HVAC retro-commissioning saves dollars, headaches

2 years ago

    

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.