Demand Controlled Ventilation Control

Introduction. Demand Controlled Ventilation (DCV) control uses zone CO2 sensors to control ventilation air. This help topic explains how HAP simulates DCV control.

HAP performs an iterative calculation to determine the steady state CO2 levels for each hour of operation simulated and the resulting control of outdoor airflow rates. This involves estimating the CO2 level in each zone. The zone CO2 levels determine how the outdoor ventilation damper is positioned. Outdoor ventilation airflow in turn influences CO2 levels in the system. Thus, elements of the analysis are interrelated and an iterative solution is needed to consider all feedback issues in the system. The following sections discuss each element in this calculation.

A. CO2 Generation by Space Occupants.

Occupants are the primary source of CO2 generation in occupied spaces. Further, occupants produce a predictable amount of CO2 based on activity level. The total CO2 generated by occupants of a zone is calculated as the sum of CO2 generated by occupants in all spaces in the zone:

Vzone = S (Vspace)(Mspace), all spaces

The CO2 generated in each space is calculated as the number of occupants present times the CO2 generated per person:

Vspace = Nocc Vocc Where:

Vzone = Total volume of CO2 produced by occupants in a zone, CFM or L/s.

Vspace = Total volume of CO2 produced by occupants in a single instance of a space, CFM or L/s

Mspace = Space multiplier. The number of spaces of this type in the zone.

Nocc = Number of occupants in the space for the current hour. This is the product of the maximum occupants times the hourly schedule factor for the current hour. Vocc = Total volume of CO2 produced by one occupant in the space, CFM or L/s. This value depends on the occupant activity level as described in the following paragraphs. The calculation of CO2 generation for a single occupant is based on the fact that human activity is related to respiration. That is, the higher the level of activity or exertion, the higher the level of respiration. During respiration, the occupant exhales air that contains CO2 generated in the lungs. In HAP the relationship between occupant activity and CO2 generation is calculated as follows:

Vocc = Qtot (K) / [M At]

English Units: Vocc = Qtot (0.00002667) CFM SI Metric Units = Qtot (0.0278) L/s

Where:

Vocc = Total volume of CO2 produced by one occupant in the space, CFM or L/s.

Qtot = Total heat gain per occupant, BTU/hr/person or W/person. This is the sum of sensible and latent heat gains defined by the occupant activity level, or directly specified by the user.

K = Curve fit coefficient. 0.00883 CFM/Met in English or 0.25 L/s/Met in Metric. Figure C-2 "Metabolic Data" from ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air

Quality, plots the relationship between metabolic level and CO2 generation. The relationship can be represented as the following equations: CO2 generation in CFM = 0.00883 x MetsCO2 generation in L/s = 0.25 x Mets

M = Heat flux for one metabolic unit or "Met", 18.4 BTU/hr/sqft or 5.39 W/sqm, where the sqft or sqm area refers to body surface area At.

At = Body surface area for one "typical" occupant, sqft or sqm. For this analysis, HAP assumes the average adult male has a body surface area of 19.4 sqft (1.8 sqm) and the average adult female has a body surface area approximately 85% of that for the male (16.5 sqft or 1.53 sqm). HAP considers the typical space occupant to be an average of male and female body areas: 18.0 sqft or 1.67 sqm.

The equations above are the combination of two separate equations. Stating the equations separately helps to understand the formulation better. First, the heat flux per person in Mets is calculated as:

M = (Qtot BTU/hr/person) [1 Met / (18.4 BTU/hr-sqft)] [1 / (At sqft)]

Example: One occupant with activity level of "office work" has a sensible heat gain of 245 BTU/hr/person and a latent heat gain of 205 BTU/hr/person. Therefore, the total heat gain is 450 BTU/hr/person and the metabolic rate per person is:

M = (450 BTU/hr/person) (1/18.4 BTU/hr/person) (1/18.0 sqft) = 1.36 Met/person

Second, the correlation between metabolic rate and CO2 generation, obtained via a curve fit of ASHRAE Standard 62 data is:

CO2 generation in CFM = 0.0088 x M CO2 generation in L/s = 0.25 x M

B. CO2 Mass Balance for Zones

Once the CO2 generation rate for zone occupants is known, a CO2 mass balance can be performed for each zone in the system to estimate the CO2 level measured by a CO2 sensor in the zone. This mass balance assumes a steady state level of CO2 will be reached each hour. The mass balance is as follows:

0 = CO2 in supply air entering zone - CO2 in direct exhaust air leaving zone.

- CO2 in return air leaving zone via the return plenum or return duct. + CO2 in infiltration air entering the zone.

- CO2 in exfiltration air leaving the zone. + CO2 generated by occupants in the zone.

To solve this equation for the zone CO2 level, the CO2 level in supply air is assumed. Later this assumption will be checked and if necessary, the calculation will be repeated with an adjusted assumption. Knowing the supply CO2 level, all the airflows and the CO2 generation by occupants, the equation can be solved for the zone CO2 level.

C. Determining Outdoor Damper Position and Outdoor Airflow

To determine outdoor damper position the DCV controller will first scan CO2 measurements by all zone CO2 sensors and identify the highest CO2 level. It then uses this CO2 reading to determine the indoor-outdoor CO2 differential:

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This CO2 differential is used with the DCV control profile to determine the required outdoor ventilation airflow.

The figure below shows a sample control profile. In this example the outdoor CO2 level is 400 ppm. The minimum CO2 differential is 100 ppm, which equates to a CO2 level of 500 ppm and corresponds to a base ventilation rate of 810 CFM. The maximum CO2 differential is 700 ppm, which equates to a CO2 level of 1100 ppm and corresponds to a design ventilation rate of 2700 CFM. The DCV controller sets the outdoor damper position as follows:

• If the CO2 differential determined by the DCV controller is less than the minimum setting of 100 ppm (which equals a CO2 level of 500 ppm), the outdoor dampers will be set to provide the base ventilation rate of 810 CFM.

• If the CO2 differential is greater than the maximum setting of 700 ppm (which equals a CO2 level of 1100 ppm), the outdoor dampers will be set to provide the design ventilation airflow of 2700 CFM.

• If the CO2 differential is between the minimum and maximum settings, then the outdoor dampers will be set to provide the airflow corresponding to the control profile. Thus, outdoor airflow is a linear function of CO2 differential in this range and ranges between the minimum (810 CFM) and maximum (2700 CFM) values.

D. Calculation of System CO2 Levels

Next, the program uses the zone CO2 data from part C and the outdoor ventilation airflow data from part D to calculate CO2 levels in the remainder of the system. This requires starting at the return grilles of all zones and working along the airflow path to determine CO2 levels at all state points in the return portion of the system, and then into the supply portion of the system.

For example, return air from the zones mixes in the return plenum or duct to yield a mixed CO2 concentration in return air. Return air mixes with outdoor air, to provide a mixed CO2 concentration in supply air, which is delivered to the zones.

E. Evaluation of Results and Iteration

Finally, the supply air CO2 level produced by part D is compared with the initial assumption for supply air CO2 level that was made at the start of the CO2 balance calculation in part B. If the two values differ by more than 10 ppm, the calculation in parts B, C and D is repeated using the new supply air CO2 value. In this way the program iterates to converge on a solution in which the outdoor air flow, system CO2 levels and zone CO2 levels are all balanced and consistent for the hour.