6.2.2.1 Breathing Zone Outdoor Airflow. The design outdoor airflow required in the breathing zone of the occupiable space or spaces in a zone, i.e., the
breathing zone outdoor airflow (Vbz), shall be determined in accordance with Equation 6-1:
Vbz = RpPz + RaAz (6-1)
where:
Az = zone floor area: the net occupiable floor area of the zone m2, (ft2).
Pz = zone population: the largest number of people expected to occupy the zone during typical usage. If the number of people expected to occupy the zone fluctuates, Pz may be estimated based on averaging approaches described in Section 6.2.6.2.
Rp = outdoor airflow rate required per person as determined from Table 6-1.
Note: These values are based on adapted occupants.
Ra = outdoor airflow rate required per unit area as determined from Table 6-1.
Note: Equation 6-1 is the means of accounting for people-related sources and area-related sources for determining the outdoor air required at the breathing zone. The use of Equation 6-1 in the context of this standard does not necessarily imply that simple addition of sources can be applied to any other aspect of indoor air quality.
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The uncertainty of indirect techniques introduces a significant level of risk. The designer, facility owner and occupants should carefully consider the method to be employed prior to implementation of any CO2-based Demand Controlled Ventilation scheme as the sole method of intake rate determination.
The new VRP in §6.2 recognizes the magnitude of building generated pollutants, therefore adding the “building component” in the zone ventilation equation. Table 6-1 and the accompanying notes specify outdoor air requirements for specific applications. Equation 6-1 (§6.2.2.1 above) is now based on the combination of ventilation rates per person (as CFM/p) PLUS ventilation rates per floor area (as CFM/ft2). Therefore, in order to claim that a ventilation system complies with the Standard through the VRP, it must meet:
(a) the minimum requirements of Table 6-1, combined with
(b) the calculated volume of outdoor air required by 6.2 and
(c) the outdoor air quality requirements set forth in Section 4 (d) while “under any load condition,”
Under ideal and very specific conditions, CO2 levels can only reflect the rate that outdoor air enters the building on a per person basis – through any and all openings. Therefore, CO2-based DCV with single ‘ppm’ set point control cannot be implemented under the requirements of Standard 62.1 since the addendum to 62-2001 was approved in mid 2003, unless applied with excessive conservatism and the accompanying increase in energy usage. Otherwise, CO2-based DCV will invariably under ventilate, over ventilate or require that the Standard be interpreted in such a way to allow the potentially large airflow errors that will result from using CO2 sensor inputs.
Indirect measurements for control typically carry such a large degree of uncertainty, one can never be secure that the controlled variable (ventilation rates) will not drop below or substantially exceed the mandated minimums under operating conditions.
CO2 methods generally produce too much risk. Compliance with the IMC using CO2 is at the discretion of the "authority having jurisdiction" based on your variance application. Compliance with ASHRAE 62.1 is indicated, but the method is left in doubt. Why risk noncompliance or excessive energy costs when there are other more reliable methods available to accomplish the same function, with similar or superior results?
Once the breathing zone outdoor air requirement is determined, the Standard requires an adjustment based on the distribution system efficiency and effectiveness. This makes complete sense since the air must reach the breathing zone to be effective. Multi-zone recirculating systems are not as efficient as 100% OA systems and are therefore required to be factored by their approximate and relative inefficiency. We are given two methods to determine this factor:
1. Table 6-3, “default Ev” method
2. Appendix A, “calculated Ev” method
These methods produce significantly different results. The more precise one is contained in Appendix A and as might be expected, is more involved. The table’s conciseness requires it to be more conservative and therefore not as efficient in many situations.
For each multiple zone recirculating system (VAV or CAV), the primary outdoor airflow fraction must be calculated for all zones that may become ‘critical’ (only one zone can be critical on CAV systems). The “critical zone” is defined as the zone that has the highest percentage of outdoor air required in the primary air stream. When analyzing a VAV system dynamically, treat it as a CAV system.
Systems that provide a variable supply of air volume to the conditioned space are influenced by everything previously discussed. In addition, outdoor airflow rates will vary as a result of changes in mixed air plenum pressure. If the design did not assume the worst-case scenario when the outdoor airflow rate for the air handler was determined, outdoor airflow rates on VAV systems may need to be reset based on calculations of the multi-space equations (6-5 through 6-8, defined in §6.2.5) in order to avoid potentially excessive over ventilation and the associated energy penalty.
Advanced VAV control strategies can dynamically satisfy the requirements of §6.2.5.1 – §6.2.5.4 and therefore be more efficient than static strategies. This can be accomplished by automatically determining the critical zone fraction to continuously calculating the corrected fraction of outdoor air. The calculation requires that the total supply airflow rate be continuously measured and the airflow rate of the critical zones are measured with permanent airflow measuring devices capable of very accurate measurement.
Airflow sensors traditionally provided with VAV boxes should not be used for these calculations. Although the OEM devices may be adequate in modulating a terminal box for thermal comfort, the combination of typically poor inlet conditions, low quality airflow pickups and low cost pressure sensors in the DDC controller will not result in the measurement accuracy necessary for proper calculation of Equations 6-1 and 6-5 through 6-8. Conservative mathematical modeling has demonstrated that typical VAV box measurement performance can be statistically exceeded by boxes without a measurement device. Accurate airflow measuring devices having a total installed accuracy better than 5% of Reading at maximum system turndown should be installed in the supply ducts for critical zones.
The result from these multi-space equations can provide wide variations in outdoor airflow requirements in some systems. Increasing the critical zone supply flow while using reheat, can reduce total outdoor airflow rates and overall energy usage. This method has been simulated at Penn State University using the multi-space equation from Standard 62-2001, with published results showing greater energy efficiency than the same system supplying the maximum, worst-case Vot continuously. The basic variables, relationships and the end results should be the same using the VRP of Standard 62.1.
§6.2.7 on Dynamic Reset addresses conditions when the ventilation control system…
"…may be designed to reset the outdoor air intake flow (Vot) and/or space or ventilation zone airflow (Voz) as operating conditions change."
6.2.7.1 Demand Control Ventilation (DCV)
6.2.7.1.1 DCV shall be permitted as an optional means of dynamic reset.6.2.7.1.1 DCV shall be permitted as an optional means of dynamic reset.
Exception: CO2-based DCV shall not be applied in zones with indoor sources of CO2 other than occupants or with CO2 removal mechanisms, such as gaseous air cleaners.
6.2.7.1.2 The breathing zone outdoor airflow (Vbz) shall be reset in response to current occupancy and shall be no less than the building component (Ra · Az) of the DCV zone.
Note: Examples of reset methods or devices include population counters, carbon dioxide (CO2) sensors, timers, occupancy schedules or occupancy sensors.
6.2.7.1.3 The ventilation system shall be controlled such that at steady-state it provides each zone with no less than the breathing zone outdoor airflow (Vbz) for the current zone population.
6.2.7.1.4 When the mechanical air-conditioning system is dehumidifying, the current total outdoor air intake flow for the building shall be no less than the coincident total exhaust airflow.
6.2.7.1.5 Documentation. A written description of the equipment, methods, control sequences, set points, and the intended operational functions shall be provided. A table shall be provided that shows the minimum and maximum outdoor intake airflow for each system.
6.2.7.2 Ventilation Efficiency. Variations in the efficiency with which outdoor air is distributed to the occupants under different ventilation system airflows and temperatures shall be permitted as an optional basis of dynamic reset.
6.2.7.3 Outdoor Air Fraction. A higher fraction of outdoor air in the air supply due to intake of additional outdoor air for free cooling or exhaust air makeup shall be permitted as an optional basis of dynamic reset.
There is no direction in the Standard on how to implement Dynamic Reset using CO2. This section of the Standard would lead one to believe that CO2 is a method of ‘counting’ and not an input to be used for direct ventilation control. Therefore, any counting method could be used to reset a flow rate established and controlled by some other measurement means. We recommend that you read the 62.1 User's Manual, particularly section 6.2 on the VRP. Because of the high risk of noncompliance and excess energy costs or insufficient dilution air, intake rates should not to be determined by the ‘counting’ device (e.g. CO2 sensors) alone.
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