Maintaining commercial HVAC economizers – Part 1

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 basic information about commercial HVAC air-side economizers as well as an expanded maintenance checklist.  See: Maintaining commercial HVAC economizers – Part 2 for specific step-by-step economizer performance testing designed to be compatible with price sensitive commercial maintenance contracts. Please Press "Read More"

Background/Notes

1.      There are two basic types of economizers

a.       Analog

b.      Digital

2.      There are two basic types of change-over sensors

a.       Dry-bulb

                                                              i.      Analog

1.      C7650 – 10°F operational dead band

2.      C7660 – 2°F dead band  +/- 1°F hysteresis

                                                            ii.      Digital

1.       Dry bulb accurate to +/- 2°F over the range of 40°F to 80°F[1]

b.      Enthalpy

                                                              i.      Analog

1.      Were accurate at 40%-70% RH +/- 5%,  +/-10% outside that range

2.      Hand calibrated at the factory

3.      Would drift and could not be field calibrated

                                                            ii.      Digital

1.      +/- 5% at 20%-80% RH[2]

2.      Relative humidity accurate to +/- 5% over the range of 20% to 80%

3.      Enthalpy accurate to +/- 3Btu/lb. over the range of 20Btu/lb. to 36Btu/lb.

3.       Repair vs. replace

The sense of those expert in economizers is that the changeover sensors available for the older analog economizers cannot meet the current ASHRAE 90, IECC or Title 24 requirements because of their design.  They suggest that all analog economizers may be considered as unacceptable because of their outdated design and their well-established high failure rates.

a.       Replace analog sensor with a newer more accurate sensor

                                                              i.      Honeywell sensors

1.      Enthalpy old sensor - C7400A1000 – new  C7400A2000

2.      Dry bulb – old sensor C7650 (10°F dead band)  – new - C7660 (2°F dead band)

                                                            ii.      Part cost >$75 *2 (mark-up) +labor = ~$300 upgrade.

                                                          iii.      What does the customer get from this?

1.      Replace when there are humidity complaints. (Humid areas)

2.      Energy benefits expected

b.      Upgrade to a digital technology

                                                               i.      Experts have determined that the snap disk type changeover is never acceptable.

                                                             ii.      Upgrade analog economizer with a digital sensor – If replacing the change-over sensor always replace with the digital version even if maintaining the analog economizer

                                                          iii.      Upgrading the economizer control, actuator, and sensors; consider upgrading to digital economizer control, even when the analog economizer appears to work if the unit is not old (less than 7 years old)

                                                          iv.      Upgrade the whole economizer to digital control when the analog economizer appears to not work. Supply houses are phasing out analog parts inventory, the service provider may only have a digital option.

4.      There are so many potential problems with analog economizers (i.e. sensor issues, actuator reliability, damper leakage issues, linkage issues) is there a set of circumstances where de-commissioning the failed economizer and going to a manual outside air intake makes sense?

a.       Yes if the economizer has failed, the units is old and the decision has been made to not repair it and there is a plan to replace it in the next few years.

b.      No if the economizer is working, there is a comfort problem and the assumption is that the outside air intake is contributing to the comfort problem.

c.       No if the economizer is non-functional and the unit is not old or does not have a replacement plan.  Economizer repair or replacement is suggested.

5.      Experts recognize that 2-stage temperature control of all units with economizers is a requirement for proper operation.

6.      Real time economizer monitoring tools – The experts consider the use of real-time economizer monitoring tools helpful if not essential for validating analog economizer function in a service or commissioning role but they are outside the scope of commercial maintenance.

7.      It is assumed that the technician will remove any jumpers and return the unit back to normal operation when testing and maintenance is complete.   

Challenges

1.      Maintenance is a fixed price activity.  Some economizers have access problems that make maintenance within the budget challenging.

2.      The customers often don’t understand the economizer or its value proposition. There currently is no data collected or metric available, derived from maintenance inspection tasking that documents the performance of the economizer.

The components of an economizer

1.      Economizer logic control

2.      Wiring and associated connectors

3.      Mechanical connection from damper actuator to dampers, which include gears, linkages, connecting rods, ball joints, and metal adapters and associated fasteners

4.      Dampers

a.       Parallel blade

b.      Opposed blade

5.      Relief damper

6.      Exhaust fan or Power exhaust accessory

7.      Weather hood or eyebrow with filter/screen to prevent precipitation, debris, and animals from entering the unit

8.      Outside air sensor

a.        Temperature or enthalpy

9.      Return air sensor, if set up for differential changeover control

a.        Temperature or enthalpy

10.  Mixed and/or supply (discharge) air sensor

11.  CO₂ Sensor, if set up for Demand Control Ventilation

12.  Remote potentiometer - optional

13.  Occupancy Sensor - optional

14.  Low ambient mechanical cooling lockout

Economizer logic 

(digital and analog economizers)

 

Suggested economizer maintenance tasking

a - Check air filter and housing integrity. Correct as needed.

Quarterly

Understanding of the meaning of the task:

1. Filter fit refers minimizing air by-passing the filters because of gaps.
2. Housing integrity refers to “economizer hood” or “eye brow” and its ability to seal out liquid moisture.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1. Any gap in the filters should be minimized.  If the gap is noticeable it should be filled.
2. If filter is damaged such that its performance is effected, it should be replaced.

Guidance to the technician when performing the task:

1.      One common way to fill an excessive gap is with a metal filler piece that may have to be fabricated.

Impact of unacceptable condition or performance:

1.      Filter bypass contributes to fouling return air filters and evaporator coils.  See this working groups document about Table 5-22 for more information about the impact of dirty return filters and a dirty evaporator.

2.      It a may also allow animals or moisture to enter into system

3.      Moisture can cause corrosion on surfaces not manufactured to protect against moisture, such as damper assembly, and may damage electronic such as economizer control or sensors.

b - Check for particulate accumulation on filters. Clean or replace as necessary to ensure proper operation.

Quarterly

Understanding of the meaning of the task:

1.      The work group understands this task relates to the outside air air-filters in the economizer housing.

2.      The task is to inspect the filters for:

a.       Fit and seal in the frame

b.      Particulate accumulation that would restrict airflow

c.       Filter damage or disintegration that would allow liquid moisture or vermin to pass

Guidance to technician when judging acceptable or unacceptable condition or performance:

1. When working on units with metal mesh air filters, assessment means removing the filter from the unit and looking at the amount of light that passes through it. If it looks dirty, it is dirty.

Guidance to the technician when performing the task:

1. The work group recognized that it is a responsibility of the technician to clean dirty economizer air filters. 
2. Economizer filters are usually cleaned similarly to an evaporator coil, with a detergent cleaner and water.

Impact of unacceptable condition or performance:

1. Dirty economizer filters restrict airflow and reduce the capacity of the economizer to provide free cooling
2. Dirty economizer filters restrict airflow and reduce the capacity of the economizer to provide ventilation air as needed
3. Damaged air filters may allow liquid moisture or vermin to enter the unit.

Best practices and minimum Standards:

1.      The sense of the Working Group is that economizer filters should be cleaned at least annually as a minimum standard and that they should be cleaned every time the condenser is cleaned.

2.      Best to clean counter to flow, providing water pressure from damper side

c1 - Communicating economizers only - Check alarms, review and document alarms, reset resolved alarms

Quarterly

Understanding of the meaning of the task:

1.      This task is to review the alarms shown in the digital economizer controller. All alarms should be documented. Alarms that relate to conditions or performance that have been resolved should be cleared.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      There is no condition or performance issues related to the alarm feature

Guidance to the technician when performing the task:

1.      This task is part of every inspection when a digital economizer is installed. All alarms should be documented. Persistent or reoccurring alarms should be referred to service for resolution.

Best practices and minimum Standards:

1.      Documenting that an alarm has been resolved and what was done to resolve the alarm is a best practice

2.      Reviewing previously documented alarms against current alarms to uncover persistent or reoccurring alarms is a best practice.

c2 - Check condition, setting and operation of outdoor sensor, return air sensor, or change-over controller against a trusted sensor. Repair, adjust, or replace components to ensure proper operation.

Annual 1

c3 digital economizers only - Check condition, setting and operation of outdoor sensor, return air sensor, or change-over controller against a reasonable assumption standard. Repair, adjust, or replace components to ensure proper operation.

Annual 2

Understanding of the meaning of the task:

1.      The changeover operation is one of the critical operations of the economizer.    This task is to test and verify that economizer changeover sensors are reasonably accurate and set to the desired set point, conditions permitting. There are two annual tasks NOT intended to be performed at the same maintenance inspection.

2.      This task applies to both digital and analog economizers.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      Analog Economizers – A simple test to determine the accuracy of the outdoor air sensor when the FREE COOLING indicator does turn on as described in conditions 4 & 5 in the analog economizer test procedure in this document.

2.      Digital Economizers – Digital economizers have status displays that show the current measured value for various sensors including outside temperature and perhaps humidity, return air temperature and humidity, mixed air temperature, supply air temperature and zone CO₂. 

Guidance to the technician when performing the task:

1.      Annual 1 – Validate the accuracy of the sensor against a trusted instrument. See: Semi-annual inspection #1 (rigorous), one of the economizer inspections each year (example: Fall)

2.      Annual 2 – Validate the accuracy of the sensor against a reasonable assumption standard. See: Semi-annual inspection #2 (cursory), a different one of the inspection each year (example: Spring)

Impact of unacceptable condition or performance:

1.      Failure or inaccuracy of the changeover operation greatly reduces the effectiveness on the economizer by missing free cooling opportunities or by allowing excessive heat and humidity into the building that then must be removed.

Timing for this task:

This task is appropriate for the following maintenance visits

1.      Economizer inspection at heat and cool start-up.

d - Check condition, setting and operation of the economizer controller and actuators.  Repair, adjust, or replace components to ensure proper operation.

Semi-Annually

Understanding of the meaning of the task:

1.      This task is to test and verify that economizer control module and actuator is functional and controller settings are appropriate for the application.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      A non-functional economizer control module is an unacceptable condition and performance.

2.      Inoperable actuators are an unacceptable condition and performance

Guidance to the technician when performing the task:

1.      Analog economizers - See the section about analog economizer inspection and test procedures in this document.

2.      Digital economizers - See the section about digital economizer inspection and test procedures in this document.

3.      Determining if the failure is a controller or actuator problem is a service task

Impact of unacceptable condition or performance:

1.      Economizer controller or actuator failure disables the economizer. This may cause excessive or insufficient indoor are for ventilation and may have a large negative impact on system energy efficiency depending of the position of the damper when failed and the indoor and outdoor climate

2.      Improper controller settings greatly reduces the effectiveness on the economizer by missing free cooling opportunities or by allowing excessive heat and humidity into the building that then must be removed.

Best practices and minimum Standards:

1.      When checking the operation of the controller/actuator/linkage, changing the minimum position setting is sometimes required.  If there is an assumption that the minimum position setting is set correctly on arrival, marking the damper shaft or marking the a measuring point on the dampers and recording the damper opening when at the original minimum position so that the minimum position setting can be reset to the original setting is a best practice

e1 analog economizers only - Check condition, setting and operation of the mixed air/discharge sensor.  Repair, adjust or replace components to ensure proper operation.

Annual

e2a digital economizers only - Check condition, setting and operation of the mixed air/discharge sensor against a trusted sensor.  Repair, adjust or replace components to ensure proper operation.

Annual 1

e2b digital economizers only - Check condition, setting and operation of the mixed air/discharge sensor against a reasonable assumption standard.  Repair, adjust or replace components to ensure proper operation.

Annual 2

Understanding of the meaning of the task:

2.      The mixed air/discharge sensor operation is used to modulate the damper position to try to maintain the desired supply air temperature.  This task is to test and verify that economizer mixed air/discharge sensor are reasonably accurate and are controlling to the designed set point, conditions permitting.

Guidance to the technician when performing the task:

1.      Analog Economizers – 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.

2.      Digital Economizers – Digital economizers have status displays that show the current measured value for various sensors including outside temperature and perhaps humidity, return air temperature and humidity, mixed air temperature, supply air temperature and zone CO₂. 

Impact of unacceptable condition or performance:

1.      Failure or inaccuracy of the changeover operation greatly reduces the effectiveness on the economizer by missing free cooling opportunities or by allowing excessive heat and humidity into the building that then must be removed.

Timing for this task:

This task is appropriate for the following maintenance visits

1.      Economizer inspection at heat and cool start-up.

f - Check dampers for proper operation, condition, and setting.  Repair, adjust, lubricate, or replace components to ensure proper operation

Semi-Annually

Understanding of the meaning of the task:

1.      Damper operation is a critical function of the economizer.  This task is to test and verify that move in response to the actuator to lubricate the dampers and linkages to maintain operability as well as to adjust the damper position to produce the air flows intended by the controller.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      The Working Group suggests that the technician should visually inspect fixed dampers and determine that they are positioned correctly and if not adjust it or document the issue as per the requirements of the maintenance contract.

Guidance to the technician when performing the task:

1.      The Working Group recognizes that dampers may require lubrication and have determined the lubricating dampers and linkages are a required maintenance function.

2.      Verify dampers close tightly, verify and correct if possible any damper seals that are damaged

3.      Technician can use potentiometer on analog units to verify damper can open properly and remove G/N to see if the close properly

4.      Digital Economizers can use test to verify damper opening and remove power to G to verify damper closes.

Impact of unacceptable condition or performance:

1.      Improperly positioned fixed outside air dampers may bring in excessive or insufficient outside air missing free cooling opportunities or by allowing excessive heat and humidity into the building that then must be removed.

g - DCV only - Check alarms, review and document alarms, reset resolved alarms

Semi-Annually

Understanding of the meaning of the task:

1.      All digital economizers and some analog economizers have the ability to operate in DCV mode.  They need CO₂ sensors and digital economizers sometimes need to be set-up in software.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      When reviewing the status menu in digital economizers, indoor CO₂ should ventilate above 1000-1200ppm of CO₂.  If the CO₂ reading is less than 300ppm or over 2500ppm the accuracy of the sensor is questionable.

Guidance to the technician when performing the task:

1.      If the CO₂ reading is below set point PPM then verify that the damper position is at or near the CO₂ min damper set point.  If CO₂ value is above set point PPM, then verify that damper position is at or near ventilation set point.

2.      If the CO₂ sensor has a display, verify that the sensor reading matches the CO₂ reading on the controller

3.      Refer any problems observed for service

Impact of unacceptable condition or performance:

1.      High CO₂ is an indoor air quality problem. It indicates that there is insufficient fresh air ventilation for building occupants health and safety.

2.      Low CO₂ is an energy efficiency problem.  It may indicate that the building is over-ventilated. Outside air must be conditioned. Conditioning hot and humid or cold outside air uses energy and adds additional load to the systems that may compromise thermal comfort in extreme conditions.

Recommended instrumentation requirements:

1.      It is not expected that a maintenance technician will have a CO₂ measuring tool.

h - Check sealing integrity of all panels on equipment. Replace fasteners and gasketing as needed.

Semi-Annually

Understanding of the meaning of the task:

1.      This task refers to inspecting panel fasteners.

2.      This task refers to inspecting unit curbs and other supporting structures.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      Panel fasteners – Panels should be secured with fasteners. 

a.       Missing fasteners should be replaced.

b.      Stripped fasteners should be replaced with oversized versions or installed in an alternate locations.                 

2.      Panels – All panels should be in place when leaving the job. Damaged or missing panels should be documented and recommended for replacement.

Guidance to the technician when performing the task:

1.      Be careful not to strip out the fasteners when reinstalling them.

2.      Be careful not to drop panels on their corners, it may puncture the roof and cause a leak.

3.      Document any holes or cracks in the curb, flashing or surrounding roof and recommend repair.

4.      It is the technician’s responsibility to secure all panels properly before leaving the unit.

Impact of unacceptable condition or performance:

1.      Unplanned air leakage into or out of the unit is an energy problem and the source of a potential comfort problem.

2.      Missing panels may cause a system reliability problem.

3.      Unsecured panels that blow off may be a liability problem and a potential safety hazard.

Definitions:

1.      This task refers to inspecting all unit panels including those that are structural, those that impact air leakage either in or out of the unit, and those that are purely decorative.

2.      Fasteners are the means used to secure the panels to the unit.  They include screws, hinges, latches or other devices.

i - Visually inspect areas of moisture accumulation for biological growth. If present, clean or disinfect as needed.

Semi-Annually

Understanding of the meaning of the task:

1.      This Working Group determined that this task calls for a visual inspection and documentation of unusual material without a determination of the identity of the material.

2.      This Working Group determined that cleaning provides no assurance of any specific outcome.

3.      The HVAC technician is very likely the only person that is going to look into the HVAC unit on a regular basis and therefore must be relied on to note unusual material.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      The Working Group recognizes that HVAC technicians are generally not trained to identify biological growth.

2.      The technician is encouraged to document, with a photograph if possible any unusual substance found in the unit.

Guidance to the technician when performing the task:

1.      The Working Group determined that the technician has the responsibility to document the existence of anything unusual in the unit.

Impact of unacceptable condition or performance:

1.      Unusual substances may be a concern to the building occupants.

Definitions:

1.      Unusual means that which the technician determines is not ordinary.

Best practices and minimum Standards:

1.      It is a best practice to document unusual substances with a photograph.

j - Check condition, setting and operation of the low limit stat.  Repair, adjust, or replace components to ensure proper operation.

Annually

Understanding of the meaning of the task:

1.      This task is to verify the operation of the outside air thermostat that disables mechanical cooling operation below the set-point regardless of any temperature control signals.

Guidance to technician when judging acceptable or unacceptable condition or performance:

1.      Mechanical cooling operation during low ambient conditions when a low ambient lock out stat is installed is an unacceptable condition

Guidance to the technician when performing the task:

1.      Checking the low limit or low ambient lockout stat requires low ambient conditions.  This task may be included in the winter mid-season inspection when low ambient conditions exist.  The test would be to produce a 2-stage call for cooling and verifying that mechanical cooling does not operate.

Impact of unacceptable condition or performance:

1.      Unadapted mechanical cooling operation during low ambient conditions is a hazard to the compressor

2.      Running mechanical cooling during low ambient conditions when an economizer is installed may unnecessarily use energy

k - Check condition, setting and operation of the relief dampers or power exhaust fan.  Repair, adjust, or replace components to ensure proper operation.

Annually

Understanding of the meaning of the task:

This task is to verify the operation of the relief dampers or power exhaust accessory

Guidance to technician when judging acceptable or unacceptable condition or performance:

Barometric dampers, if existing must operate freely

Power exhaust accessories must operate according to the economizer sequence of operation

Guidance to the technician when performing the task:

Power exhaust accessory fans must operate at some point prior to the economizer reaching 100% outside air.  When it begins to operate is application specific.

Impact of unacceptable condition or performance:

Inoperable relief dampers or power exhaust accessories may cause the building to be over-pressurized during economizer operation.

Inoperable relief dampers or power exhaust accessories may cause the zone to insufficiently ventilated for indoor air quality or free cooling.

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[1] Digital dry bulb and  Enthalpy accuracy from T24 and 90.1 2013 requirements

[2] Digital dry bulb and  Enthalpy accuracy from T24 and 90.1 2013 requirements