Lowering Energy Costs with Demand Control Ventilation
One of the vital standards all buildings must be designed for is ASHRAE Standard 62.1 – Ventilation for Acceptable Indoor Air Quality. Engineers and Designers use these standards to ensure all areas of any building receive the minimum mandated ventilation rate, also known as fresh or outside air.
Modern Heating, Ventilation, and Air Conditioning systems will deliver the minimum amount of fresh air for a buildings occupants. Unlike a multi-residential building that is occupied continuously 24 hours a day, the majority of buildings are only occupied for a set period through the day and some portions (i.e. meeting or conference rooms) may only be occupied for a few hours a week. To keep such areas fully ventilated when not in use is a tremendous waste of energy as the building is conditioning spaces with heat, cooling, humidification, dehumidification.
Knowing this, and carefully working within the limits of section 6.2.7 “Dynamic Reset” of ASHRAE 62.1, a building can be configured to vary the amount of fresh air and zone/area flow as operating conditions change. Operating conditions in this case refer to occupancy state of the zone. Earlier systems only considered whether the zone was occupied or not but now we can directly or indirectly monitor the actual number of people in a zone.
A simple method to adjust ventilation rate is to simply schedule the building’s occupancy to reduce ventilation when the building is unoccupied, and increase it to the setpoint when it is occupied. This is an occupancy schedule type of control where the building automation system knows the population based on time of day.
Occupancy Sensors may also be used to indicate if a zone is occupied. Note however that it would not necessarily provide sufficient resolution to identify the specific number of people in a zone. The building automation system would not know exactly how much to ventilate and would likely default to the maximised calculated for that zone.
Carbon Dioxide (CO2) Sensors can monitor in real-time the amount of CO2 in any given zone. The zone’s total CO2 is a combination of the CO2 present in the ambient air, blended with the CO2 that is continuously generated by the occupants.
The BAS (Building Automation System) can be programmed to dynamically adjust the amount of fresh air based on the measured CO2 reading. However, CO2 setpoint is not constant, and the ratio of total fresh air per person varies with the amount of people in the occupied zone as illustrated below.
|Zone Area||1000 sq ft|
|Ventilation Rate:||0.06 CFM/sqft + 7.5 CFM/person|
|Ambient CO2 Levels||400 ppm|
|Occupant CO2 generation rate,||Metabolic activity level = 1.25
Base CO2 generation rate: 0.0084 cfm / met / person
Ventilation Rate @ occupancy of 20: 1000 * 0.06 + 20*7.5 = 210 CFM (10.5 CFM/person)
CO2 Concentration = 400 ppm + (0.0105 CFM CO2/person)*20 / 210 CFM = 1400
Ventilation Rate rate @ occupancy of 65: 1000 * 0.06 + 65*7.5 = 547.5 CFM (8.4 CFM/person)
CO2 Concentration = 400 ppm + (0.0105 CFM CO2/person)*65 / 547.5 CFM = 1646
The higher the occupancy, the higher the zone CO2 level at steady state conditions, assuming minimum fresh air is being supplied into the building. It is now possible to calculate and regulate the required amount of fresh air for this zone using a CO2 reading.
For zero occupancy, the target CO2 concentration is 400 ppm, and Voamin = 1000*0.06 = 60 CFM
At maximum occupancy, the target CO2 concentration is 1646 ppm, and Voat = 547.5 CFM
The building automation system can be programmed to adjust the fresh air and return air dampers to vary the Oa flow between Voamin and Voat . For a majority of rooftop or indoor air handling units additional balancing and verification will be required to verify damper position corresponding to the required fresh air flow and minimum and maximum for occupancy.
Using two data points of CO2 to drive the fresh are rate is not without its pitfalls. The chart illustrates two values for determining fresh air rate; one using the linear interpolation between min and maximum CO2 readings, and the other using equation A-I from the ASHRAE 62 User’s Manual, which yields the fresh air rate at each CO2 reading in the zone. Using a straight interpolation results in consistent over-ventilation of the space at all partial occupancies. In reality, the CO2 reading should be used in conjunction with the formula A-I from the Users Manual to determine the correct damper position. This is for a fixed zone size, activity level and ventilation rates.
A simplified method would be to control the rate of fresh air to a fixed CO2 concentration. This results in significant over-ventilation, in some cases approach 2x fresh air values recommended by ASHRAE 62 as shown below.
We can calculate the fresh air for a fixed differential of indoor less ambient CO2 concentration with:
Voac = Occupant CO2 generation rate * # Occupants * 10^6
CO2 Concentration differential (ppm)
With a 1000 PPM target, the fully occupied zone in the previous example would be ventilated at 1137 CFM of fresh air, more than double the 547.5 CFM advised per ASHRAE 62.
The above chart shows the calculated fresh air per occupant and the corresponding steady state CO2 concentration that a sensor would be expected to record. The third line is the required fresh air required to maintain a constant 1000 PPM in the space. Please note steady state concentrations over 1000 PPM only occur with six or more occupants in the space.
Different approaches may be used depending on the situations and needs of the building. Areas need to be assessed and investigated prior to making any design or programming modifications. Misapplication could lead to zones being under ventilated creating undesirable indoor air quality.
User’s Manual for ANSI/ASHRAE Standard 62.1-2007
Trane Engineers Newsletter volume 34-5 CO2-Based Demand-Controlled Ventilation