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September 2, 2021

Economizer Missteps: Keep your Energy Consumption Low and Cost Savings High

This post was originally featured in Issue 29 of our Energy Optimization Newsletter, exploring valuable energy conservation measures, incentive programs and regional legislative updates, recent case studies, news roundups, and more. Click here to view and download a copy of the full newsletter.


What are economizers?

One of the most common energy conservation measures (ECMs) uncovered by Edison Energy’s Energy Optimization Services team is a malfunctioning or poorly controlled air-side economizer. Economizers allow for free cooling by opening dampers to bring in more outside air when the outdoor conditions are favorable, reducing the need for mechanical cooling and saving energy. When an economizer is not functioning or controlled properly, it can lead to increased energy usage, reduced indoor air quality, and impact equipment life.

Economizers are commonly used in HVAC systems for commercial, institutional, and industrial buildings, and at times in residential settings. In fact, energy codes in most of the United States require that economizers be installed on any forced air cooling units with a cooling capacity greater than 4.5 tons, with the exception of very hot and humid areas (ASHRAE climate zones 1A and 1B).

What causes economizer failure?

Figure 1: Economizer damper failure resulting in a simultaneously closed outside and return air damper, severely restricting airflow through the unit.

An economizer can fail in many different ways; some of the most common are discussed below.

  • Mechanical failures, such as rusted or corroded dampers, loose or broken damper linkages, and failed or mis-calibrated actuators prevent the economizer from responding to control system commands.
  • Sub-optimal controls can reduce economizing effectiveness, prevent economizing at all, and even increase energy consumption. Common control mis-steps include:
    • Lockout setpoints which have been set too low (ASHRAE recommends high-limit outside air temperature lockout setpoints of 65°F to 75°F, depending on climate zone)
    • Economizers modulating in parallel with mechanical cooling, rather than maximizing free cooling first, before mechanical cooling is staged on
    • Economizers permitted to operate simultaneously with heating
    • Economizing controls which are not integrated into the building automation system, or never configured in the first place (a common issue for packaged rooftop units)
  • Sensors which are used to control the economizer can fail or drift over time, causing the control system to make poor economizing or ventilation decisions.

A poorly setup or failed economizer is guaranteed to waste energy, but can also lead to more serious issues including coil freezing, duct collapse, decreased compressor life, and loss of ability to provide proper ventilation.

 

Figure 2: Return fan dead-heading against closed return and exhaust dampers due to poor economizing/ventilation controls. This condition ultimately led the return duct to blow out due to high pressure.

How can these issues be addressed?

Proper maintenance, routine review of controls, and regular testing of economizers can help address issues before they become serious. A more thorough systematic review of the HVAC system by an energy professional is recommended every 5 to 7 years, through retro-commissioning.

For new systems or buildings, initial commissioning of the economizer is also highly recommended to ensure that the controls and actuators are wired correctly, the economizer settings are configured optimally, the dampers move through their full range of motion, and the BMS integration is done properly.

What can I do to avoid economizer failure?

Look out for the following warning signs that may indicate you have an economizer issue:

  • A sudden increase in mechanical cooling or heating can signal that your economizer dampers are stuck closed or open. Look at your mixed air temperature, if available – does it make sense relative to the outside and return air temperatures and the damper command?
  • Review your outside air damper positions on a cool day – are your units economizing? If not, are they in heating mode (ok), or cooling mode (red flag)? Dampers that are always at their minimum setting can signal that the lockout setpoints are set too low, or outside air sensors are unreliable.
  • Is your supply fan suddenly unable to meet it’s duct static pressure setpoint, or has your HVAC unit suddenly become very noisy? Damper/actuator failures can cause severe airflow restrictions that can collapse ductwork on large systems if not promptly addressed.
  • Economizer dampers closing when mechanical cooling is enabled or dampers open with heating enabled can signal issues with optimal economizer damper sequencing.

In addition to regular maintenance and periodic retro-commissioning, the best way to prevent economizer failures and missteps from sneaking up on you is via a monitoring-based continuous commissioning system, or other fault detection and diagnostic tool that will automatically alert you when issues arise.

Why does this matter?

When an economizer is not functioning or controlled properly, it can lead to increased energy usage, reduced indoor air quality, and impact equipment life. Keeping your economizers tuned up saves your bottom line, as well as improving your occupant experience.


Want to learn more? Contact Edison Energy today.

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