13.5. NA7.5.1.2 Constant Volume Systems Outdoor Air Acceptance

NA7.5.1.2 Constant Volume Systems Outdoor Air Acceptance

Use Document NRCA-MCH-02-A

Purpose of the Test

This test ensures the constant volume air handling unit provides adequate outdoor air ventilation to the spaces served under all operating conditions.

Systems requiring demand ventilation controls per §120.1(c)3 must conform to §120.1(c)4E regarding the minimum ventilation rate when the system is in occupied mode.

Related acceptance tests for these systems include the following:

NA7.5.2 Constant-Volume, Single-Zone, Unitary Air Conditioners and Heat Pump Systems

NA7.5.4 Air Economizer Controls (if applicable)

NA7.5.5 Demand-Controlled Ventilation Systems Acceptance (if applicable)

Instrumentation

Performance of this test will require measuring outdoor air flow. If the system was installed with an airflow monitoring station (AFMS) on the outdoor air, then it can be used for the measurements if it has a calibration certificate or is field-calibrated. The instrumentation needed to perform the task may include, but is not limited to:

A means to measure airflow (typically either a velocity pressure probe or hot wire anemometer).

A watch or some equivalent instrument to measure time in minutes

Test Conditions

To perform the test, override the control system of the air handling unit. The control system of the air handling unit must be complete.

All systems must be installed and ready for system operation, including:

Air economizer controls.

Duct work.

Control sensors (temperature, flow, thermostats, and so forth).

Electrical power to air handling unit and control system components.

Completion of air handling unit start-up procedures, per manufacturer’s recommendations.

Documentation of the initial conditions before overrides or manipulation of the set points and schedules. All systems must be returned to normal at the end of the test.

Note: Systems requiring demand ventilation controls per §120.1(c)3 must conform to §120.1(c)4E regarding the minimum ventilation rate (refer to NA7.5.5 Demand Controlled Ventilation Systems Acceptance Test).

Estimated Time to Complete

Construction inspection: 0.5 hours

Functional testing: 1 hour (depending on difficulty in measuring outdoor air flow)

Acceptance Criteria

System demonstrates a means of maintaining the minimum outdoor air damper position.

Minimum damper position is marked on the outdoor air damper

Measured outdoor air flow is within 10 percent of the total value found on the Energy Standards mechanical plan check document NRCC-MCH-03-E Column M.

Potential Issues and Cautions

Do not attempt to set the minimum damper position and perform the acceptance test at the same time. The acceptance test verifies the outdoor airflow of the system after calibration and system set-up is complete. Testing costs can be reduced by conducting the acceptance test immediately after set-up is concluded.

• Newly constructed and additions/alterations: Applies only to new variable air volume (VAV) systems

• Newly constructed and additions/alterations: Applies only to new constant air volume (CAV) systems

1. Reference the supporting documentation as needed. Reference NRCC-MCH-03-E or the mechanical equipment schedules to determine the total required outdoor airflow for the system.

2. Indicate method and equipment used to measure airflow during the functional test (for example, hot-wire anemometer) on the acceptance document. Note calibration date; calibration date must be within one year.

3. Check the system type (VAV or CAV) on the acceptance document (the following instructions apply only to CAV systems).

4. Check that the system is designed to provide a fixed minimum OSA when the unit is on and has a means of maintaining a minimum outdoor air damper position. Minimum position is marked on the outdoor air damper. The intent is that if the damper position is moved for any reason, it can be returned to the proper position to maintain design minimum outdoor air flow requirements.

• Packaged HVAC systems without an economizer will most likely have a fixed outdoor air damper that can be adjusted manually.

• Small packaged HVAC systems (< 20 tons) with an economizer will most likely have a controller/actuator that will control the outside and return air dampers (for example, a Honeywell W7459A economizer control package). The economizer control package is responsible for maintaining a minimum ventilation damper position as necessary and will most likely receive operation signals from either a thermostat or through a connection to a central DDC system.

• Large packaged HVAC systems (≥ 20 tons) will most likely have either a stand-alone economizer controller/actuator package (for example, a Honeywell W7459A) or a control package similar to a built-up system (that is, outside and return air dampers controlled by a DDC signal). The stand-alone economizer package may receive operation signals from a thermostat, an internal DDC controller, or a central DDC system. The “built-up” style economizer will most likely be controlled by an internal DDC controller or a central DDC system. Some large package systems may also have a dedicated outdoor air damper/actuator, independent of the economizer control strategy.

• Built-up HVAC system can control the outside and return dampers through a single actuator and damper linkages or through independent actuators and control signals. The control signals will most likely come from a central DDC system. Some built-up systems may also have a dedicated outdoor air damper/actuator, independent of the economizer control strategy.

5. Indicate the method being used to deliver outside air to the unit (for example, duct, return air plenum). For systems where return air plenum is used to distribute outside air to a zonal heating or cooling unit, confirm that outside air supply is connected either:

• Within 15 feet of the unit, with the air directed substantially toward the unit, and with a discharge velocity of at least 500 feet per minute.

6. Confirm that preoccupancy purge has been programmed into the system for the 1-hour period immediately before the building is normally occupied per §120.1(c)2. Confirmation is most easily accomplished by scheduling the unit to start one hour prior to actual occupancy. The purge amount must be the lesser of the minimum outdoor air rate or three complete building air changes (ACH).

• Follow the best practice guidelines below to increase accuracy of outdoor air flow measurements. Traverse measurements taken in supply, return, or outdoor air ducts should be located in an area of steady, laminar flow. Where possible, take measurements at least six duct diameters away from turbulence, air intakes, bends, or restrictions.

• When using face velocity measurements to calculate outdoor air flow, care should be taken to accurately measure free area dimensions of intake.

• When velocity measurements are taken at the plane of the intake between damper blades where flow is restricted (to achieve faster flows), free area should be measured as the actual open space between dampers and should not include frames or damper blades. See Figure 13-2 for illustration of free opening measurements.

• Hot wire anemometers are more appropriate than velocity pressure probes for measuring low-speed flows (that is, less than 250 feet per minute). When measuring flow with a hot wire anemometer, make sure to position the measurement device perpendicular to flow direction.

• Take multiple measurements and average results to minimize effects of fluctuations in system operation and environmental conditions (in other words, wind).

Your body can obstruct air flow and effect measurements. Position your body away from the intake and flow of air.

Step 2: Disable the air economizer when applicable and test at full supply airflow

When the system has an outdoor air economizer, force the economizer to the minimum position and stop outside air damper modulation.

For systems with an air economizer, disabling the economizer will prevent the outdoor air damper from modulating during the test due to atmospheric conditions rather than supply airflow variations. Disabling the economizer is necessary only if the system is in cooling mode and outdoor air temperature is below the economizer high limit setpoint. The economizer can be disabled in several ways, depending on the control strategy used to modulate the outdoor air dampers:

1. Use the high-limit switch by reducing the setpoint (return air value or outdoor air value if a comparative or changeover strategy, respectively, is used) below the current OSA dry-bulb or enthalpy measurement

2. Disable the economizer damper control loop through software if it is a DDC system.

• Document the measured outdoor air reading. Document the required outdoor airflow rate found on mechanical plan check document NRCC-MCH-03-E Column M, or mechanical equipment schedules. In the “Testing Calculation and Results” section of the acceptance document, confirm that measured outdoor air flow is within 10 percent of design outdoor air flow

Outdoor air flow can be measured directly, or indirectly, in a variety of ways. Acceptable methods for measuring outdoor air flow include, but are not limited to, the following techniques:

Read the outdoor air flow value measured by an air flow monitoring station if one is installed.

1. Traverse across the outdoor air duct to measure duct velocity, measure duct size, and calculate flow.

2. Measure face velocity at various points across outdoor air intake, measure intake damper size, and calculate flow.

3. Traverse across the supply and return ducts to calculate flow. (Outdoor airflow can be estimated as the difference between the supply and return airflow rates.)

Ensure all schedules, setpoints, operating conditions, and control parameters are placed back at the initial conditions. Release any overrides on the economizer or demand ventilation controls.