Demand Control Ventilation (For Dummies!)

In the past building ventilation was based on the maximum estimated number of occupants along with the building’s envelope and a number of other items. This is where we hear things like 2 air changes an hour. It is the estimated air changes needed hourly to keep the indoor air quality safe.

As you can imagine, conference rooms that can hold hundreds of people require more air changes than a single room office. But what happens when the conference room isn’t being used?

With many existing systems the number of air changes is the same regardless of if the room is being used or not. This means systems bring in much more outside air than is needed and you end up paying to condition that air. If only there was a way to monitor changes in the indoor environment and adjust air changes based on the results. (In comes Demand Control Ventilation)

Demand Control Ventilation

As the name implies Demand Control Ventilation (DCV)  looks at the demand for ventilation using sensors and supplies the outside air as needed. This type of system can work in small and large buildings alike.

Consider this example: As employees arrive to a building in the morning for work, a DCV system will increase the number of air changes in occupied rooms.  This is necessary due to the increase in CO2 created by having the building occupied. The DCV system will also decrease demand for air changes when employees leave at the end of the day, because there is now less CO2 being produced in the building. With  a DCV system your ventilation will adjust automatically during occupancy changes like this. The results are reduced energy costs, improved indoor air quality, and increased occupancy comfort.

Three Key Benefits to Demand Control Ventilation:

1.Reduced Energy Costs

2.Improve Indoor Air Quality

3.Increased Occupant Comfort

 

How The Sensor Works

The sensor used to adjust the ventilation rates is most commonly a CO2 sensor that is hyper sensitive to CO2 levels in the air. As the number of people in a space increases, so do the CO2 levels. Seeing a spike in CO2 the system will increase the number of air changes until it hits the appropriate rate.

In 2004 ASHRAE updated the amount of outdoor air necessary for a given space. They state that 7.5 cfm of outdoor air per person is required, in addition to 0.06cfm of outdoor air per sq/ft of floor space. To calculate the airflow rate for your space simply take:

 (number of occupants) x (7.5cfm) + (the # of sq/ft in the space) x (.06cfm)

Factoring in the average amount of CO2 released by a human, the CO2 monitors in the room use this equation to make real time adjustments to airflow.

Alternatively, some DCV systems will use an occupancy counting system to adjust rates. Turnstiles, ticket sales, security swipes or other methods to obtain an estimated count of occupants can be used and relayed to the system. Based on the total occupancy of that event or meeting the DCV system adjusts appropriately.

Important Considerations

It is important to avoid these three common pitfalls when installing a DCV system. One, as you adjust the amount of outdoor ventilation going into the building it is also important to ensure your building’s exhaust systems (common in kitchens, restrooms, and copy rooms) are taken into account. You want to be careful not to reduce the outdoor air flow rate so low that it results in unwanted building pressurization. By accounting for the exhaust systems, you can avoid this issue.

Secondly, you want to be cognizant of where you place the CO2 sensor. It is important that the system gets an accurate representation of the CO2 in the room. Placing the sensor by door, windows or in return air ducts can result in false CO2 readings. By staying away from these “hot spots” your system will accurately adjust the ventilation rates.

Third, you need to maintain the sensors just like you maintain your HVAC system. CO2 sensors require calibration over time and should be adjusted during annual maintenance to ensure your system is operating and adjusting the way it should.

Have Questions?

Not sure if a DCV makes financial sense for your facility? Want to chat with an expert? Call us at 1-800-237-3141, email sales@rasmech.comchat with a support agent, or contact us online.  We would be happy to help you out.

For a more in-depth look at DCV’s also check out “Applying Demand-Controlled Ventilation” by ASHRAE and “Code Notes” by the US Department of Energy.

 

 

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