened for the zone with the greatest ventilation demand, while the air dampers are adjusted for all other zones.
Demand-controlled ventilation (DCV) should not be confused with variable air volume (VAV) ventilation. VAV systems can also adjust airflow, but they only qualify as DCV if airflow is controlled based on occupancy or air pollution. If a VAV system uses manual control or a predetermined routine, it is not considered DCV.
When deployed correctly, DCV provides better energy savings and indoor air quality. If the system is designed to always supply the minimum airflow required by construction codes, and control is based on air pollution monitoring, DCV can reduce electricity bills a great deal while creating a healthier indoor environment.
How Ventilation Design Can Benefit from the Local Climate
Air conditioning systems work against the heating effect of both internal and external sources in buildings. Internal sources include people and appliances, while external sources include solar radiation and warm air. However, in temperate climate zones, there are times of the year when the air conditioner is only removing indoor heat, and the outdoor air can actually cool the building:
- In these cases, it makes sense to increase the outdoor air (OA) to take advantage of the cooling effect. This reduces air conditioning costs, since the outdoor air provides “free cooling”.
- Even if the ventilation cost increases, the energy savings achieved through air conditioning are much higher.
- This function can be accomplished with a device called an “economizer”.
There is a very important consideration when an economizer is used along with DCV. Under normal conditions, the DCV system can reduce airflow based on occupancy or air pollutants. However, when this machine is active, it overrides the DCV system to increase the outdoor airflow. While this suspends the ventilation savings temporarily, it achieves energy savings through air conditioning savings that are higher. A smart system should be able to switch from DCV to economizer mode when it’s relevant.
Designing DCV systems based on building area and occupancy has been a common practice in the HVAC industry. However, this approach misses a very important detail: air pollution levels often change regardless of floor space and occupancy. The methods to monitor occupancy range from simple people counters to more sophisticated CO2 sensors, but none of them tracks air pollution directly.
DCV designs can optimize energy efficiency and air quality when they are equipped with air pollution sensors. This has two positive outcomes: Ventilation is increased when air pollutant levels rise, even if the occupancy has remained constant. A conventional DCV system is incapable of this response. At the same time, over ventilation is avoided because the building is only ventilated according to its needs, but no more than that.
Finally, DCV improves the efficiency of heating and air conditioning systems, since they don’t have to condition more outdoor air than necessary.