CO 2 concentration and occupancy density in the critical zones served by the VAV system

This article presents the results obtained from monitoring a VAV system with highly diversified zone occupancy density are presented in the article. The investigated VAV system meets the load for 72 zones (68 perimeters and 4 interiors) consisting of classrooms, offices, conference rooms, etc. with highly diversified occupancy densities from 1.875 to 2.5 m2/person for the classrooms and from 10 to 15 m2/person for the offices. The monitoring shows that the CO2 concentration can exceed the set point in the critical rooms. Simulation results are also presented in the article to show that it is often impossible to adjust the operation of such VAV systems because the adjusted System Outdoor Air Fractions, % OA, can reach 100% even where the zone CO2 concentration is not respected. The presented monitoring and simulation results were obtained in the winter, with the VAV system operating at partial load and with the minimum outdoor air flowrate required by the economizer system. As shown in the article, to respect the zone set point CO2 concentration in such period, the VAV system must operate mostly at a %OA equal to 100% instead of its minimum value. To circumvent this, the supply zone air flow rate may have to be designed taking into account the CO2 concentration resulting from the critical zones occupancy density.


Introduction
A VAV (variable air volume) system, recognized for its energy efficiency, is a system which controls the fan air flow rate according to the thermal comfort of occupants.The supply air temperature is held relatively constant, but could be moderately reset depending on the season.It must always be low enough to meet the cooling load in the most demanding zone.VAV terminal units measure supply airflow rate and control that flow in response to room temperature.The static pressure regulator controls motor-fan speed to maintain the inlet pressure in the VAV boxes.The minimum throttling ratio in the zones is 20%, while the system ratio is 50%.The supply zone air flow rate required is maintained by a specific control loop and depends on the zone temperature.In the perimeter zones, the supply zone air flow rate is at its minimum before the electric baseboard starts to heat.The main disadvantage of a VAV system is that when it is operating at partial load, the outdoor air flow rate supplied to the zones served by the system could be too low, and could result in poor air quality [2,3].This paper presents a study of CO2 concentration in the zones of the existing VAV system, operated with an air-side economizer cycle to control the outdoor air flow rate.

VAV system description and monitoring
The investigated VAV system meets the load for 72 zones (68 perimeters zones with electric baseboards and 4 interiors zones), consisting of classrooms, offices, conference rooms, etc.The design system air flowrate Vps,design is 17.314 m 3 /s and the minimum system outdoor air fraction is 15% of this Vps,design flowrate.The system outdoor air flowrate Voa is controlled by economizer cycle, but the measurement of CO2 concentration sensor is also used to control the Voa flowrate if the 1000 ppm is exceeded.The system supply air temperature set point changes linearly within the 14 to 18°C range as a function of the outdoor temperature.It is corrected by adding a value of between -2 and +2°C when the fan airflow rate varies from 90 to 50%.The zone occupancy density is hifhly diversified, and varies from 1.875 to 2.5 m 2 /person for the classrooms and from about 10 to 15 m 2 /person for the offices.The system operates by applying RACO2-DAV (Return Air CO2 Control -Demand Controlled Ventilation) [3,6] with only one CO2 concentration sensor to maintain the return CO2 concentration set point.Since, in the VAV systems, the system outdoor air fraction %OA is the same for all zones served, and since the CO2 is only generated by occupants of these zones, the CO2 concentration could respect the set point in the return duct by exceeding it in the critical zones with high occupancy density.To demonstrate the problems encountered in this system, we monitored of this CVCA system with the following considerations and steps:  The winter is chosen for this monitoring because the VAV system operates during this period at the minimum Voa,s flowrate required by the economizer system;  The classrooms Z1 to Z7 are identified as the critical zones in terms of occupancy density, and are presented in The system outdoor air fraction %OAmon (Voa,s/Vps) is calculated by applying the mass conservation and the energy balance using Tmix, Tret, Toa and Vps data.It should be noted that the minimum %OA value of the investigated VAV system was 15%, but sometimes Tret and Tmix were close or very close, and gave %OAmon values lower than 15%.In those cases the minimum 15% was used in the monitoring data.
The zone outdoor airflow rate Voaz,i is calculated as Voaz,i=%OAmon*Vdz,i and is used to determine the Voaz,i,pers =Voaz,i/Pz,i when Pz,i is the zone population.The critical zones characteristics are as follows: The data acquisition was realized during the following periods: 2015: February 12, 13, and 25, and March 10 and 11; 2016: January 28, 29 and 30, February 1, 2 and 3, and March 9 and 10.Each day of monitoring covers three courses periods: 8:30 AM to 12:00 PM, 1:30 PM to 5:00 PM and 6:00 PM to 9:30 PM.  Figure 2 shows the results obtained in zone Z5 from noon on February 12 to noon on February 13, 2015, the day when the worst results for Cz,i are recorded.Figure 3 shows the min and max values of CO2 concentration Cz,i only the zones Z3, Z4, Z5 and Z7, obtained in 2015.The mean values of population, %OAmon, and of return air CO2 concentration, Cret,mon, are also presented in this figure .The following observations may be drawn from an analysis of the results in Figure 2:  The CO2 concentration Cz,5 is very high and widely exceeds the 1000 ppm limit.It varies with the population and with the Voaz,i using %OAmon and Vdz,i presented in the Figure 2. Finally, it depends on the Voaz,i,pers (l/s*person);  The maximum CO2 concentration value Cret,mon is 1033 ppm, but the Cret,mon mostly respects the 1000 ppm;  The population Pz,i is determined as the number of students registered in the courses;  As mentioned above, when the %OAmon presented in the figure is equal to 15%, it means that it could be lower according to the energy balance used in the calculation of %OAmon;  The results show that Vdz is really a function of the zone temperature because it is not at its maximum (600 l/s) even if the CO2 concentration exceeds the 1000 ppm limit. The fan is in operation during the night because of low outdoor air temperatures.
Concerning Figure 3, it can be noted that:  The maximum values of the zone CO2 concentration Cz,i often exceed 1000 ppm;  Zone Z4 is the largest and most ventilated zone, and the Cz,i of this zone exceeds the limit only slightly;  The best factor explaining the CO2 concentration overrun may be the l/s/person ratio, but it is calculated by taking into account the zone Voaz,i and the Pz,i.As already mentioned, %OAmon (used to determine Voaz,i) is calculated using the energy balance, and is sometimes corrected to 15%.The Pz,i is based on the number registered students in the courses and it could be different from the real Pz,i.The l/s/person ratio can therefore sometimes be inaccurate. The CO2 concentration of zone Z3 is not normal during the PM period when the population is zero.This zone could very well have been occupied even if according to the course schedule, it should not have been.The question now is how the operation of this VAV system should be adjusted in order to avoid exceeding the concentration in critical zones.

Simulation of investigated system 3.1 Cz,i model and calculation
The CO2 zone concentration Cz,i is determined through the model proposed by [1]: where the zone air supply air flowrate Vdz,i, and the zone population Pz,i are recorded by monitoring, V is the zone volume, Δτ is the calculation step, and 0.0049 is the CO2 generated by a person in l/s (4900 ppm).The CO2 concentration of supply air Cs,mon is calculated by equation 2 with Voa,s according to %OAmon and Cret,mon from monitoring, and the CO2 concentration of outdoor air Coa,s is equal to 450 ppm.

VAV-F system and application of SACO2-DCV control system
The VAV-F system was created to simulate the corrected operation by applying the new SACO2-DCV (Supply Air CO2 Control-Demand Controlled Ventilation) [4,5] control strategy in order to respect the zone CO2 concentration set point.This system includes only the critical zones presented in Table 1.
Since the CO2 concentration is widely exceeded in the monitoring, the first question is: was the investigated VAV system well designed to respect the 1000 ppm set point?To answer this question, the Cz,i,design was calculated for each critical zone, i.e., for all zones of the VAV-F system operating in design conditions.The model applied was that of the ASHRAE 62.01 Standard (Ventilation Rate Procedure).The following assumptions were used: (i) Zone Air Distribution Effectiveness (Ez) and Occupation Diversity (D) are equal to 1.0, and (ii) the zone air supply air flow rate Vdz,i, and zone population Pz,i are the design values.This model determines factors such as the Zone Ventilation Efficiency, the System Ventilation Efficiency, the Supply Outdoor Air Fraction for each zone and finally the System Outdoor Air Fraction (Y) required in the air supply by system Vps.If the CO2 outdoor air concentration Coa,s is considered to be 450 ppm, the Cz,i,design could be determined by the following equation: The results in Table 2 effectively show that the CO2 concentration Cz,i,design in the zones is not respected, particularly in zones Z1, Z2 and Z3.It therefore follows that these design parameters are not well selected for the investigated system.

Simulation results
The monitoring results show that the Cz,i is widely exceeded in the critical zones.The principal question is then whether it is possible to adjust the CO2 concentration of supply air Cs to meet the Cret,mon 1000 ppm set point?To answer this question, the following methodology for each monitoring instant is applied:  Calculation of CS,mon for the operation during monitoring by equation 2 with Vps, Cret,mon, and %OAmon from monitoring;  Calculation of Cz,i,sim of each zone by equation 1 using Cs,mon determined before and Vdz,i and Pz,i from monitoring.This is presented in the Figure 5;  Determination of the set point of Cz,i for the zones.This could be the maximal value of Cz,i,sim obtained in the preceding step but, as indicated in Figure 6, it mostly exceeds 1000 ppm, and the Cz,i set point is taken as 1000 ppm and as 1400 ppm, which is the maximal value of Cz,i,design in Table 2;  Calculation of Cs,min,1000 and Cs,min,1400 i.e., the minimum values required to meet the Cz,i set points for the zones (1000 and 1400 ppm) determined in the previous step;  Calculation of the new adjusted System Outdoor Air Fractions %OAsim in each instant (t) by equation 2 using Cs,min,1000 or Cs,min,1400 and Cret,1000 and Cret,1400 equal to 1000 and 1400 ppm, respectively.The Cret,mon from monitoring is also used for this calculation, and three curves of %OAsim (%OAsim,1000, %OAsim,1400, %OAsim,mon) are presented in Figure 6.
− •         (4) The results obtained by applying this methodology are presented in the Figures 5 and  6 for zone Z5 and the VAV-F system respectively.The following remarks may be made:  Cz,5 exceeds 1000 or 1400 ppm even if %OAsim is 100%, but we see that %OAsim is also 100% even if the Cz,5 is lower than 1000 or 1400 ppm.This happens for two reasons:  5;  Taking into account the Cret,mon, as shown in Figure 6, the %OAsim equals to 100% is also required by Cs,sim determined using Cret,mon.
The question now is why the application of SACO2-DCV is not sufficient to respect the CO2 concentration Cdz,i in the critical zones.One reasons for this is that the Vdz,i, which is controlled by the control loop, depends only on the zone temperature Tz,i and, on the other hand, the design (maximum) supply zone air flowrate Vdz,i,design is determined only as a function of the zone load.To meet the Cz,i set point with the existing cooling charge in the critical zones when the %OA is already at 100%, the Vdz,i should be increased.To that end the Ts (the system supply air temperature) must also be increased, but, depending on the cooling or heating demand, this can lead to overheating or overcooling of other zones.In the case of VAV systems with diversified zone occupancy density, the design and the operation control of these systems must be optimized.

Conclusion
When the VAV systems are operating at partial load, the outdoor air flow rate supplied to the zones could be too low and result in poor air quality.
The results obtained when monitoring a VAV system with a highly diversified zone occupancy density show that the CO2 concentration can exceed the set point in critical rooms.One reasons for this is that the Vdz,i, which is controlled by the control loop, depends only on the zone temperature Tz,i and, on the other hand, the design (maximum) supply zone air flowrate Vdz,design is determined only as a function of the zone load.
It is sometimes impossible to adjust the operation of such a VAV system because the adjusted System Outdoor Air Fractions % OA could reach 100% even if the zone CO2 is not respected, as can be seen in this article.
The monitoring and simulation results presented were obtained in the winter during the operation of the VAV system at partial load, and with the minimum %OA outdoor air flowrate required by the economizer system.As shown in the article to respect the zone set point CO2 concentration in such a period, this VAV system must operate mostly at an %OA equal to 100% instead of minimum.It would seem that to avoid such operations when the VAV system must meet the loads of zones with diversified occupancy densities, the design supply zone air flow rate should be determined while taking into account the CO2 concentration resulting from the critical zones occupancy density.

Fig. 1 .
Fig. 1.Diagram of the VAV system with variables used.

Fig. 4 .
Fig. 4. Comparison between the calculated and measured CO2 concentration in zone Z5.

Table 2 .
Data used to determine the CO2 concentration in the design condition.
The Coa,s is considered and limited to 450 ppm; o The %OAsim equal to 100% is required by other zones that Z5 presented in Figure