Demand Control Ventilation (DCV) vs Airside Economizer: Can They Work Together?
Demand controlled ventilation is among the most effective HVAC efficiency measures, reducing airflow when occupancy, normally measured from a CO2 sensor, decreases. However, an airside economizer (according to ASHRAE standard 90.1) is also effective in many climate zones. Thanks to its temperature sensor, this device is capable of detecting when the outdoor air (OA) has a suitable temperature to provide “free” cooling, and so increasing airflow to save on air conditioning.
- Both measures can save energy, but they interfere with each other when used simultaneously.
- A control system must determine which measure offers the greatest savings at a given time, overriding the other system.
DCV systems and airside economizers can both affect IAQ positively or negatively, but the interaction is different in each case. With an adequate design, both airing measures can help improve IAQ.
Depending on how it is designed, a DCV system can be beneficial or detrimental for air quality. If the system responds to both number of people (carbon dioxide) and air pollutants, it can increase airflow to compensate for pollution, even when number of people has not changed. On the other hand, a DCV system that just relies on a CO2 sensor will only respond to air pollution sources that depend on occupants. For example, when only this source is monitored, the airing system will not respond to off-gassing from fresh paint or new furniture.
Since an economizer increases the outdoor air supply, through an oa damper, it will tend to improve air quality when active. Note that indoor air is 2 to 5 times more polluted than outdoor air, according to the US Environmental Protection Agency. However, special care is needed in representative locations, like industrial areas, that suffer spikes in outdoor air pollution. If the economizer activates at this time, untreated outdoor air may worsen IAQ.
How an Airside Economizer Saves Energy
To understand how this device saves energy, the first step is analyzing the heat sources in buildings. These can be internal or external:
- When the weather is warm, the outdoor air supplied by the system introduces heat. The building’s exterior surfaces also gain heat from the air by convection, and from the sun by radiation.
- A building also has internal heat sources. Some examples are human metabolism, home and office appliances, and many types of equipment.
- External heat gain is significant during the hottest days of summer. However, the air conditioning system deals mostly with indoor heat when the weather is moderate.
In some climate zones, the outdoor air often has a suitable temperature to provide free cooling without air conditioning. Then the economizer temperature sensor detects this condition, and so increasing the outdoor air supply to reduce the air conditioning load. Depending on the weather, it may be capable of fully replacing air conditioning with outdoor air.
However, when an airside economizer and DCV are used in the same installation, they tend to compete with each other. While the latter reduces the outdoor air supply according to the occupants count, the former increases it when free cooling is possible.
DCV can save energy under any weather conditions, as long as the building has a variable number of people schedule that allows for less air circulation. However, economizers have limited use in regions with warm and humid climates. The potential hours of free cooling are reduced in these cases, and installing this device achieves little or no savings.
Controlling HVAC Systems with an Economizer and DCV
With the right controls in place, both systems can independently meet the ventilation requirements of a building. However, since DCV and an economizer oppose each other, a control system must give priority to the measure with the highest savings under the current conditions.
- The economizer should override the DCV system when it can offer higher savings. This happens when the cooling savings with extra outdoor air exceed the additional fan power.
- Otherwise, DCV must be allowed to operate normally.
Economizers are normally inactive during winter, since there is no benefit from “free cooling” at this time of the year. Therefore, the DCV system operates normally along with space heating equipment. In some specific applications, an economizer may be useful all year long. For example, large data centers may require cooling even during winter, and so the device can continue operating.
How Ventilation Efficiency Measures Benefit HVAC Systems
Space heating and air conditioning units normally have a higher energy consumption than air ventilating systems. For this reason, the direct savings from airing measures tend to be lower than the savings from heating and cooling upgrades. However, there is an important factor: efficient ventilation can reduce the workload on heating and cooling systems as well.
When rooftop units introduce outdoor air, it may require heating or cooling depending on the time of the year. In this case, an airflow reduction also reduces the workload on the equipment currently active. In the case of an economizer, the fan power is increased to bring more cfm of outdoor air into the building, but the free cooling effect saves on air conditioning.
Ideally, airing systems should optimize the overall building efficiency, even if that means consuming more fan power. HVAC installations with both a DCV system and an economizer are a great example of this. The DCV system will try to reduce cfm, but the economizer will override DCV to increase airflow when the cooling savings are higher.
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