Maintaining commercial HVAC economizers – Part 2

Introduction

Economizers bring in outside air for ventilation and for free cooling.  However outside air may be too warm or too humid to offer free cooling or very cold.  Additional outside air that must be conditioned increases the cooling and/or heating load on the building.  Economizers that are properly designed, maintained and set can save significant amounts of energy. However economizers that are not functional, leaking, have sensors out of calibration or miss-set can have large energy penalties.  Maintaining economizers is critical to their function and to their impact on energy use.

This document sets out some specific step-by-step economizer performance testing designed to be compatible with price sensitive commercial maintenance contracts.  See: Maintaining commercial HVAC economizers – Part 1 for basic information about commercial HVAC air-side economizers as well as an expanded maintenance checklist.

Minimal visual inspection tasks (common to all economizers)

1.      Visually inspect the integrity and conditions of economizer components and wiring.

a.       Inspect wiring and wiring connections including the main economizer connection is plugged in, all wires connected to sensors, actuators and controllers

b.      Inspect components for excess corrosion, wear, and mechanical problems such as linkages and economizer hardware.

c.       Check eyebrow / economizer hood and filter assembly for proper sealing, and inspect integrity of moisture screen (bird Screen)

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Testing the Honeywell analog non-DCV economizer during maintenance

Ensure to mark location of existing Shaft or damper position to ensure they can be returned if deemed in the original location if desired.

Single outdoor enthalpy changeover method

Minimal Functionality Checks

Only proceed with minimal functionality checks when no visual problems are found

Note: The potentiometers in economizer controllers are easily broken. If the potentiometer turns freely 360 degrees or beyond stops, it is broken.

Condition 1 - Units blower is running AND no call for 1st stage cooling AND the outside air damper dampers is at the Minimum position

1.      Disconnect main economizer wiring plug and ensure outside damper returns to its closed position

2.      If Economizer is hard wired to AC, remove the wire from terminal TR1 and insure that outside damper returns to its closed position

3.      If the damper is not open to a minimum position gently turn the minimum position potentiometer to open the OA damper slightly and wait for the damper to move. When the damper has moved gently turn the minimum position potentiometer back to the original state or to the proper ventilation position for the space.

Condition 2 - Units blower is running AND no call for 1st stage cooling AND the economizer controller IS indicating FREE COOLING,

1.      Place a jumper at appropriate T-Stat terminals in control section to simulate a call for first stage cooling and ensure that outside air damper moves in response to the call for free cooling.

Condition 3 - Units blower is running AND there is a call for first stage of cooling AND the economizer controller IS indicating FREE COOLING

1.      Remove the call from the control section for first stage cooling. The damper should close to minimum position within a few minutes.

a.       If the damper does not close check the mechanical connection and wiring or for obstructions.

Condition 4 - Units blower is running AND there is a call for first stage of cooling AND NOT second stage cooling AND the economizer controller IS NOT indicating FREE COOLING

1.      Turn the A, B, C, D change over potentiometer to the A position.

a.       The FREE COOLING indicator should turn on;

b.      The mechanical cooling should be off

c.       The OA damper should modulate toward open.

2.      If the FREE COOLING indicator does not turn on then the conditions are too high for economizing and the test cannot be performed.

                                                              i.      An alternative is that the sensor has failed. If a failed changeover is suspected (test fails with an ambient temperature under 70°F or when the technician thinks it is obvious based on the chart on the next page that the conditions are appropriate for economizing)

                                                            ii.      If the changeover doesn’t switch and the assumption is that condition are not favorable, please note the outside air temperature and relative humidity or wet bulb temperature measured at the time the test was taken.

Condition 5 - Units blower is running AND there is a call for first stage of cooling AND second stage cooling AND the economizer controller IS NOT indicating FREE COOLING

1.      Remove call for second stage cooling and then perform test in condition 4.

Condition 6- Units blower is running AND no call for 1st stage cooling AND the economizer is NOT indicating FREE COOLING

1.      Place a jumper at appropriate T-Stat terminals in control section to simulate a call for first stage cooling.

2.      Turn the A, B, C, D change over potentiometer to the A position.

3.      The FREE COOLING indicator should turn on.

a.       If the economizer controller IS indicating FREE COOLING with an A setting then perform test in condition 3.

b.      If the economizer controller IS NOT indicating FREE COOLING with an A setting the conditions are too high for economizing and the test cannot be performed.  

                                                        i.      An alternative is that the sensor has failed. If a failed changeover is suspected (test fails with an ambient temperature under 70°F or when the technician thinks it is obvious that the conditions are appropriate for economizing)

                                                      ii.      If the changeover doesn’t switch and the assumption is that condition are not favorable, please note the outside air temperature and relative humidity or wet bulb temperature measured at the time the test was taken.

Verifying the functionality of the changeover sensor

A simple test to determine the functionality of the outdoor air sensor when the FREE COOLING indicator does turn on as described in conditions 4 & 5 above. The working group suggests that testing the accuracy of sensors is part of the proposed more rigorous of the semi-annual inspections and not part of the more cursory inspection.

Quick-test for sensor functionality

·         Using the chart in Figure 1, determine where the current outdoor conditions are compared to the curves labeled A, B, C & D.

·         Slowly adjust the change-over setting and observe where in the A, B, C, D range the controller switched.

·         Compare where the controller switches to the expectation from the chart and judge if the controller changed over at the expected setting.  If not, suspect a bad sensor.

Analog Economizer single enthalpy curves

Figure 1 Modified psychrometric chart

The graph above is used with an analog economizer.  You may recognize the A, B, C, D lines used to configure the economizer.  These were used with analog economizers for many years.

This graph is what the controller uses to choose whether to economize or use mechanical cooling.  The outdoor temperature is on the bottom, and the humidity is shown on the curved lines. The set point for enthalpy is a point on a graph where the outdoor air temperature and humidity intersect.

Once you have set the economizer to an A, B, C, or D setting, you can look at this chart to determine if the economizer will use free cooling or mechanical cooling. Any time the outdoor air conditions are to the left of the curve, the conditions are good for economizing.   When the conditions are to the right, the economizer will use mechanical cooling.

For example: If you choose the B curve, and the outdoor temperature is 70 degrees,  and the relative humidity  is 40 percent, the intersection of those lines is at a point to the left of the B curve. This means the controller would be expected to economize.  

If you changed the setting on the controller to the C curve, that same 70 degree, 40 percent humidity point is now to the right of the C curve and conditions are not good to economize.   On a call for cooling from the thermostat the mechanical cooling would be expected to turn on.

So a technician will use these curves to choose when to controller will use free air and when to use mechanical cooling on a call for cooling.  Additionally observing what settings cause the controller to switch may be used to validate the accuracy of the changeover sensor and the functionality of the controller.

Conclusion

If the economizer and its components do not respond as indicated above to the minimum inspection and functionality checks, there appears to be an operational problem with the system and a Service Referral should be generated for follow-up and repair.

Modifications to the test procedure for various other economizer types

Single dry-bulb changeover method

Assumption – Snap disk sensors are not acceptable.  The working group strongly suggest using the later versions of the dry bulb sensor. The single dry bulb test method is the same as the single enthalpy method with the exception that another reference is used for expected values.

The working group suggests using the later versions of the dry bulb sensor.  The C7660 is only valid in Single Dry Bulb. The C7650 can be used for differential dry bulb and should not be used for single dry bulb.  In differential dry bulb operation the sensors compare mA output and chooses higher (lower temp) value.  The differential method does not work with the 10 degree F dead band

Differential enthalpy changeover method

When testing an economizer with a differential enthalpy changeover scheme, convert it to a single enthalpy changeover system.

·         Disconnect return air sensor

o   Jump SR+ SR with a 620Ω resister

Placing the jumper from SR+ to SR, makes the system a single outside enthalpy changeover system and eligible for testing as described above.

Differential enthalpy changeover – Applying the Quick-test for sensor accuracy to the return air sensor.

·         To test the return air sensor remove the outside enthalpy sensor from SO+ and SO and replace them with the wires from the return air sensor. See: Verifying the functionality of the changeover sensor and follow the procedure defined there.

·         It is critically important that the sensors are correctly re-installed when testing is complete. The test described has the risk of making a functional economizer non-functional by breaking terminals or wiring during the test or by not leaving the sensors installed correctly when the test is complete.

Testing the digital economizer during maintenance

The sense of the experts is that digital economizers need three levels of maintenance in a yearly cycle

1.      Semi-annual inspection #1 (rigorous), one of the economizer inspections each year (example: Fall):

§  Review, note and reset alarms (when issues are resolved)

§  Review the Status Menu (Honeywell) or Monitor Live Conditions (Belimo) – control signals

§  Review the status menu – sensor values tested against a trusted sensor

§  Run the check-out sequence (Honeywell) or Economizer Acceptance Test (Belimo) provided by the digital economizer controller

2.      Semi-annual inspection #2 (cursory), a different one of the inspection each year (example: Spring):

§  Review, note and (reset alarms when issues are resolved)

§  Review the status menu – control signals

§  Review the status menu – sensor values tested against a common sense standard

3.      An inspection coincidental with every other maintenance inspection during the annual cycle:

§  Review, note and reset alarms (when issues are resolved)

Note – Semi-annual inspection #1 and Semi-annual inspection #2 are separate tasks in the table. Add footnote that they are not done at the same inspection.

Review the status menu– control signals

1.      Make sure that there is a call for fan and 1^st stage cooling at the unit’s thermostat terminal block.

2.      Verify that the controller sees a call for one stage of cooling and not two stages of cooling.

Review the status menu– sensor values

1.      Make sure no expected values are blank or not giving an expected display

2.      Compare the outside temperature value to the actual outside air temperature

3.      Evaluate the other sensor values against a common sense standard (Semi-annual inspection #1) –or-

4.      Use a trusted tool to independently measure the values of each sensor and validate against the ASHRAE 90.1, IECC, Title 24 or other operative requirements.  (Semi-annual inspection #2)

a.       Temperatures must match within +/- 5°F

b.      If there are offsets available, calibration may be performed.

Run the check-out sequence provided by the digital economizer controller

1.      Perform the economizer checkout sequence

a.       Verify that the actuator and dampers move to the positions indicated by the controller

b.      Verify that the mechanical cooling and exhaust functions operate as indicated by the controller.