ASHRAE 62.1
Systems and Equipment

Section 5 specifies the minimum systems and equipment required under Standard 62.1. §5.3 currently states:

"Mechanical ventilation systems shall include controls, manual or automatic, that enable the fan system to operate whenever the spaces served are occupied. The system shall be designed to maintain no less than the minimum outdoor airflow as required by Section 6 under any load condition. Note: Variable Air Volume (VAV) systems with fixed outdoor air damper positions must comply with this requirement at minimum system primary airflow."

A recently proposed addendum suggests changes to the language as follows: "Mechanical ventilation systems shall include controls in accordance with the following:

5.3.1 All systems shall be provided with manual or automatic controls to maintain no less than the outdoor air intake flow (Vot) required by Section 6, under all load conditions or dynamic reset conditions.

5.3.2 Systems with fans supplying variable primary-air (Vps) or variable mixed-air flow, including single-zone VAV and multiple-zone-recirculating VAV systems shall be provided with one or more of the following: a. Outdoor air-intake, return air dampers or

a. combination thereof that modulate to maintain no less than the outdoor air intake flow (Vot)

b. Outdoor-air injection fans that modulate to maintain no less than the outdoor air intake flow (Vot)

c. Other means of ensuring compliance with Section 5.3.1

Because the requirements in the Standard for compliance are set forth "under any load condition," we are being asked to maintain a constant rate of outdoor airflow in dynamic systems. Logically, there should also be a requirement for continuous airflow measurement at the intake of all air-handling units with automatic controls providing a space with a constant rate of outdoor air, regardless of the system size, type or point of operation. The Standard and its companion User's Manual encourages us to use direct measurement feedback for continuous control on all VAV designs, even those using a powered outdoor air system (i.e. injection fan, HRV/ERV, smaller DOAS, etc.). Although not contained in the Society's `minimum' standard, ASHRAE is highlighting the potential source of problems and a few of the more obvious means to avoid them.

Installing a -P airflow measuring device upstream of your intake damper is essential to maintain the minimum ventilation rates specified by ASHRAE Standard 62.

Caution: Maintain minimum flow rates above 200 fpm to avoid "false", wind induced airflow readings when the airflow station is located close to the intake louver.