HVAC retro-commissioning saves dollars, headaches

HVAC retro-commissioning saves dollars, headaches Retro-commissioning of the HVAC system at Shenandoah, Iowa, High School led to unit ventilators getting updated controls rather than being replaced, solving a longstanding humidity problem and saving $500,000 that was invested in other needs — like carpeted halls. (CANCO photo)

1 year, 4 months ago

Building Value

Testing of heating, ventilation and air-conditioning systems is a crucial step for school renovation, as well as new construction projects.


By Cindy Larson, project manager and licensed architect
Carl A. Nelson & Company

An effective design for renovation of an existing building should start with not only understanding the client’s needs, but also understanding the building itself. This includes the mechanical system and what is working and not working. If funds unlimited, it is easy to replace everything with new. However, most clients would prefer to spend their funds on building elements that directly impact learning, rather than buying new HVAC equipment. That is where retro-commissioning comes in. Retro-commissioning is a thorough review of the existing equipment including the controls to determine what works and what doesn’t, and what can be retained, repaired or should be replaced. Retro-commissioning supports cost-effective solutions.

Mechanical systems are complex, and the failures can be complex. There is rarely a silver bullet with a single, simple fix. Equipment fails because of age, deferred maintenance, controls, and improper original installation. However, retro-commissioning can determine what the problems are and inform a focused, school-specific solution. This can save a significant amount of money.

In one example where we are working in a Construction Manager Agency (CMa) role, the Mount Pleasant Community School District wanted to add cooling to the high school gym. If the existing air-handling unit (AHU) was to be replaced, it would have had — due to the very large size — a lead time of close to one year and cost between $150,000 and $175,000. This would have put the project in jeopardy due to limited funding and a grant deadline of June 30, 2024. The existing AHU had a section already set up for additional cooling coils, and the original drawings confirmed the AHU was designed for future cooling of the gym. It was assumed the fan was sized properly to meet the added cooling load. When doing the retro-commissioning, however, it was discovered the static pressure for the fan dropped too much to handle the added friction of blowing through the cooling coils. A new fan would cost upwards of $80,000. When reviewing the pressure drop at various locations of the AHU, we found there was a larger-than-normal drop at the return air damper and/or the outside air dampers. This led to discovering the return air damper was too small. Instead of replacing the fan, the return air damper will be replaced for a significant savings to the district. Retaining the existing AHU improved the schedule and the budget for the Owner.

In other instances, we see that equipment has been added, replaced and upgraded throughout a building’s life but resulting in poor communication between the system components and with the components themselves. As an example of this from the Shenandoah Community School District, some of the high school HVAC equipment had controls based on programming from Windows 95, while other controls were based on more recent code. The school’s provider for servicing the controls could not even look at the programming of the unit ventilators to see how the equipment was working, and therefore, could not diagnose why there was a humidity issue. The retro-commissioning agent used a laptop with Windows 95 to read the code and determined the fresh air dampers were not working properly in the unit ventilators. After replacing the controls and reprogramming the unit ventilators, the unit ventilators worked as intended, which was one part of the solution to the larger humidity issue. The design based on the retro-commissioning meant much of the existing HVAC equipment was able to be reused. This saved the district $500,000, which allowed for the budget to be used elsewhere. The savings funded upgrades including remodeling of the locker rooms with private showers, new restrooms for the teachers, new doors throughout the school, all new LED lights, IT upgrades, and a refresh of paint and flooring in most of the school.

Frequently, issues found during retro-commissioning can be traced to the equipment’s installation. That is why commissioning on new projects is so important. A trio of recent Carl A. Nelson & Company projects with significant mechanical upgrades show the benefits. The Shenandoah High School renovation described above is one. Two others are at Keokuk Community School District’s Hawthorne Elementary School and at Lincoln Elementary School in the Washington Community School District. 

At Shenandoah High School, commissioning found a quick solution to a series of chilled-water flow alarms that triggered multiple shutdowns of the cooling system. The commissioning agent was able to point to a malfunctioning sensor in the chiller as the source of the problem.

Bid DateSchool Name$/SFNote
2019Shenandoah, IA; High School$20.19Retro-commissioning, reusing all AHUs and terminal units with new chiller and boilers
2021Keokuk, IA; Hawthorne Elementary$46.28Reusing two AHUs and duct work, with new terminal units, new chiller, new boilers and new AHU.
2021Washington, IA; Lincoln Elem.$76.52Completely new HVAC system, geothermal

Hawthorne Elementary, while not undergoing a formal retro-commissioning process as part of an HVAC upgrade to replace a system with parts dating to the 1957 construction and several more-recent additions, did include thoughtful reuse of some existing infrastructure and equipment. Commissioning at the school following completion noted several issues in need of correction, a small segment of which could have caused future headaches if not discovered early — including omission of a control sequence to run pumps if the glycol-free water in the pipes is not drained after the cooling season and prior to an anticipated freeze. Others included three instances of damper control sequence errors in air-handling units that would have reduced operating efficiency if not addressed. Lincoln Elementary School, built in 1960, had a whole new HVAC system installed, as well as installation of a geothermal heating and cooling system. Commissioning of the system at Lincoln is nearing completion, and thus far has not uncovered any noteworthy issues.

As demonstrated above, making sure the equipment is working per the design intent improves energy efficiency and prevents problems that can lead to expensive maintenance issues and unnecessary equipment replacement in the future. However, if you are just starting a new project in an existing building, retro-commissioning is the first step to create focused, cost-effective solutions to HVAC issues. To learn more about retro-commissioning as a beneficial part of making upgrades to your schools’ HVAC systems, contact Carl A. Nelson & Company at (319) 754-8415, or email canco@carlanelsonco.com.


In Building Value, Carl A. Nelson & Company’s team of construction experts will address topics aimed at helping educational facility owners to get the most out of their projects before, during and after construction. Have questions? Submit it by email to canco@carlanelsonco.com for a prompt reply and possible use as a topic in an upcoming installment.