NA7.5.4 Air Economizer Controls Acceptance

Use Form NRCA-MCH-05-A

Purpose of the Test

The purpose of functionally testing an air economizer cycle is to verify that an HVAC system uses outdoor air to satisfy space cooling loads when outdoor air conditions are acceptable. There are two types of economizer controls; Stand-alone packages and DDC controls. The stand-alone packages are commonly associated with small unitary rooftop HVAC equipment and DDC controls are typically associated with built-up or large packaged air handling systems. Test procedures for both economizer control types are provided.

Cooling fan systems > 54,000 Btu/hr must have an economizer. Air economizers must be able to provide 100% of the design supply air with outside air; water economizers must be able to provide 100% of the design cooling load at 50°F dry-bulb and 45°F wet-bulb.

For units with economizers that are factory installed and certified operational by the manufacturer to California Energy Commission economizer quality control requirements, the in-field economizer functional tests do not have to be conducted. A copy of the manufacturer's certificate must be attached to the NRCA-MCH-05-A.However, the Construction Inspection, including compliance with high temperature lockout temperature setpoint, must be completed regardless of whether the economizer is field or factory installed.

Instrumentation

Instrumentation to perform the test includes:

•      Hand-held temperature probe (must be calibrated within the past year)

•      Device capable of calculating enthalpy (must be calibrated within the past year)

•      1.2 k Ohm resistor (when specified by the manufacturer)

Test Conditions

Equipment installation is complete (including HVAC unit, duct work, sensors, control system, thermostats).

Non-DDC DX systems are required to have a two-stage thermostat.

HVAC system must be ready for system operation, including completion of all start-up procedures per manufacturer’s recommendations.

For those units having DDC controls, it may be necessary to use the building automation system (BAS) to override or temporarily modify the variable(s) to achieve the desired control. BAS programming for the economizer, cooling valve control, and related safeties must be complete.

For built-up systems all interlocks and safeties must be operable--for example, freeze protection, limit switches, static pressure cut-out, etc.

Document the initial conditions before overrides or manipulation of the settings. All systems must be returned to normal at the end of the test.

Prior to conducting the test, demand control ventilation systems must be disabled, if applicable.

Estimated Time to Complete

Construction Inspection: 0.5 to 1 hours (depending on familiarity with the controls)

Functional testing: 0.5 to 2 hours (depending on familiarity with the controls and issues that arise during testing)

Acceptance Criteria

If the economizer is factory installed and certified, a valid factory certificate is required for acceptance. No additional equipment tests are necessary.

Air Economizer lockout setpoint complies with Standards Table 140.4-B per §140.4(e)3.  This table is located below in this document in Section 13.63 Test Procedure.

Outside sensor location accurately reads true outdoor air temperature and is not affected by exhaust air or other heat sources.

All sensors are located appropriately to achieve the desired control.

During economizer mode, the outdoor air damper modulates open to a maximum position and return air damper modulates 100 percent closed.

The outdoor air damper is 100 percent open before mechanical cooling is enabled and remains at 100 percent open while mechanical cooling is enabled (economizer integration when used for compliance with Section 140.4(e)2B). The economizer is capable of providing partial cooling even when additional mechanical cooling is required to meet the load. For unit controls, the outdoor air damper may not begin to close until the leaving air temperature is below 45°F.).

When the economizer is disabled, the outdoor air damper closes to a minimum position, the return damper modulates 100 percent open, and mechanical cooling remains enabled.

If the unit has heating capability, the outdoor air damper remains at minimum position when heating is enabled. When the unit is turned off or otherwise disabled, the outdoor air damper closes.

Potential Issues and Cautions

If conditions are below freezing when test is performed, coil(s) may freeze when operating at 100 percent outdoor air.

Outdoor air and relief dampers should be closed when the system is in unoccupied and warm-up modes, preventing problems with unconditioned air entering the building during unoccupied hours.

If the damper interlocks fail and the outdoor air damper does not open before the return damper closes, damage to the air handling unit or associated duct work may occur.

Air Economizers with poor mixing can have excessively stratified air streams that can cause comfort problems or freeze stat trips. Mixing problems are more likely to occur as the VAV system reduces flow, leading to reduced velocities in the mixing box and through the dampers.

Check for exterior doors standing open and other signs of building over-pressurization when all units are on full economizer cooling (100 percent OSA).