Retro-commissioning
- Resolute Team Member
- Aug 16, 2022
- 5 min read
Updated: Jun 2
Summary: This article explains how retro-commissioning and HVAC trend data can help identify hidden building performance issues, including boiler hunting, oversized bypass valves, PID tuning problems, and VAV reheat valve cycling. Resolute Building Intelligence uses existing building automation system data to help facility teams, design engineers, and commissioning agents understand what is happening, identify root causes, and prioritize corrective action.
Key Takeaways
Retro-commissioning helps facility teams verify that HVAC systems are operating as intended after construction, renovation, or system changes.
Building analytics can identify hidden HVAC performance issues that may not be obvious during day-to-day operation.
Common issues include boiler hunting, oversized bypass valves, PID tuning problems, VAV reheat valve cycling, inefficient heating hot water operation, and avoidable equipment wear.
Resolute Building Intelligence uses existing building data to help facility teams, engineers, and commissioning agents identify issues, understand root causes, and prioritize corrective action.
There is a famous quote from Phaedrus, an Athenian Philosopher,
"Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden."
Many people believe a new Heating, Ventilation, and Air Conditioning (HVAC) System should operate perfectly when new, but do they? The answer depends on the details you find. Many parts and pieces must work together to create a functional HVAC System that can keep a building comfortable.
Resolute’s analytic and fault detection software can pinpoint when these components are not performing in an optimal way and provide insights to make the necessary adjustments.
Here are examples of findings our software uncovered in a newly constructed and commissioned building:
Oversized Bypass Valve
A Heating hot water boiler plant is designed to supply hot water at a specific temperature to all heating coils and terminal units in the system. In one situation, our trend information indicated that these high-efficiency firing rate boilers were not correctly modulating. Instead, the boilers cycled on and off to maintain the heating hot water supply temperature setpoint (Figure 1). With its integral boiler control system, this behavior was uncharacteristic.
The boiler manufacturer was contacted and informed of the trend data. A technician was sent to look at the boiler operation and determined that there was a dysfunctional boiler that was disabled until it could be replaced. Afterward, the firing data was reviewed, and it showed a definite improvement.
However, there were times when the boiler firing rate would ramp up and down repeatedly, trying to reach the correct setpoint. This is called “hunting”. Upon further investigation of the boiler plant trend data, we noticed a heating hot water bypass valve opening and closing only slightly to maintain the minimum water flow through the boilers.
When the bypass valve began to open to keep the minimum water flow through the boiler there was a dramatic increase in the hot water return temperature that entered the boilers. The reverse would occur then the bypass valve began to close. The boiler firing rate cycled on and off since the bypass valve was hunting, thus creating a chain reaction. The ever so slightly hunting of the bypass valve and the dramatic temperature swings led us to question if the bypass valve was oversized.
We were able to confirm the bypass valve was 4 inches, the same size as the heating hot water mains, which is not ideal. It was determined that the 4-inch bypass valve was capable of 1,000 gpm with a 5-psig pressure drop when a minimum total boiler flow rate of 120 gpm and a minimum constant flow rate of 31 gpm was all that was needed.
Therefore, the bypass valve should modulate between 0-89 gpm; the recommended solution was to replace the 4-inch bypass valve with a 2-inch valve to solve the hunting problem.
PID Tuning
Variable Air Volume (VAV) boxes with a hot water reheat coil are designed to maintain the zone temperature within the serving space. In the example below, during operation, the VAV boxes kept the zone temperature setpoint however, the trend information showed the reheat control valves were hunting (Figure 2).
The hunting of the control valves will cause premature wear on the control valve and the actuator, leading to more frequent control valve and/or actuator replacements and increased maintenance costs.
Our recommended solution was for the controls contractor to come back and Proportional, Integral, and Derivative (PID) tune the reheat control valves.
By PID tuning the control valves, the reactions of the control valves were adjusted so that when there are zone temperature changes within one space the valves will modulate with the variations.
Although the zone heating hot water temperatures were functioning, the existing method was not the most efficient or effective way to achieve the desired temperatures. Our analytics and fault detection software has the ability to trend, analyze, and report on these HVAC System performances and faults.
Our software will provide the data to help facilities personnel, design engineers, and commissioning agents create a better-performing HVAC System. The result saves time by troubleshooting problems, reducing the need for parts replacement, leads to fewer occupant complaints, and provides a proactive approach to operating your HVAC System.
What types of HVAC issues can retro-commissioning identify? Retro-commissioning can help identify issues such as equipment hunting, incorrect valve sizing, poor PID tuning, excessive cycling, inefficient reheat operation, sensor problems, airflow issues, and systems that no longer match the intended design sequence.
How does building analytics support retro-commissioning? Building analytics helps by reviewing trend data from the building automation system. This makes it easier to see patterns over time, compare actual operation to intended performance, and identify issues that may not be visible during a single site visit.
Why does boiler hunting matter? Boiler hunting can lead to inefficient operation, unnecessary cycling, temperature swings, added wear on equipment, and more difficult troubleshooting for facility teams.
Why is PID tuning important for VAV reheat valves? PID tuning helps control valves respond more smoothly to temperature changes. Poor tuning can cause valves to open and close too frequently, which can waste energy and shorten actuator or valve life.
Who benefits from retro-commissioning? Facility managers, building owners, commissioning agents, design engineers, service providers, and operations teams can benefit because retro-commissioning helps connect system data to practical corrective action.
Topics covered: retro-commissioning, HVAC retro-commissioning, building analytics, building automation system data, BAS data, boiler hunting, oversized bypass valves, heating hot water systems, PID tuning, VAV reheat valves, commercial HVAC performance, facility operations, commissioning, and fault detection.
Reviewed and updated: June 2, 2026
Reviewed by: Resolute Building Intelligence
Topic areas: HVAC analytics, retro-commissioning, building automation system data, boiler operation, VAV reheat valves, PID tuning, and commercial building performance.
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