Most "model" and local building codes, plus all Federal construction requirements, reference the "national" consensus standard for ventilation and/or the ventilation rate tables that it contains. The latest version of the standard became effective January 4, 2000, wherein Section 5.1 states: "Ventilating systems may be mechanical or natural. When mechanical ventilation is used, provision for air flow measurement should be included." (Par. 5.1, p.4)

When a single air handler serves multiple zones, the Standard provides a formula for compliance (Equation 6-1, par. 6.1.3.1). Outside air intake setpoints are dynamic in VAV systems and must be continuously reset as zone flow rates change. This requires accurate airflow measurement: at the intake, total supply, and into each occupied zone.

With precision airflow measurement instrumentation installed, meeting the requirements of ASHRAE 62-1999 is assured, under all internal and external operating conditions.

The "Multi-Zone Equation" quantifies the zone dilution air requirement based on the outside air portion of all zones at a given airflow rate to each, and rewards the system for unused recirculated outside air from over-ventilated areas. The dynamic quantification and calculations required indirectly demand the accurate measurement of the airflow rates at the outside air, supply air and individual zone air volumes. And, in order to have any chance of optimizing control in VAV systems these airflow rates should at least be continuously monitored.

EBTRON's Technical Bulletin TB202.1-99 offers mathematically conservative examples of the impacts that can be expected due to inaccurate airflow measurements on multi-zone systems. The examples offered assume that errors at a typical VAV box can easily exceed ± 20% of reading when the system turn-down is greater than 2:1 or 50% of maximum. Based on a more normal turndown ratio of 4:1, error could be as great as 100%.

If a ± 20% error can be assumed as probable, the impacts on the outside air setpoint will be dramatic. The example in the Technical Bulletin produced a range of setpoint error rates based on an assumed error at the VAV box, as follows:

These errors are beyond the point one can reasonably predict or make allowances for in system design and component selection. Therefore, for compliance, building ventilation design and operation MUST provide:

1. The ability to determine the outside airflow rate at all times;

2. The ability to determine the supply airflow rate at all times, so that the outside air fraction can be controlled;

3. The ability to accurately measure the airflow rate to each zone and report that information back to a controller, so that the outside air fraction can be calculated.

No other method used to control dilution ventilation rates is more effective, stable or reliable than direct airflow measurement.

The "standard of care" in ventilation design has always included compliance with ASHRAE Standard 62, but the importance of conformance with this codified standard has increased with: the national focus on Indoor Air Quality issues, the exploding numbers in personal injury litigation and national energy conservation efforts.

Direct measurement provides a positive and secure means of control, as well as a simple method to document measurements in real-time with trend-logs. Positive, continuous control and documentation are both required in ASHRAE 62-1999.

The documentation provisions of Section 6.3 can be satisfied by logging the data, as collected from each measurement point for control input. Logging the data automatically will significantly reduce the designer's long-term exposure to liability or immediately highlight any unforeseen deficiencies, allowing timely corrections. Any long-term degradation observed in the system, if it actually exists, could determine who should bare the burden of responsibility - building operations or the designer.

Steps for Ventilation Code Compliance

1. Obtain code design documentation from a design professional, listing outside air ventilation rates for each occupied zone. Keep documentation current. 
2. Have your design professional review your HVAC system and recommend any changes that may be required. 
3. Install permanently mounted, high accuracy airflow sensors at every outside air intake (and at each occupied space in multi-zone VAV systems). 
4. Continuously control and reset the outside air intake to meet the dilution air requirements of each occupied space. 
5. Maintain building pressure by maintaining a positive pressurization flow to avoid moisture buildup within the building envelope, thus reducing the potential for mold and fungal growth. 
6. Have all the monitored data regularly reviewed by a design professional.

Following these steps will allow your building's occupants to feel secure that all code requirements are being met continuously and provides the flexibility to meet most future changes.

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