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It is significant to recognize the magnitude of influence that the IMC carries. At last count, the IMC2000 or 2003 is effective in a total of 47 states and territories which have or soon will require compliance to the IMC. About 95-98% of the U.S. population will be subject to the ICC's Mechanical Code.
SECTION 403 - MECHANICAL VENTILATION.
When mechanical ventilation is used, specific minimum rates of ventilation must be supplied, based on occupancy and structure type and defined in Table 403.3. The section continues into 403.3.1 to specify that the ventilation requirements are allowed to be based on rates "per person" and restricted to the actual number of occupants present. This language allows ventilation reset for variable occupancy, or what is known as Demand Controlled Ventilation (DCV).
The retained language (until 2006) is as follows:
"403.3.1 System Operation. The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in Table 403.3 and the actual number of occupants present."
Regardless of the "interpretations" presented by proponents of CO2-based demand controlled ventilation (DCV) with CO2, using the very generalized allowances for acceptance of modifications for individual cases in Section 105, and for qualified statements made in the Commentary for Section
"403.3.1; this determination currently disallows the "traditional" use of CO2-based DCV strategies that have been proven to be problematic and to only "approximate" occupancy. CO2 levels have no direct relationship to the minimum intake rates required in table 403.3 or the actual rates introduced at the air handler."
"403.3.3.1 Variable Air Volume System Control.
Variable air volume [VAV] air distribution systems...shall be provided with controls to regulate the flow of outdoor air. Such control system shall be designed to maintain the flow of outdoor air at a rate of not less than that required by Section 403 over the entire range of supply air operating rates."
"403.3.4 Balancing. Ventilation systems shall be balanced by an approved method. Such balancing shall verify that the ventilation system is capable of supplying the airflow rates required by Section 403."
To rely on a one-time manual system set-up and balance requires the contractor or user to select the worst-case condition for continuous operation and thereby increase the outside air delivered to normally excessive and unnecessary rates. When reading these IMC sections on VAV systems and performance verification, it is difficult to believe that any means other than direct measurement of outside air intakes will satisfy this requirement, if the objective includes optimizing the energy used and minimizing operating costs.
SECTION 405 - SYSTEMS CONTROL
"405.1 General. Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied.
Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air supply rate during occupancy."
IMC2003 CONCLUSIONS AND RECOMMENDATIONS
When distilled, this chapter of the IMC requires that the minimum rates specified in the tables must be provided "continuously" during occupation and controlled "...to automatically maintain the required outdoor air supply rate...over the entire range of supply air operating rates."
The word "maintain" is unambiguous and does not mean "get close to" or "average" the required rate. At the most, it could be interpreted to mean "not less than" that required. In fact, VAV requirements (403.3.3) provide that the control systems will maintain the required ventilation at a rate "not less than" that in the tables. Therefore, any indirect method of control (fixed intake damper, adiabatic mixing, supply-return calculation, return fan speed-slaving, CO2, etc.) will not be capable of insuring that the rates never drop below the minimums specified in the table and cannot provide verification that any specific level of intake volume has been provided.
An easy but often neglected conclusion is that the best and most efficient way to insure specific rates of outdoor air are introduced to the air handling system is to simply measure them directly. With the appropriate instrumentation and subject to the conditions of measurement, one can choose to control a combination of system variables to maintain a fixed intake set point, at least to within the limitations of the instruments proven accuracy.
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